Chapter 6D
The Value of Astrobiology with or without Specimens
The lack of extraterrestrial specimens is an objection to the pursuit of astrobiology only if we accept a narrow definition of the field. Astrobiology goes beyond the search for extraterrestrial life: It is largely the application of space science and technology to understand how life may originate and evolve anywhere in the cosmos. As a practical matter, astrobiology often devotes itself to investigating how life originated and evolved on this planet. Astrobiology tries to determine, for example, what the Earth was like 3.5 - 4.5 billion years ago — what was the ultraviolet flux? What were the volcanic and other tectonic activity? How much molecular oxygen was in the atmosphere? And how much ozone? How much carbon, hydrogen, and nitrogen were "recycled" through the Earth's crust and how much were brought to the Earth by asteroids and comets? To decide these issues we must go away from the Earth to study the older surfaces of the Moon and Mars, the presumably still primordial atmosphere of Titan, and the largely untouched chemistry of comets.[1] Astrobiology is thus inseparable from comparative planetology. (Figure 3).
This connection is all the more evident when we remember that to understand the nature of an environment well we need to understand its origin and evolution. In the global environment of the Earth life has played a crucial part (e.g., in the density, temperature, and composition of the atmosphere, cf. Ch. 4). How life originated is thus a question of great importance if we are to understand how our global environment came to be as it is. At the same time, we cannot begin to settle that question without making some critical determinations about how the planet was formed, how its atmosphere was created, how much energy it received from the sun in its early evolution, and in general all those questions that form integral part of comparative planetology.
In trying to answer the question of the origin of life, however, there are great difficulties of substance and of method. For example, some investigators would not be satisfied with an answer unless it demonstrated that life was somehow inevitable or at least very likely. They would require that we specify the early conditions in the planet (e.g., a reducing atmosphere and later a primordial soup of organic materials), and then show that by processes that should be expected (e.g., radiation, lightening) organic evolution towards life was highly probable. Another school of thought would have a series of extraordinary coincidences bring life about. Thus even if organic matter was abundant on the early Earth, it would have taken an accident, or accidents, to get organic evolution on the road to life. To illustrate the sort of disputes involved let me consider a hypothesis that requires the presence of a large moon for life to begin. Of course, if that hypothesis is right, we should conclude that life is probably very rare in the galaxy and not to be found in the solar system at all, except for our own kind.
According to this hypothesis, clays served as the templates for the amino acids to combine into the first complex organic molecules. Presumably the amino acids would be floating in shallow waters over clay surfaces, but still required a mechanism that could make them more complex. That mechanism is provided by having two amino acids form a peptide bond — a bond through which carbon and nitrogen in separate organic chains become linked. This bond can form when, for example, a nitrogen atom loses its bond with a hydrogen atom (H) and a carbon nearby loses its hydroxide bond (OH). Since the H and the OH combine to form water (H20), the formation of the peptide bond requires a loss of water. The problem is how to lose water in the presence of all that water in which the amino acids are suspended.
The tides created by the Moon provide the solution to the problem. When the tides go out a residue of amino acids is left on the clays, and the heavier concentration in drier surroundings allows the peptide bond to form. Once formed, the peptide bond is stable in water. After many repetitions of this process, organic molecules of an increasing complexity can be formed. The function of the clays is to provide a mechanism for replication, but that matter will wait a few paragraphs. For the time being, the issue is the role of the Moon in the origin of life.
The sun produces tides, too, but they are much smaller and perhaps not sufficient to bring about the needed peptide bonds. If a planet were closer to the star it would enjoy greater solar tides, but unfortunately it may also become locked into a very slow rotation (the Moon always offers the same face to the Earth, Mercury rotates every 58.6 Earth days, and Venus' day is longer than its year). Having a day-night cycle seems important because the opportunity to move away from equilibrium gives the pre-biological molecules a chance to vary, and this opportunity for variation is an essential characteristic of evolution. Cyclical events are in general favorable because they permit the molecules to reach a state of equilibrium to consolidate their gains before having to change again. Apart from the tides and the day-night cycle we have the concomitant temperature fluctuations. And we have seasons. Now, the Earth has seasons because its axis is tilted in just the right way. Then we must also take into account the gravitational influence of other planets, the role of the magnetosphere and many other factors whose possible relevance or even their very existence may escape the experts at this time.
Insofar as any of these are large factors in allowing life to gain a foothold on Earth, the origin of life becomes an improbable event. But we simply do not know. As plausible as hypotheses such as this may seem today, they may sound very quaint in two or three decades, let alone in a few centuries. And even if the events in question were indeed factors in bringing life into our world, on further examination they may turn out to be just some among the many alternative mechanisms that could have provided for the evolution of ever-more complex molecules in a variety of other worlds. It happens all too often that when a mechanism cannot be immediately proposed to explain a particular step on the way to life, people who ought to know better jump to the conclusion that life on Earth was an extraordinary coincidence.
Many biochemists, for example, have felt that the problem of the origin of macromolecules is insoluble. At the most basic level, life consists of nucleic acids (such as DNA and RNA) that contain the genetic information, and of functional proteins (such as enzymes). If we imagine that DNA or RNA was the original macromolecule we have to explain how it could replicate in the absence of enzymes, which are essential in modern living systems. On the other hand if we imagine that the proteins came first, how could they have built around themselves the nucleic acids that would carry the information necessary for future coding of the same proteins?
