Hypotheses of origins.
Perhaps the most fundamental and at the same time the least understood
biological problem
is the origin of life. It is central to many scientific and philosophical
problems and to any
consideration of extraterrestrial life. Most of the hypotheses of the origin
of life will fall
into one of four categories:
1. The origin of life is a result of a supernatural event; that is, one
permanently beyond the
descriptive powers of physics and chemistry.
2. Life--particularly simple forms--spontaneously and readily arises from
nonliving matter
in short periods of time, today as in the past.
3. Life is coeternal with matter and has no beginning; life arrived on
the Earth at the time
of the origin of the earth or shortly thereafter. (see also Index: spontaneous
generation)
4. Life arose on the early Earth by a series of progressive chemical reactions.
Such
reactions may have been likely or may have required one or more highly
improbable
chemical events.
Hypothesis 1, the traditional contention of theology and some philosophy,
is in its most
general form not inconsistent with contemporary scientific knowledge, although
this
knowledge is inconsistent with a literal interpretation of the biblical
accounts given in
chapters 1 and 2 of Genesis and in other religious writings. Hypothesis
2 (not of course
inconsistent with 1) was the prevailing opinion for centuries. A typical
17th-century view
follows:
[May one] doubt whether, in cheese and timber, worms are generated, or,
if
beetles and wasps, in cowdung, or if butterflies, locusts, shellfish, snails,
eels,
and such life be procreated of putrefied matter, which is to receive the
form
of that creature to which it is by formative power disposed[?] To question
this
is to question reason, sense, and experience. If he doubts this, let him
go to
Egypt, and there he will find the fields swarming with mice begot of the
mud
of the Nylus [Nile], to the great calamity of the inhabitants.
It was only in the Renaissance, with its burgeoning interest in anatomy,
that such
transformations were realized to be impossible. A British physiologist,
William Harvey,
during the mid-17th century, in the course of his studies on the reproduction
and
development of the king's deer, made the basic discovery that every animal
comes from an
egg. An Italian biologist, Francesco Redi, in the latter part of the 17th
century, established
that the maggots in meat came from flies' eggs, deposited on the meat.
And an Italian
priest, Lazzaro Spallanzani, in the 18th century, showed that spermatozoa
were necessary
for the reproduction of mammals. But the idea of spontaneous generation
died hard. Even
though it was proved that the larger animals always came from eggs, there
was still hope
for the smaller ones, the microorganisms. It seemed obvious that, because
of their ubiquity,
these microscopic creatures must be generated continually from inorganic
matter.
Meat could be kept from going maggoty by covering it with a flyproof net,
but grape juice
could not be kept from fermenting by putting over it any netting whatever.
This was the
subject of a great controversy between the famous French bacteriologists
Louis Pasteur
and F.A. Pouchet in the 1850s, in which Pasteur triumphantly showed that
even the
minutest creatures came from germs floating in the air, but that they could
be guarded
against by suitable filtration. Actually, Pouchet was arguing that life
must somehow arise
from nonliving matter; if not, how had life come about in the first place?
(see also Index:
germ theory)
Toward the end of the 19th century Hypothesis 3 gained currency, particularly
with the
suggestion by a Swedish chemist, S.A. Arrhenius, that life on Earth arose
from
panspermia, microorganisms or spores wafted through space by radiation
pressure from
planet to planet or solar system to solar system. Such an idea of course
avoids rather than
solves the problem of the origin of life. In addition, it is extremely
unlikely that any
microorganism could be transported by radiation pressure to the Earth over
interstellar
distances without being killed by the combined effects of cold, vacuum,
and radiation.
Pasteur's work discouraged many scientists from discussing the origin of
life at all.
Moreover they were anxious not to offend religious feeling by probing too
deeply into the
subject. Although Darwin would not commit himself on the origin of life,
others
subscribed to Hypothesis 4 more resolutely, notably the famous British
biologist T.H.
Huxley in his Protoplasm, the Physical Basis of Life (1869), and the British
physicist
John Tyndall in his "Belfast Address" of 1874. Although Huxley and Tyndall
asserted that
life could be generated from inorganic chemicals, they had extremely vague
ideas about
how this might be accomplished. The very phrase "organic molecule" implies
that there
exists a special class of chemicals uniquely of biological origin, despite
the fact that organic
molecules have been routinely produced from inorganic chemicals since 1828.
In the
following discussion the word organic carries no imputation of biological
origin. In fact the
problem largely reduces to finding an abiological source of appropriate
organic molecules.
(see also Index: Darwinism)
The primitive atmosphere.
Darwin's attitude was: "It is mere rubbish thinking at present of the origin
of life; one might
as well think of the origin of matter." The two problems are, in fact,
curiously connected,
and modern scientists are thinking about the origin of matter. There is
convincing evidence
that thermonuclear reactions and subsequent explosions in the interiors
of stars generate all
the chemical elements more massive than hydrogen and helium and then distribute
them into
the interstellar medium from which subsequent generations of stars and
planets form.
Because of the commonality of these thermonuclear processes, and because
some
thermonuclear reactions are more probable than others, there exists a cosmic
distribution of
the major elements, so far as is known, throughout the universe. Table
1 compares, for
some atoms of interest, the relative numerical abundances in the universe
as a whole, on the
Earth, and in living organisms. There is of course some variation in composition
from star to
star, from place to place on the Earth, and from organism to organism,
but such
comparisons are nevertheless very instructive. The composition of life
is intermediate
between the average composition of the universe and the average composition
of the Earth.
Ninety-nine percent both of the universe and of life is made of the six
atoms, hydrogen
(H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne).
Can it be that life
on Earth arose when the chemical composition of the Earth was much closer
to the average
cosmic composition, and that some subsequent events have changed the gross
chemical
composition of the Earth?
The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are much closer
to cosmic
composition than is the Earth. They are largely gaseous, with atmospheres
composed
principally of hydrogen and helium. Methane (CH4) and ammonia (NH3) have
been
detected in smaller quantities, and neon and water are suspected. This
circumstance very
strongly suggests that the Jovian planets were formed out of material of
typical cosmic
composition. They have very large masses, and because they are so far from
the sun their
upper atmospheres are very cold. Therefore it is impossible for atoms in
the upper
atmospheres of the Jovian planets to escape from their gravitational fields;
escape was
probably very difficult even during planetary formation. The Earth and
the other planets of
the inner solar system, however, are much less massive and most have hotter
upper
atmospheres. It is possible for hydrogen and helium to escape from the
Earth today, and it
may well have been possible for much heavier gases to have escaped during
the formation
of the Earth. It is reasonable to expect that in the very early history
of the Earth a much
larger abundance of hydrogen prevailed, which has subsequently been lost
to space. Thus
the atoms carbon, nitrogen, and oxygen were present on the primitive Earth,
not as CO2
(carbon dioxide), N2, and O2 as they are today but rather in the form of
their fully
saturated hydrides, CH4 (methane), NH3 (ammonia), and H2O. In the geological
record,
the presence of such reduced minerals as uraninite (UO2) and pyrite (FeS2)
in sediments
formed several billions of years ago implies that conditions then were
considerably less
oxidizing than they are today.
In the 1920s J.B.S. Haldane in Britain and A.I. Oparin in the Soviet Union
recognized that
the abiological production of organic molecules in the present oxidizing
atmosphere of the
Earth is highly unlikely; but that, if the Earth once had more reducing
(in this context,
hydrogen-rich) conditions, the possible abiogenic production of organic
molecules would
have been much more likely. If large numbers of organic molecules were
somehow
synthesized on the primitive Earth, there would not necessarily be much
trace of them
today. In the present oxygen atmosphere, largely produced by green-plant
photosynthesis,
such molecules would tend, over geological time, to be oxidized to carbon
dioxide,
nitrogen, and water. In addition, as Darwin recognized, the first microorganisms
would
consume prebiological organic matter produced prior to the origin of life.
Production of simple organic molecules.
The first deliberate experimental simulation of these primitive conditions
was carried out in
1953 by a U.S. graduate student, S.L. Miller, under the guidance of the
eminent chemist
H.C. Urey. A mixture of methane, ammonia, water vapour, and hydrogen was
circulated
through a liquid water solution and continuously sparked by a corona discharge
elsewhere
in the apparatus. The discharge may be thought to represent lightning flashes
on the early
Earth. After several days of exposure to sparking, the solution changed
colour. Subsequent
analysis indicated that several amino and hydroxy acids, intimately involved
in
contemporary life, had been produced by this simple procedure. The experiment
is in fact
so elementary, and the amino acids can so readily be detected by paper
chromatography,
that the experiment has been repeated many times by high school students.
Subsequent
experiments have substituted ultraviolet light or heat as the energy source
or have altered
the initial abundances of gases. In all such experiments amino acids have
been formed in
large yield. On the early Earth there was much more energy available in
ultraviolet light than
in lightning discharges. At long ultraviolet wavelengths, in which methane,
ammonia, water,
and hydrogen are all transparent, but in which the bulk of the solar ultraviolet
energy lies,
the gas hydrogen sulfide (H2S) is a likely ultraviolet absorber. (see also
Index: ultraviolet
radiation)
Following such reasoning, a U.S. astrophysicist, Carl Sagan, and his colleagues
made
amino acids by long wavelength ultraviolet irradiation of a mixture of
methane, ammonia,
water, and H2S. The amino acid syntheses, at least in many cases, involve
hydrogen
cyanide and aldehydes (e.g., formaldehyde) as gaseous intermediaries formed
from the
initial gases. It is quite remarkable that amino acids, particularly biologically
abundant amino
acids, can be made so readily under simulated primitive conditions. When
laboratory
conditions become oxidizing, however, no amino acids are formed, suggesting
that reducing
conditions were necessary for prebiological organic synthesis.
Under alkaline conditions, and in the presence of inorganic catalysts,
formaldehyde
spontaneously reacts to form a variety of sugars, including the five-carbon
sugars
fundamental to the formation of nucleic acids and such six-carbon sugars
as glucose and
fructose, which are extremely common metabolites and structural building
blocks in
contemporary organisms. Furthermore, the nucleotide bases as well as porphyrins
have
been produced in the laboratory under simulated primitive Earth conditions
by several
investigators. While there is still debate on the generality of the experimental
synthetic
pathways and on the stability of the molecules produced, most if not all
of the essential
building blocks of proteins, carbohydrates, and nucleic acids can be readily
produced
under quite general primitive reducing conditions, plus probably ATP as
well.
Production of polymers.
The construction of polymers, long-chain molecules made of repeating units
of these
essential building blocks, however, is a much more difficult experimental
problem.
Polymerization reactions are generally dehydrations, in which a molecule
of water is lost in
the formation of a two-unit polymer. Dehydrating agents must be used to
initiate
polymerization. The polymerization of amino acids to form long protein-like
molecules was
accomplished through dry heating by a U.S. investigator, S.W. Fox. The
polyamino acids
that are formed are not random polymers and have some distinct catalytic
activities. The
geophysical generality of dry heating and return to solution, however,
has been questioned.
Long polymers of amino acids can also be produced from hydrogen cyanide
and
anhydrous liquid ammonia. Some evidence exists that nucleotide bases and
sugars can be
combined in the presence of phosphates or cyanides under ultraviolet irradiation.
Some
condensing agents such as cyanamide are efficiently made under simulated
primitive
conditions. Despite the breakdown by water of molecular intermediates,
condensing agents
are often quite effective in inducing polymerization, and polymers of amino
acids, sugars,
and nucleotides have all been made this way.
A famous British scientist, J.D. Bernal, suggested that adsorption of molecular
intermediates on clays or other minerals may have concentrated these intermediates.
Such
concentration could offset the tendency for water to break down polymers
of biological
significance. Of special interest is the possibility that such concentration
matrices included
phosphates, for this would help explain how phosphorus could have been
incorporated
preferentially into prebiological organic molecules at a time when biological
concentration
mechanisms did not yet exist. Mineral catalysis implies that organic synthesis
could also
occur in deep water where ultraviolet light had been filtered out.
Quite apart from concentration mechanisms, the primitive waters themselves
may have
been a not very dilute solution of organic molecules. If all the surface
carbon on the Earth
were present as organic molecules in the contemporary oceans, or if many
known
ultraviolet synthetic reactions producing organic molecules were permitted
to continue for a
billion years with products dissolved in the oceans, a 1 percent solution
of organic
molecules would result. For similar reasons, Haldane suggested that the
origin of life
occurred in a "hot dilute soup." In addition, concentration mechanisms
do exist, such as
evaporation or freezing of pools or adsorption on interfaces or the generation
of colloidal
enclosures called coacervates.
The origin of the code.
It has been shown that all the essential building blocks for life and their
polymers may have
been produced in some fair concentration on the primitive Earth. This possibility
is certainly
relevant to the origin of life, but it is not the same thing as the origin
of life. By the genetic
definition of life discussed above in Definitions of life, a self-replicating,
mutable
molecular system, capable of interacting with the environment, is required.
In contemporary
cells the nucleic acids are the sites of self-replication and mutation.
Laboratory experiments
have already shown that polynucleotides can be produced from nucleotide
phosphates in
the presence of a specific enzyme of biological origin and a pre-existing
"primer" nucleic
acid molecule. If the primer molecule is absent, polynucleotides are still
formed, but they of
course contain no genetic information. Once such a polynucleotide spontaneously
forms, it
then acts as primer for subsequent syntheses. (see also Index: genetic
code)
Imagine a primitive ocean filled with nucleotides and their phosphates
and appropriate
mineral surfaces serving as catalysts. Even in the absence of the appropriate
enzyme it
seems likely, although not yet proved, that spontaneous assembly of nucleotide
phosphates
into polynucleotides occurred. Once the first such polynucleotide was produced,
it may
have served as a template for its own reproduction, still of course in
the absence of
enzymes. As time went on there were bound to be errors in replication.
These would be
inherited. A self-replicating and mutable molecular system of polynucleotides,
eventually
leading to a diverse population of such molecules, may have arisen in this
way.
Alternatively, the primitive hereditary material may have involved some
other molecule
altogether, but no concrete suggestion for such a molecule has ever been
proposed.
In any case, a population of replicating polynucleotides cannot quite be
considered alive
because it does not significantly influence its environment. Eventually,
all the nucleotides in
the ocean would have been tied in polynucleotides and the entire synthetic
process would
then have ground to a halt. So far as is known, polynucleotides have no
an catalytic
properties, and proteins have no reproductive properties. It is only the
partnership of the
two molecules that makes contemporary life on Earth possible. Accordingly,
a critical and
unsolved problem in the origin of life is the first functional relation
between these two
molecules, or, equivalently, the origin of the genetic code. The molecular
apparatus
ancillary to the operation of the code--the activating enzymes, adapter
RNAs, messenger
RNAs, ribosomes, and so on--are themselves each the product of a long evolutionary
history and are produced according to instructions contained within the
code. At the time
of the origin of the code such an elaborate molecular apparatus was of
course absent.
It has been proposed that a weak but selective chemical bonding does exist,
even in the
absence of any of this apparatus, between amino acids and nucleotides.
There need not be
a very great selectivity; a given nucleotide sequence might in primitive
times have coded for
many different amino acids or, conversely, the same amino acid may have
been coded for
by several different nucleotide sequences. All that is required is that
a particular linear
sequence of nucleotides must code for some nonrandom sequence of amino
acids. The
active sites largely responsible for the catalytic activity of contemporary
enzymes are
generally only five or six amino acids long; the remainder of the enzyme
is devoted to more
sophisticated functions, such as arranging for the enzyme to be turned
on and off by the
machinery of the cell. With, say, 20 different varieties of amino acids
available in the
primitive environment, the chance of any given active site being produced
by a random
sequence of nucleotides is one in 205, or one in about 3,000,000. But 3,000,000
combinations to form units five amino acids long is not a very large number
for the
chemistry and time periods in question. To conclude this speculation, then,
if
polynucleotides were initially capable of crude, nonenzymatic replication,
and if a crude
primitive genetic code existed, then any one of a very large number of
catalytic properties
was available to some self-replicating polynucleotides on the primitive
Earth. This situation
is all that would be necessary for the origin of life; those polynucleotides
that could code
for a primitive protein having catalytic properties furthering the replication
of the
polynucleotide would preferentially replicate. Other polynucleotides coding
for less
effective proteins would have replicated more slowly. The foregoing is
one of several
possibilities for the origin of the first living systems. Many separate
and rather diverse
instances of the origin of life may have occurred on the primitive Earth,
but competition
eventually eliminated all but one line. Every organism on Earth today would
be a
descendant of that line.
The earliest living systems.
One curious feature of biological organic molecules is their optical activity:
they rotate the
plane of a beam of plane- polarized light. Organic molecules produced abiologically
do not
show optical activity. Molecules made of the same units can be put together
in
complementary ways like a left- and right-handed glove. The same building
blocks can be
used to produce molecules that are three-dimensional mirror images of each
other. This
asymmetry is responsible for optical activity. At the time of the origin
of life, organic
molecules, corresponding both to left- and right-handed forms, were produced.
The
laboratory simulation experiments always produce both types. But the first
living systems
could have been made only of one type, for the same reason that carpenters
do not use
random mixtures of screws with left- and right-handed threads. Whether
left- or
right-handed activity was adopted was probably purely a matter of chance,
but once a
particular asymmetry was established in the first living systems, it maintained
itself. This
belief implies that optical activity should be a feature of life on any
planet, and also that the
chances should be equal of finding a given terrestrial organic molecule
or its mirror image
molecule in extraterrestrial life forms. (see also Index: polarization)
The first living systems probably resided in a molecular garden of Eden,
where all the
building blocks that contemporary organisms must work hard at synthesizing
were available
free. Under such conditions the numbers of organisms must have increased
very rapidly.
But such increases cannot go on indefinitely. In time the supply of some
molecular building
block must have become short. Those primitive organisms that had the ability
to synthesize
the scarce building block, say A, from a more abundant one, say B, clearly
had a
competitive advantage over those organisms that could not perform such
a synthesis. In
time, however, the secondary source of supply, B, would have also become
depleted and
those organisms that could produce it from a third building block, C, would
have
preferentially replicated. A U.S. biochemist, N.H. Horowitz, proposed that
in this way the
enzymatic reaction chains of contemporary organisms--each step catalyzed
by a particular
enzyme--originally evolved.
Even the evolution of enzymatic reaction chains may have occurred in free
nucleic acids
before the origin of the cell. The cell may have arisen in response to
the need for
maintaining a high concentration of scarce building blocks or enzymes,
or as protection
against the gradually increasing abundance of oxygen on the primitive Earth.
Oxygen is a
well-known poison to many biological processes, and in contemporary higher
organisms
the mitochondria that handle molecular oxygen are kept in the cytoplasm,
far from contact
with the nuclear material. Even today processes are known whereby polyamino
acids form
small spherical objects, microns to tens of microns across, with some of
the properties of
cells. These objects, called proteinoid microspheres by Fox, are certainly
not cells, but they
may indicate processes by which the ancestors of cells arose. Procaryotic
cells almost
certainly preceded eucaryotic cells, and the evolution of so extremely
complex an
apparatus as the mitotic spindle (which ensures equal segregation of replicated
chromosomes) must have taken very long periods of time to evolve. The development
of
mitochondria and chloroplasts (each of which contains its own DNA) in the
eucaryotic cell
may have been the result of a symbiosis, a cooperative arrangement entered
into at first
tentatively by originally free-living cells.
As the competition for building blocks increased among early life forms,
and also perhaps
as the abiological production of organic molecules dwindled because of
the increasing
oxygen abundance, the strictly heterotrophic way of life became more and
more costly.
The utilization of porphyrins, which are also made abiologically, by primitive
photoautotrophs would have had great selective advantage. Many of the intermediates
and
enzymes in photosynthesis and in the anaerobic breakdown of carbon compounds
are
similar, but there is no generally accepted view of the origin of the photosynthetic
process.
Photosynthesis in procaryotes is more primitive than in such eucaryotes
as green plants. In
bacteria, water is not the ultimate source of hydrogen atoms for reducing
carbon dioxide,
and therefore oxygen is not produced. In addition, when a chlorophyll-containing
cell is
exposed both to light and to oxygen, it is killed unless it also contains
an accessory
carotenoid pigment. Thus green-plant photosynthesis had to wait until the
appearance of
carotenoids while bacterial photosynthesis, which does not produce oxygen,
could function
without carotenoids. (see also Index: carotene)
The antiquity of life.
Among the oldest known fossils are those found in the Fig Tree chert from
the Transvaal,
dated at 3,100,000,000 years old. These organisms have been identified
as bacteria and
blue-green algae. It is very reasonable that the oldest fossils should
be procaryotes rather
than eucaryotes. Even procaryotes, however, are exceedingly complicated
organisms and
very highly evolved. Since the Earth is about 4,500,000,000 years old,
this suggests that
the origin of life must have occurred within a few hundred million years
of that time. (see
also Index: fossil record)
By performing chemical analyses on the oldest sediments, it is possible
to say something
about the sorts of organic molecules produced, either biologically or abiologically,
in
primitive times. Thus, amino acids and porphyrins have been identified
in the oldest
sediments, as have pristane and phytane, typical breakdown products of
chlorophyll. There
are several indications that these organic molecules, dating from 2,000,000,000
to more
than 3,000,000,000 years ago, are of biological origin. For one thing their
long-chain
hydrocarbons show a preference for a straight chain geometry, whereas known
abiological
processes tend to produce a much larger proportion of branched chain and
cyclic
hydrocarbon molecular geometries than have been found in these sediments.
Abiological
processes tend to produce equal amounts of long-chain carbon compounds
with odd and
with even numbers of carbon atoms. But the oldest sediments show a distinct
preference
for odd numbers of carbon atoms per molecule, as do products of undoubted
biological
origin. Finally, a C12 enrichment, for which no abiological process seems
able to account,
has been discovered in the oldest sediments, evidence that suggests that
plantlike life,
which concentrates the carbon isotope C12 preferentially to C13, was present
very early.
These departures from thermodynamic equilibrium are often considered to
be compelling
signs of biological activity. Such evidence again points to the great antiquity
of life on
Earth.
The fossil record, in any complete sense, goes back only about 600,000,000
years. In the
layers of sedimentary rock known by geological methods and by radioactive
dating to be
that old, most of the major groups of invertebrates appear for the first
time. All these
organisms appear adapted to life in the water, and there is no sign yet
of organisms
adapted to the land. For this reason, and because of a rough similarity
between the salt
contents of blood and of seawater, it is believed that early forms of life
developed in
oceans or pools. With no evidence for widespread oxygen-producing photosynthesis
before this time, and for cosmic abundance reasons described above, the
oxygen content
of the Earth's atmosphere in Precambrian times was very likely less than
today.
Accordingly, in Precambrian times, solar ultraviolet radiation, especially
near the
wavelength of 2,600 Å, which is particularly destructive to nucleic
acids, may have
penetrated to the surface of the Earth, rather than being totally absorbed
in the upper
atmosphere by ozone as it is today. In the absence of ozone, the ultraviolet
solar flux is so
high that a lethal dose for most organisms would be delivered in less than
an hour. Unless
extraordinary defense mechanisms existed in Precambrian times, life near
the Earth's
surface would have been impossible. Sagan suggested that life at this time
was generally
restricted to some tens of metres and deeper in the oceans, at which depths
all the
ultraviolet light would have been absorbed, although visible light would
still filter through.
As the amount of atmospheric oxygen and ozone increased, due both to plant
photosynthesis and to the photodissociation of water vapour and the escape
to space of
hydrogen from the upper atmosphere, life increasingly close to the Earth's
surface would
have been possible. It has been suggested that the colonization of the
land, about
425,000,000 years ago, was possible only because enough ozone was then
produced to
shield the surface from ultraviolet light for the first time.
Life then had insinuated itself between the sun and the Earth. It diverted
solar energy to its
own uses and contrived more and more ways of exploiting more and more environments.
Some experiments were faulty and the lines became extinct; others were
more successful
and the lines filled the Earth. Evolution through natural selection directed
the proliferation of
a growing array of life forms throughout the biosphere (see EVOLUTION;
THE
THEORY OF: The concept of natural selection).
EXTRATERRESTRIAL LIFE
It is notknown what aspects of living systems are necessary in the sense
that living systems
everywhere must have them; it is not known what aspects of living systems
are contingent
in the sense that they are the result of evolutionary accident, so that
somewhere else a
different sequence of events might have led to different characteristics.
In this respect the
possession of even a single example of extraterrestrial life, no matter
how seemingly
elementary in form or substance, would represent a fundamental revolution
in biology. It is
not known whether there is a vast array of biological themes and counterpoints
in the
universe, whether there are places that have fugues, compared with which
our one tune is a
bit thin and reedy. Or it may be that our tune is the only tune around.
Accordingly the
prospects for life on other planets must be considered in any general discussion
of life.
(see also Index: exobiology)
The chemistry of extraterrestrial life.
What are the methods and prospects for a search for life beyond the Earth?
Each of the
definitions of life described in Definitions of life (see above) implies
a method of
searching for life. Particular physiological functions, particular metabolic
activities, such
specific molecules as proteins and nucleic acids, self-replication and
mutation, processes
not in closed-system thermodynamic equilibrium--all these might be sought.
All the search
methods significantly depend upon chemistry.
Life on Earth is structurally based on carbon and utilizes water as an
interaction medium.
Hydrogen and nitrogen have significant accessory structural roles; phosphorus
is important
for energy storage and transport, sulfur for three-dimensional configuration
of protein
molecules, and so on. But must these particular atoms be the atoms of life
everywhere, or
might there be a wide range of atomic possibilities in extraterrestrial
organisms? What are
the general physical constraints on extraterrestrial life?
In approaching these questions several criteria can be used. The major
atoms should tend
to have a high cosmic abundance. A structural molecule for making an organism
at the
temperature of the planet in question should not be extremely stable, because
then no
chemical reactions would be possible; but it should not be extremely unstable,
because then
the organism would fall to pieces. There should be some medium for molecular
interaction.
Solids are not appropriate because the diffusion times are very long. Such
a medium is
most likely a liquid (but could possibly be a very dense gas) that is stable
in a number of
respects. It should have a large temperature range (for a liquid, the temperature
difference
between freezing point and boiling point should be large). The liquid should
be difficult to
vaporize and to freeze; in fact, it should be very difficult to change
its temperature at all. In
addition it should be an excellent solvent. There should also be some gas
on the planet in
question that could be used in various biologically mediated cycles, as
CO2 is in the carbon
cycle on Earth.
The planet, therefore, should have an atmosphere and some near-surface
liquid, although
not necessarily an ocean. If the intensity of ultraviolet light or charged
particles from the sun
is intense at the planetary surface, there must be some place, perhaps
below the surface,
that is shielded from this radiation but that nevertheless permits useful
chemical reactions to
occur. Since after a certain period of evolution, lives of unabashed heterotrophy
lead to
malnutrition and death, autotrophs must exist. Chemoautotrophs are, of
course, a
possibility but the inorganic reactions that they drive usually require
a great deal of energy;
at some stage in the cycle, this energy must probably be provided by sunlight.
Photoautotrophs, therefore, seem required. Organisms that live very far
subsurface will be
in the dark, making photoautotrophy impossible. Organisms that live slightly
subsurface,
however, may avoid ultraviolet and charged particle radiation and at the
same time acquire
sufficient amounts of visible light for photosynthesis.
Thermodynamically, photosynthesis is possible because the plant and the
radiation it
receives are not in thermodynamic equilibrium; for example, on the Earth
a green plant may
have a temperature of about 300 K while the sun has a temperature of about
6,000 K. (K
= Kelvin temperature scale, in which 0 K is absolute zero; 273 K, the freezing
point of
water; and 373 K, the boiling point of water at one atmosphere pressure.)
Photosynthetic
processes are possible in this case because energy is transported from
a hotter to a cooler
object. Were the source of radiation at the same (or at a colder) temperature
as the plant,
however, photosynthesis would be impossible. For this reason the idea of
a subterranean
plant photosynthesizing with the thermal infrared radiation emitted by
its surroundings is
untenable, as is the idea that a cold star, with a surface temperature
similar to that of the
Earth, would harbour photosynthetic organisms.
It is possible to approach some of the foregoing chemical requirements
and see just which
atoms are implied. When atoms enter into chemical combination, the energy
necessary to
separate them is called the bond energy, a measure of how tightly the two
atoms are bound
to each other. Table 2 gives the bond energies of a number of chemical
bonds, mostly
involving abundant atoms. The energies are in electron volts (eV; 1 eV
= 1.6 × 10-12
ergs). The symbols are as follows: H, hydrogen; C, carbon; N, nitrogen;
O, oxygen; S,
sulfur; F, fluorine; Si, silicon; Bi, bismuth (very underabundant, biologically
uninteresting,
and present only as an illustration of the relatively weak chemical bonds
in some metals).
Bond energies generally vary between 10 eV and about 0.03 eV; double and
triple bonds
where two or three electrons are shared between two atoms tend to be more
energetic
than single bonds, single bonds more energetic than hydrogen bonds where
a hydrogen
atom is shared between two other atoms, and hydrogen bonds more energetic
than the
very weak (van der Waals) forces that arise from the attraction of the
electrons of one
atom for the nucleus of another. At room temperature, atoms, free or bound,
move with an
average kinetic energy corresponding to about 0.02 eV. Some of the atoms
have greater
energies, some lesser. At any temperature a few will have energies greater
than any given
bond energy; hence bonds occasionally will break. The higher the temperature,
the more
atoms there are moving with sufficient energy to spontaneously break a
given bond. (see
also Index: chemical bonding)
Suppose it is decided arbitrarily (although the decision will not critically
affect the
conclusions) that for life to exist at any time the fraction of bonds broken
by random
thermal motions must be no larger than 0.0001 percent. It then turns out
that any
hypothetical life where the structural bonds are based upon van der Waals
forces can only
exist where the temperature is below 40 K, for hydrogen bonds below about
400 K, for
bonds of 2 eV below 2,000 K, and for bonds of 5 eV below 5,000 K. Now,
2,000 to
5,000 K are typical surface temperatures of stars; 400 K is somewhat above
the highest
surface temperature found on Earth; and 40 K is about the cloud-top temperature
of
distant Neptune. Thus, over the entire range of temperatures, from cold
stars to cold
planets, there seem to exist chemical bonds of appropriate structural stability
for life, and it
would appear premature to exclude the possibility of life on any planet
on grounds of
temperature.
Life on Earth lies within a rather narrow range of temperature. Above the
normal boiling
point of water, much loss of configurational structure or three-dimensional
geometry
occurs. At these temperatures proteins become denatured, in part because
above the
boiling point of water the hydrogen bonding and van der Waals forces between
water and
the protein disappear. Also, similar bonds within the protein molecule
tend to break down.
Proteins then change their shapes, their ability to participate in lock-and-key
enzymatic
reactions is gravely compromised, and the organism dies. Similar structural
changes, some
of them connected with the stacking forces between adjacent nucleotide
bases, occur in the
heating of nucleic acids. But it is significant that these changes are
not fragmentations of the
relevant molecules but rather changes in the ways they fold. There appears
to be no reason
that configurational bonds should not have been evolved that are stable
at higher
temperatures than terrestrial organisms experience. On planets hotter than
the Earth there
seems to be no reason that slightly more stable configurational forces
should not be
operative in the local biochemistry.
Molecular factors.
While the bonds that characterize life on Earth are too weak at high temperatures,
they are
too strong at low temperatures, tending to slow down the rates of chemical
reactions
generally. There are less stable bonds (e.g., hydrogen bonds, silicon-silicon
bonds, and
nitrogen-nitrogen bonds), however, that might play structural roles at
significantly lower
temperatures. At higher temperatures, multiple bonds (e.g., in aromatic,
or ring-shaped,
hydrocarbons) might be utilized for life. There clearly is a rich variety
of little-studied
chemical reactions that proceed at reasonable rates either at much lower
or at much higher
temperatures than those on Earth.
Except for bismuth and fluorine, all the atoms in Table 2 have relatively
high cosmic
abundances. At terrestrial temperatures, carbon is the unique atom for
biological structure.
Not only does it have high abundance but it forms a staggering variety
of compounds of
great stability, it lends itself to compounds that are configured by weaker
bonds, and it
enters into multiple bonds. These double- and triple-bonded molecules,
among other useful
properties, absorb long-wavelength ultraviolet light, a process leading
to the synthesis of a
variety of more complex molecules. A photon of ultraviolet light at a wavelength
of 2,000
Å has an energy of 6.2 eV, capable of breaking many bonds, and permitting
more complex
reactions among the resulting molecular fragments. Photons of blue light
have energies of
about 3 eV, and of red light about 2 eV.
Silicon compounds do not form double bonds. Silicon-oxygen bonds are slightly
more
stable than carbon-carbon bonds, but they tend to produce molecules like
the silicates,
which are crystals of the same unit repeated over and over again, rather
than molecules
with aperiodic side chains with potential information content. On low-temperature
planets,
silicon-silicon bonds are more promising than carbon bonds in terms of
reaction times, but
they do not form double bonds and the carbon abundance is likely to be
greater.
Nevertheless, silicon compounds may be of limited biological importance
both on
high-temperature and low-temperature worlds.
Hydrogen bonding confers on liquids the stability properties necessary
for life. There seem
to be very few reasonable candidates for liquid interaction media. By all
odds water is the
most suitable. The other candidates, all to some extent hydrogen bonded,
are ammonia,
hydrogen fluoride, hydrogen cyanide, and mixtures of liquid hydrocarbons.
Hydrogen
fluoride can be excluded because it is too scarce cosmically. The hydrocarbons
are not
good solvents of salts, but life elsewhere may not be based on the same
acid-base
chemistry as life on Earth. The liquid range of water is larger than commonly
thought,
ranging from about 210 K in saturated salt solutions to 647 K at enormous
atmospheric
pressures. Water is the biological liquid medium of choice above 200 K,
particularly in
view of its extremely high cosmic abundance. At lower temperatures ammonia
or hydrogen
cyanide could serve as a liquid medium. (see also Index: molecule)
There are functional roles for specific atoms in biology, but except for
considerations of
structure and a liquid interaction medium they do not seem fundamental.
For example, the
energy-rich phosphate bonds in ATP are in fact of relatively low energy;
they are about as
energetic as the hydrogen bonds (see Table 2). The cell must store up large
numbers of
these bonds to drive a molecular degradation or synthesis. On high-temperature
worlds the
energy currency may be much more energetic per bond, and on low-temperature
worlds
much less energetic per bond.
It may be concluded that, in our present state of ignorance, it is premature
to exclude life
on grounds of temperature on any other planet, particularly when account
is taken of the
temperature heterogeneity of the other planets. But life does require an
interaction medium,
an atmosphere, and some protection from ultraviolet light and from charged
particles of
solar origin.
The conclusion that for the Earth, carbon-based aqueous life is the most
appropriate may
be slightly suspect, since terrestrial life is manifestly carbon-based
and aqueous. In 1913 a
U.S. biochemist, L.J. Henderson, published The Fitness of the Environment
in which the
biological advantages of carbon and water were stressed for the first time
in terms of
comparative chemistry. He was struck by the fact that those very atoms
that are needed
are just those atoms that are around; it remains a remarkable fact that
atoms most useful for
life do have very high cosmic abundances. (see also Index: carbon cycle,
carbon)
The search for extraterrestrial life.
Exobiology, a term coined by a U.S. biologist, J. Lederberg, for the study
of
extraterrestrial life, has been called a science without a subject matter.
It is certainly true
that, as yet, no strong evidence for life beyond the Earth has been adduced.
Exobiology,
however, has deep significance even if extraterrestrial life is never found.
The mere design
of exobiological experiments forces man to examine critically the generality
of his
assumptions about life on Earth. In addition, a lifeless neighbouring planet
presents a very
interesting quandary: How is it that life has originated and evolved on
Earth, but not on the
planet in question? There is an entire spectrum of possibilities. A given
planet may be
lifeless and have no vestiges of primitive organic matter and no fossils
of extinct life. It may
be lifeless but may have either organic chemical or fossil relics. It may
possess life of a
simple sort or life of a quite complex biochemistry, physiology, and behaviour.
It may
possess intelligent life and a technical civilization. Establishment of
any one of these five
possibilities would be of fundamental biological importance.
The difficulties and opportunities inherent in exobiological exploration,
in determining which
of these five possibilities applies to a given planet, is most clearly
grasped by imagining the
situation reversed, with man on some neighbouring planet, say Mars, examining
the Earth
for life with the full armoury of contemporary scientific instrumentation
and knowledge.
First a distinction must be made between remote and in situ testing. In
remote testing light
of any wavelength reflected from or emitted by the target planet can be
examined, but with
in situ studies samples of the planet must be acquired by visiting them
or by sending
instruments that land on the planet, perform experiments, and radio back
their findings.
Since biological exploration involves the detailed characterization of
any life found, rather
than its mere detection, in situ experiments are necessary. (see also Index:
space
exploration)
The bulk of the remote sensing methods are directed toward finding some
thermodynamic
disequilibrium on the planet. This may be a chemical disequilibrium, a
mechanical
disequilibrium, or a spectral disequilibrium. For example, it would be
quite easy to
determine spectroscopically from Mars that the Earth's atmosphere contains
large amounts
of molecular oxygen and about one part per million (106) of methane. It
would also be
possible to calculate that, at thermodynamic equilibrium, the abundance
of methane should
be less than one part in 1035. This huge discrepancy implies the existence
of some process
continuously generating methane on the Earth so rapidly that methane increases
to a very
large steady-state abundance before it can be oxidized by oxygen. Now such
a
methane-production mechanism need not be biological. It is conceivable
that relatively
stable aromatic hydrocarbons were produced abiologically in the early history
of the Earth
and that their slow thermal degradation leads to a continuous loss of methane
from the
planetary subsurface. But this and similar nonbiological explanations of
the observed
disequilibrium are unlikely. From Mars this thermodynamic discrepancy would
be
considered not as proof of life on Earth but as a significant hint of life
on Earth. In fact the
methane abundance on the Earth is produced by bacteria that, in the course
of the
reduction of a more oxidized form of carbon, release methane. Some methane
bacteria live
in swamps (hence, the term marsh gas for methane), and others--a significant
fraction--live
in the intestinal tracts of cows and other ruminants. The methane abundance
over India is
probably larger than over most other areas of the world, and if an extraterrestrial
observer
knew how to interpret the methane disequilibrium accurately (which is unlikely)
it would be
possible for him to deduce cows on Earth by spectrochemical analysis. The
existence of
relatively large quantities of methane in the presence of an excess of
oxygen would remain a
tantalizing but enigmatic hint of life on Earth. Similarly, the large amount
of oxygen might
itself be a sign of life if one could reliably exclude the possibility
that the photodissociation
of water and the escape to space of hydrogen were the source of oxygen.
Also such
relatively complex reduced organic molecules as terpenes, a hydrocarbon
given off by
plants, might conceivably be detected spectroscopically, perhaps by a spectrometer
in
orbit about the Earth. Not only would the chemical disequilibrium of terpenes
in an excess
of oxygen be suggestive of life, but equally suggestive would be the fact
that terpenes are
much more abundant over forested areas than over deserts. (see also Index:
isoprenoid)
Photographic observation.
Photographic observations of the daytime Earth from Mars would give equivocal
results.
Even with a resolution of 100 metres (that is, an ability to discriminate
fine detail at high
contrast only if its components are more than 100 metres apart), it would
be extremely
difficult to discern cities, canals, bridges, the Great Wall of China,
highways, and other
large-scale accoutrements of the Earth's technical civilization. In satellite
photographs with
100-metres (one metre = 1.0936 yards) resolution only about one in a thousand
random
photographs of the Earth yields features even suggestive of life. As the
ground resolution is
progressively improved, it becomes increasingly easy to make out the regular
geometrical
patterns of cultivated fields, highways, airports, and so on. But these
are only the products
of a civilization recently developed on Earth, and even photographs of
the Earth with a
ground resolution of 10 metres, but taken 100,000 years ago, would still
have shown no
clear sign of life. The lights of the largest cities might be just marginally
detectable from
Mars at night. Seasonal changes in the colour or darkness of plants would
be detectable
from Mars, but such cycles might easily have nonbiological explanations.
(see also Index:
celestial photography, satellite communication)
To detect individual animals a ground resolution of a few metres is required,
and even here
a low sun and long shadows are generally necessary. This detection could
be accomplished
with a large telescope in Earth orbit. It would then be possible to determine,
for example,
that objects with the general shape of cows are frequent on the Earth.
But suppose that
members of the civilization examining the Earth thus remotely are not even
approximately
quadrupedal and do not immediately associate the shape of cows with life.
They would
nevertheless be able to deduce life. They would observe that certain locales
on Earth have
a quantity of raised lumps connected to the ground by four stilts. It would
be possible to
calculate that wind and water erosion would cause the lumps to topple to
the ground in
geologically short periods of time. Such stilted lumps are mechanically
unstable; they are
not in equilibrium; if pushed hard, they fall. Accordingly, there must
be a process for
generating stilted lumps on the Earth in short periods of time. It would
be very difficult to
avoid the implication that this generation process is biological.
A third detection technique arises upon scanning the radio spectrum of
the Earth. Because
of domestic television transmission, the high-frequency end of the AM broadcast
band, and
the radar defense networks of the United States and various other countries,
the amount or
energy put out by the Earth to space at certain radio frequencies is enormous.
At some
frequencies, if this radiation were to be interpreted as ordinary thermal
emission, the
temperature of the Earth would have to be hundreds of millions of degrees,
according to an
estimate made by a Russian astrophysicist, I.S. Shklovskii. Moreover, it
would be possible
to determine that this radio "brightness temperature" of the Earth had
been steadily
increasing with time over the last several decades. Finally, it would be
possible to analyze
the frequency and the time variation of these signals and deduce that they
were not purely
random noise. (see also Index: radio source)
Now imagine in situ studies by vehicles that enter the Earth's atmosphere
and land at some
predetermined locale. There are many places on the Earth (the ocean surface,
the Gobi
Desert, Antarctica) where large organisms are infrequent and a life-detection
attempt
based solely on television searches for large life forms would be a risky
investment. On the
other hand, if such an experiment were successful (the camera records a
dolphin cavorting,
a camel chewing its cud, a penguin waddling), it would provide quite convincing
evidence
of life.
Although the oceans, the Gobi Desert, and Antarctica are relatively devoid
of large life
forms, they are in many places replete with minute life forms. Therefore,
microorganism
detectors would be a good investment. A television camera coupled to a
microscope
(optical or electron) would be a promising life detector if the sample
acquisition problem
could be solved: the early Dutch microscopist Antonie van Leeuwenhoek had
no difficulty
at all in identifying as alive the little "animalcules" that he found in
a drop of water, although
nothing similar had previously been seen in human history.
In addition to morphological criteria for the detection of microorganisms,
there are
metabolic and chemical criteria. For example, a sample of terrestrial soil,
or seawater, say,
might be acquired and introduced into a chamber containing food the investigators
guess
the earthlings might find tasty. Such food might be an abundant product
of prebiological
organic synthetic experiments. It could then be determined whether any
characteristic
molecules, such as carbon dioxide or ethanol, are produced metabolically
or whether the
medium containing food and terrestrial sample changes its acidity or becomes
cloudy
because of the growth of microorganisms, or it might be investigated whether
there is heat
given off in the chamber containing sample and food. Alternatively, photosynthesis
could be
tested by measuring the fixation of some gas, say carbon dioxide, as a
function of
illumination provided artificially to the sample by the instrument. Along
chemical lines a
direct test of terrestrial soil or seawater for optical activity might
be made. Organic
molecules could certainly be searched for with a combined gas chromatograph
and mass
spectrometer or by a remote analytic chemistry laboratory. The detection
of any amount of
organic matter would of course be interesting and relevant, whether or
not it was biological
in origin. Such criteria as have been used in the analysis of Precambrian
sediments
(described in The antiquity of life, above) might be used to test for biological
origin.
Ambiguities of tests for life.
It is remarkable, however, that many of these tests are ambiguous. It would
be possible,
for example, for the Martian investigator to guess wrong about what terrestrial
organisms
eat and to make incorrect assumptions about their structural chemistry
or their interaction
medium. If forms of regular geometry that do not move were detected microscopically,
there might be serious questions of biological versus mineralogical origin.
Chemical criteria
(such as the expectation that if odd-numbered carbon chains are more prominent
than
even-numbered carbon chains, then life is detected) might not be valid
unless it was certain
which processes actually occurred in the prebiological organic chemistry
of the planet in
question. In addition, there might be the galling problem of contamination.
The Martians'
spacecraft might carry living organisms from their own planet and report
them as detected
on the planet Earth. For this reason great care would have to be taken
that spacecraft were
rigorously sterilized.
In fact, many of these problems have already arisen in an analysis of a
variety of meteorite
called carbonaceous chondrites. These meteorites, which fall on the Earth
probably from
the asteroid belt, contain about 1 percent organic matter by mass, far
too much to be
largely the result of terrestrial contamination. The most abundant organic
molecules,
however, are not clearly of biological origin, and some of the biologically
more interesting
molecules may be contaminants. Reports of optical activity have been contested
and might
alternatively be due to contamination. Geometrically interesting microscopic
inclusions have
been detected in these bodies. The most abundant inclusions, however, are
probably
mineralogical in origin, while the most highly structured and lifelike
are very rare and, at
least in some cases, are obviously the result of contamination (in one
case by ragweed
pollen). Finally, claims have been made of the extraction of viable microorganisms
from the
interiors of carbonaceous chondrites. These meteorites are porous, however,
and "breathe"
air in and out during their entry into the atmosphere. There also have
been significant
opportunities for their contamination after arrival on the Earth. Moreover,
one of the
organisms extracted was a facultative aerobe. Since, as yet, no planet
in the solar system
besides the Earth is known to contain significant quantities of molecular
oxygen, it seems
quite curious that the complex electron-transfer apparatus required for
oxygen metabolism
would be evolved out on the asteroid belt in expectation of ultimate arrival
on the Earth.
Here, again, contamination has proved a serious hazard. The large amounts
of organic
matter that are found in carbonaceous chondrites, however, suggest that
the production of
organic molecules occurred with very great efficiency in the early history
of the solar
system.
From such a hypothetical exercise as the instrumental detection of life
on Earth by an
extraterrestrial observer and from the actual experience acquired in the
analysis of
carbonaceous chondrites, the following conclusions can be drawn: There
is no single and
unambiguous "life detector." There are instruments of great generality
that make few
ambiguous assumptions about the nature of extraterrestrial organisms, particularly
their
chemistry. These systems, however, require a fair degree of luck (an animal
must walk by
during the operating lifetime of the instrument), or they require the solution
of difficult
instrumental problems (such as the acquisition and preparation of samples
for remote
microscopic examination). Other instruments, such as metabolism detectors,
have great
sensitivity and are directed at the more abundant microorganisms. They
are quite specific,
however, and are critically dependent upon certain assumptions (for example,
that
extraterrestrial organisms eat sugars) that are no better than informed
guesses. Therefore,
an array of instruments, both very general and very specific, seems required.
Stringent
sterilization of such spacecraft appears necessary, both to avoid confusion
of the
life-detection experiments and to prevent interaction of contaminants with
the indigenous
ecology. Many of the instruments and strategies discussed in the preceding
paragraphs
continue to be adapted by the United States in attempts to search for life
on the Moon and
the nearby planets.
An exobiological survey of the solar system.
A brief survey of the physical environments and biological prospects of
the moons and
planets of the solar system, so far as is known, follows. The Moon's surface
seems
inhospitable to life of any sort. The diurnal temperatures range from about
100 to about
400 K. In the absence of any significant atmosphere or magnetic field,
ultraviolet light and
charged particles from the Sun penetrate unimpeded to the lunar surface,
delivering in less
than an hour a dose lethal to the most radiation-resistant microorganism
known. For other
reasons already mentioned, the absence of an atmosphere and of any liquid
medium on the
surface also argues against life. The subsurface environment of the Moon
is not nearly so
inclement. About a metre or so subsurface there is no penetration of ultraviolet
light or solar
protons, and the temperature is maintained at a relatively constant value
about 230 K. Even
there, however, the absence of an atmosphere and the probable absence of
abundant
liquids make the biological prospects rather dim.
It is not out of the question, however, that prebiological organic matter,
produced in the
early history of the Moon, might be found sequestered beneath the lunar
surface. Such
organic matter may have been produced either in an original lunar atmosphere
that has
subsequently been lost to space, or in a secondary lunar atmosphere produced
by release
of gases after the formation of the Moon, and also subsequently lost to
space. The depth at
which such organic matter may be found depends upon the unknown history
of the early
lunar atmosphere, if any, and upon whether the Moon has, on the whole,
gained or lost
matter due to meteoritic impact. An apparent gaseous emission near the
lunar crater
Alphonsus was recorded in 1958 and a spectral identification was made of
the molecule
C2, a likely organic fragment, but this identification subsequently has
been disputed.
Because of contamination by unmanned spacecraft, the lunar surface had
accumulated a
microbial load estimated by the late 1960s at some 100,000,000 microorganisms.
Since
such organisms will be immediately killed unless shielded from radiation,
and since the
likelihood of their growth seems remote, such contamination may not be
a serious problem
in subsequent microbial analysis of returned lunar samples. A much more
serious
contamination problem occurs during the acquisition of such samples by
astronauts.
Samples obtained during the historic Apollo 11 Moon landing in July 1969
were tested for
possible organic molecules, but results were inconclusive. Such a finding
might shed
significant light on the early history of organic molecules in the solar
system.
The environment of Mercury is rather like that of the Moon. Its surface
temperatures range
from about 100 to about 620 K, but about a metre subsurface the temperature
is constant,
very roughly at comfortable room temperature on Earth. But the absence
of any significant
atmosphere, the unlikelihood of bodies of liquid, and the intense solar
radiation make life
unlikely.
Martian "vegetation" and "canals."
Direct evidence for life on Mars has been claimed for many decades. The
first such
argument was posed by a French astronomer, E.L. Trouvelot, in 1884: "Judging
from the
changes that I have seen to occur from year to year in these spots, one
could believe that
these changing grayish areas are due to Martian vegetation undergoing seasonal
changes."
The seasonal changes on Mars have been reliably observed, not only visually
but also
photometrically. There is a conspicuous springtime increase in the contrast
between the
bright and dark areas of Mars. Accompanying colour changes have been reported,
but
their reality has been disputed. While such changes have been attributed
to the growth of
vegetation, seasonally variable dust storms are an equally convincing possibility.
The most famous case, historically, for life on Mars is the discovery of
the "canals," a set
of apparent thin straight lines that cross the Martian bright areas and
extend for hundreds
and sometimes thousands of kilometres. They change seasonally as do the
Martian dark
areas. These lines, first systematically observed by an Italian astronomer,
G.V. Schiaparelli,
in 1877, were further cataloged and popularized by a U.S. astronomer, Percival
Lowell,
around the turn of the century. Lowell argued from the unerring straightness
of the lines that
they could not be of geological origin but must instead be the artificial
constructs of a race
of intelligent Martians. He suggested that they might be channels carrying
water from the
melting polar caps to the parched equatorial cities of Mars. While considerable
skepticism
has been expressed about these straight lines, there is no doubt that approximately
rectilinear features do exist on the Martian surface. More probable explanations,
however,
include crater chains, terrain contour boundaries, faults, mountain chains,
and ridges
analogous to the suboceanic ridge systems that are features of the Earth.
(see also Index:
Mars, canals of)
In July and August 1976, two U.S. probes bearing equipment designed to
detect the
presence or remains of organic material made successful landings on Mars.
Analyses of
atmospheric and soil samples met with procedural difficulties and yielded
initially ambiguous
and inconclusive results, although the data were later generally interpreted
as negative, at
least for the vicinity of the probe (see SOLAR SYSTEM: The quest for life
on Mars).
(see also Index: Mariner)
Venus and the superior planets.
According to both ground-based and space-borne observations, the average
surface
temperatures of Venus are around 750 K. It does not seem likely, either
at the poles or on
the tops of the highest Venus mountains, that the surface temperature will
be below 400 K,
and noontime temperatures are probably significantly hotter than 700 K.
Thus, quite apart
from the other surface conditions, the temperatures on Venus seem too hot
for terrestrial
life. It is still not possible to exclude a Venus surface life with a rather
different chemistry,
although hydrogen bonding would be much less suitable for the geometrical
configuration of
polymers on Venus than it is on Earth. The clouds of Venus, however, are
another matter.
There, carbon dioxide, sunlight, and (according to the results of the Venera
space vehicles)
water are to be found. These are the prerequisites for photosynthesis.
Some molecular
nitrogen also is expected at the cloud level, and some supply of minerals
can be expected
from dust convectively raised from the surface. The cloud pressures are
about the same as
on the surface of the Earth, and the temperatures in the lower clouds also
are quite
Earthlike. Despite the fact that there is little oxygen, the lower clouds
of Venus are the most
Earthlike extraterrestrial environment known. While there are no recorded
cases of
organisms on Earth that lead a completely airborne existence throughout
their life cycle, it
is not impossible that such organisms could exist in the vicinity of the
Venus clouds,
perhaps buoyed, as is a fish by its swim bladder, to avoid downdrafts carrying
them to the
hotter lower atmosphere.
A similar speculation can be entertained with regard to the lower clouds
of Jupiter. On
Jupiter the atmosphere is composed of hydrogen, helium, methane, ammonia,
and probably
neon and water vapour. But these are exactly those gases used in primitive-Earth
simulation
experiments directed toward the origin of life. Laboratory and computer
experiments
have been performed on the application of energy to simulated Jovian atmospheres.
In
addition to the immediate gas-phase products, such as hydrogen cyanide
and acetylene,
more complex organic molecules, including aromatic hydrocarbons, are formed
in lower
yield. The visible clouds of Jupiter are vividly coloured, and it is possible
that their hue is
attributable to such coloured organic compounds. There is also an apparent
absorption
feature near 2,600 Å, in the ultraviolet spectrum of Jupiter, which
has been attributed both
to aromatic hydrocarbons and to nucleotide bases. In any event it is likely
that organic
molecules are being produced in significant yield on Jupiter; it is possible
that Jupiter is a
vast planetary laboratory that has been operating for 5,000,000,000 years
on prebiological
organic chemistry. (see also Index: ultraviolet radiation)
The other Jovian planets, Saturn, Uranus, and Neptune, are similar in many
respects to
Jupiter, although much less is known about them. Their cloud-top temperatures
progressively decrease with distance from the Sun. In the case of Saturn,
microwave
studies have indicated that the atmospheric temperature increases with
depth below the
clouds; similar situations are expected on Jupiter, Uranus, and Neptune.
Thus, it is by no
means clear that the low temperatures of the upper clouds of the Jovian
planets apply to
the lower clouds, or to the underlying atmosphere. The environment of Pluto
is almost
completely unknown. In addition to these planets, the solar system contains
32 natural
satellites, some of which, such as Titan, a satellite of Saturn, and Io,
a satellite of Jupiter,
appear to have atmospheres. There are also tens of thousands of comets,
which, judging
from their spectra, contain organic molecules, as well as some thousands
of asteroids and
asteroidal fragments revolving about the Sun between the orbits of Mars
and Jupiter. These
are the presumed sources of the carbonaceous chondrites, which contain
organic matter.
In short, there is a wide range of environments of biological interest
within the solar system.
There is no direct evidence for extraterrestrial life on these planets,
but, on the other hand,
there is no strong evidence against life on many of these worlds. Beyond
this is the near
certainty that biologically interesting organic molecules will be found
throughout the solar
system.
Intelligent life beyond the solar system.
For thousands of years man has wondered whether he is alone in the universe
or whether
there might be other worlds populated by creatures more or less like himself.
The common
view, both in early times and through the Middle Ages, was that the Earth
was the only
"world" in the universe. Nevertheless, many mythologies populated the sky
with divine
beings, certainly a kind of extraterrestrial life. Many early philosophers
held that life was
not unique to the Earth. Metrodorus, an Epicurean philosopher in the 3rd
and 4th centuries
BC, argued that "to consider the Earth the only populated world in infinite
space is as
absurd as to assert that in an entire field sown with millet, only one
grain will grow." Since
the Renaissance there have been several fluctuations in the fashion of
belief. In the late 18th
century, for example, practically all informed opinion held that each of
the planets was
populated by more or less intelligent beings; in the early 20th century,
by contrast, the
prevailing informed opinion (except for the Lowellians) held that the chances
for
extraterrestrial intelligent life were insignificant. In fact the subject
of intelligent
extraterrestrial life is for many people a touchstone of their beliefs
and desires, some
individuals very urgently wanting there to be extraterrestrial intelligence,
and others wanting
equally fervently for there to be no such life. For this reason it is important
to approach the
subject in as unbiased a frame of mind as possible. A respectable modern
scientific
examination of extraterrestrial intelligence is no older than the 1950s.
The probability of
advanced technical civilizations in our galaxy depends on many controversial
issues. (see
also Index: Milky Way Galaxy)
A simple way of approaching the problem, which illuminates the parameters
and
uncertainties involved, has been devised by a U.S. astrophysicist, F.D.
Drake. The number
N of extant technical civilizations in the galaxy can be expressed by the
following equation
(the so-called Green Bank formula): (see also Index: star, Green Bank equation)
N = R*fpne fl fi fcLwhere R* is the average rate of star formation over
the lifetime of the
galaxy; fp is the fraction of stars with planetary systems; ne is the mean
number of planets
per star that are ecologically suitable for the origin and evolution of
life; fl is the fraction of
such planets on which life in fact arises; fi is the fraction of such planets
on which intelligent
life evolves; fc is the fraction of such planets on which a technical civilization
develops; and
L is the mean lifetime of a technical civilization. What follows is a brief
consideration of the
factors involved in choosing numerical values for each of these parameters,
and an
indication of some currently popular choices. In several cases these estimates
are no better
than informed guesses and no very great reliability should be pretended
for them.
There are about 2 ×1011 stars in the galaxy. The age of the galaxy
is about 1010 years. A
value of R* = 10 stars per year is probably fairly reliable. While most
contemporary
theories of star formation imply that the origin of planets is a usual
accompaniment of the
origin of stars, such theories are not well enough developed to merit much
confidence.
Through the painstaking measurement of slight gravitational perturbations
in the proper
motions of stars, it has been found that about half of the very nearest
stars have dark
companions with masses ranging from about the mass of Jupiter to about
30 times the mass
of Jupiter. The nearest of these dark companions orbit Barnard's star,
which is only six
light-years from the sun and is the second nearest star system. The most
direct indication
that planetary formation is a general process throughout the universe is
the existence of
satellite systems of the major planets of our own solar system. Jupiter,
with 16 satellites,
Saturn with 20 or more, and Uranus with five each closely resemble miniature
solar
systems. It is not known what the distribution of distances of planets
from their central star
are in other solar systems and whether they tend to vary systematically
with the luminosity
of the parent star. But considering the wide range of temperatures that
seem to be
compatible with life, it can be tentatively concluded that fpne is about
one.
Likelihood of life.
Because of the apparent rapidity of the origin of life on Earth, as implied
by the fossil
record, and because of the ease with which relevant organic molecules are
produced in
primitive-Earth simulation experiments, the likelihood of the origin of
life over a period of
billions of years seems high, and some scientists believe that the appropriate
value of fl is
also about one. For the quantities of fi and fc the parameters are even
more uncertain. The
vagaries of the evolutionary path leading to the mammals, and the unlikelihood
of such a
path ever being repeated has already been mentioned. On the other hand,
intelligence need
not necessarily be restricted to the same evolutionary path that occurred
on the Earth;
intelligence clearly has great selective advantage, both for predators
and for prey.
Similar arguments can be made for the adaptive value of technical civilizations.
Intelligence
and technical civilization, however, are clearly not the same thing. For
example, dolphins
appear to be very intelligent, but the lack of manipulative organs on their
bodies has
apparently limited their technological advance. Both intelligence and technical
civilization
have evolved about halfway through the relevant lifetime of the Earth and
Sun. Some, but
by no means all, evolutionary biologists would conclude that the product
fi fc taken as
10-2 is a fairly conservative estimate.
Still more uncertain is the value of the final parameter, L, the lifetime
of a technical
civilization. Here, fortunately for man, but unfortunate for the discussion,
there is not even
one example. Contemporary world events do not provide a very convincing
counterargument to the contention that technical civilizations tend, through
the use of
weapons of mass destruction, to destroy themselves shortly after they come
into being. If
we define a technical civilization as one capable of interstellar radio
communication, our
technical civilization is only a few decades old. If then L is about 10
years, multiplication of
all of the factors assumed above leads to the conclusion that there is
in the second half of
the 20th century only about one technical civilization in the galaxy--our
own. But if technical
civilizations tend to control the use of such weapons and avoid self-annihilation,
then the
lifetimes of technical civilizations may be very long, comparable to geological
or stellar
evolutionary time scales; the number of technical civilizations in the
galaxy would then be
immense. If it is believed that about 1 percent of developing civilizations
make peace with
themselves in this way, then there are about 1,000,000 technical civilizations
extant in the
galaxy. If they are randomly distributed in space, the distance from the
Earth to the nearest
such civilization will be several hundred light-years. These conclusions
are, of course, very
uncertain.
How is it possible to enter into communication with another technical civilization?
Independent of the value of L, the above formulation implies that there
is about one
technical civilization arising every decade in the galaxy. Accordingly,
it will be
extraordinarily unlikely for man soon to find a technical civilization
as backward as his.
From the rate of technical advance that has occurred on the Earth in the
past few hundred
years, it seems clear that man is in no position to project what future
scientific and technical
advances will be made even on Earth in the next few hundred years. Very
advanced
civilizations will have techniques and sciences totally unknown to 20th-century
man.
Nevertheless man already has a technique capable of communication over
large interstellar
distances. This technique, already encountered in the discussion of life
on Earth, is radio
transmission. Imagine that we employ the largest radio telescope available
on Earth, the
1,000-foot-diameter dish of Cornell University, the Arecibo Observatory
in Puerto Rico,
and existing receivers, and that the identical equipment is employed on
some transmitting
planet. How distant could the transmitting and receiving planets be for
intelligible signals to
be transmitted and received? The answer is a rather astonishing 1,000 light-years.
Within a
volume centred on the Earth, with a radius of 1,000 light-years, there
are more than
10,000,000 stars. (see also Index: communication system, extraterrestrial
life , radio
astronomy)
There would of course be problems in establishing such radio communication.
The choices
of frequency, of target star, of time constant, and of the character of
the message would all
have to be selected by the transmitting planet so that the receiving planet
would, without
too much effort, be able to deduce the choices. But none of these problems
seem
insuperable. It has been suggested that there are certain natural radio
frequencies (such as
the 1,420-megacycle line of neutral hydrogen) that might be tuned to; the
first choice might
be to listen to stars of approximately solar spectral type; in the absence
of a common
language there nevertheless are messages whose intelligent origin and intellectual
content
could be made very clear without making many anthropocentric assumptions.
Because of the expectation that the Earth is relatively very backward,
it does not make
very much sense to transmit messages to hypothetical planets of other stars.
But it may very
well make sense to listen for radio transmissions from planets of other
stars. Project Ozma,
a very brief program of this sort, oriented to two nearby stars, Epsilon
Eridani and Tau
Ceti, was organized in 1960 by Drake. On the basis of the Green Bank formula,
it would
be very unlikely that success would greet an effort aimed at two stars
only 12 light-years
away, and Project Ozma was unsuccessful. It remains, however, the first
pioneering
attempt at interstellar communication. Related programs were organized
on a larger scale
and with great enthusiasm in the 1960s in the U.S.S.R., where a state scientific
commission
devoted to such an effort was organized. Other communication techniques
including laser
transmission and interstellar spaceflight have been discussed seriously
and may not be
infeasible, but if the measure of effectiveness is the amount of information
communicated
per unit cost, then radio is the method of choice.
The search for extraterrestrial intelligence is an extraordinary pursuit,
in part because of the
enormous significance of possible success, but in part because of the unity
it brings to a
wide range of disciplines: studies of the origins of stars, planets, and
life; of the evolution
of intelligence and of technical civilizations; and of the political problem
of avoiding man's
self-annihilation. But at least one point is clear. In the words of Loren
Eiseley (also from
The Immense Journey),
Lights come and go in the night sky. Men, troubled at last by the things
they
build, may toss in their sleep and dream bad dreams, or lie awake while
the
meteors whisper greenly overhead. But nowhere in all space or on a thousand
worlds will there be men to share our loneliness. There may be wisdom;
there
may be power; somewhere across space great instruments, handled by
strange, manipulative organs, may stare vainly at our floating cloud wrack,
their owners yearning as we yearn. Nevertheless, in the nature of life
and in
principles of evolution we have had our answer. Of men [as are known on
earth] elsewhere, and beyond, there will be none forever.