For years none of the mechanisms proposed seemed satisfactory, not even co-evolutionary mechanisms because the chemical association of nucleotides (the building blocks of nucleic acids) and amino acids (the building blocks of proteins) was just too problematic. On the face of this situation some people thought that life was a stroke of luck. And some others even suggested that life had probably come from elsewhere to the Earth, as if removing the problem of the origin of life a few light years amounted to a solution.
Nevertheless some plausible mechanisms were proposed in the late 1960s and have been refined ever since. One of them, suggested by A.G. Cairns-Smith, is that microscopic crystals in clays can serve to replicate molecules. Such clays have a large capacity for adsorption, which causes tiny bits of proteins to stick to them, just as particles of meat do to the surface of a frying pan. The crystals in these clays would then grow and reproduce the patterns of the amino acids adsorbed in the clays.
These processes can be repeated millions of times, until with the development of enzymes, as J.D. Bernal notes, we would also see the appearance of co-enzymes, some of which are identical to the nucleotides of RNA. As the co-enzymes are adsorbed, their efficiency in chemical energy transfer would give clear reproductive advantages to their associated enzymes. Under these conditions, a co-evolution of functional proteins and nucleic acids becomes possible. And this result presumably paves the road to the eventual origin of the first cells. This view has been buttressed by the work of the space scientist James Lawless, who has shown that clays do select precisely the amino acids that can form biologically active proteins.
There are many other hypotheses, many of them buttressed by experimental work that fill in some of the steps deemed necessary to take us from atmospheric gases to living cells (e.g., proteins that can "make" their own RNA) But what is necessary and what is not depends on the approach one takes to explain the origin of life. First, there are different starting points. Some want, even demand, a reducing atmosphere (poor in oxygen, rich in hydrogen and other gases like methane). Others think that the original atmosphere was composed largely of carbon dioxide. Second, then comes a story of the evolution of organic matter, a story that may involve thousands of steps, of which only some are specified. And of course, there could be alternative plausible stories. What makes them plausible is that some of the steps that may have seemed baffling at one time can be produced in the laboratory now, while others can be explained theoretically. For example, the first generally accepted story gained its plausibility from the Miller-Urey experiment, in which a reducing atmosphere in a flask was subjected to electrical discharges. Presumably the result of the experiment was a soup containing the building blocks of life. Apparently, however, only very few interesting organic molecules were actually produced in such an experiment, and those were of very little complexity. The steps from there to, say, a self-replicating molecule, let alone a cell, are truly gigantic.
Since there are so many ways to tell the story, many equally unconstrained by the scant evidence, it is not surprising that the intuitions of different investigators differ on what is crucial and what is not. And even if most of the apparently necessary steps of a particular story can be accounted for by experiments, there remains the difficulty that the answer to one part of the puzzle is often at odds with the proposed answer to the next part (e.g., a molecule used as a building block for a more complex molecule is produced in an alkaline solution, but the more complex molecule has to be produced in its opposite, an acidic environment; this is not a fatal setback, since in living things the product of a reaction can be transported to a different internal environment to be used to build something else, and in general we find that natural processes have co-evolved in the living world to accomplish just this transport, cf. the example about elephants and their environments given above). The problem is that it all seems just too convenient. What we want to know now is not just how it could have been, but how it was -- we want the "real" story.
To go from just-so stories to compelling hypotheses we need a better understanding of the initial conditions on the Earth, and as we develop our hypotheses accordingly, we will get ideas of what sorts of evidence about the subsequent evolution of the global environment we may want to look for. One helpful way to proceed is to examine those worlds where according to some approaches life might have started, or at least where we should expect some small amount of organic evolution. To the extent that organic evolution has taken place we learn much about our own, once we factor in the relevant differences. To the extent to which organic evolution has not taken place, we also learn much about the failure of some forms of reasoning about the origin of life and perhaps get some clues about more appropriate forms of reasoning.
Astrobiology and comparative planetology will merge in many other contexts. Take, for example, the search for the origin of the organic carbon on the Earth, surely a needed background to make a definitive determination of how life started on the Earth. To have organic compounds, we first need to trace the carbon and the other relevant elements (hydrogen is normally easy, since it is almost everywhere, with the glaring exception of the Moon and Mercury). We begin the search for carbon in the solar system and then see how it was apportioned to the Earth. If the Earth had a disproportionate amount of carbon, we must deal with a certain set of scenarios in which the Earth comes to occupy a privileged position. If carbon is very common in the solar system, as indeed it is, our scenarios are of a different sort, but we still want to know how the Earth came by the amounts that it has: Did it happen during the initial accretion of the planet, or was most of the carbon brought in by the subsequent cometary and asteroidal bombardment?
How do we trace the carbon in the solar system? We will look for clues in the comets and asteroids, as well as in the inner planets and all the other rocky objects of respectable size in the solar system, particularly the large moons of Jupiter and Saturn. If we wish, space exploration will make them all available to us. In the meantime we may also examine the meteorites that we do have. In some of these meteorites we find that carbon is associated with two sets of rare gases — apparently the carbon serves as a casing that keeps the rare gases trapped. When the carbon is released from the meteorite it comes out in two installments. In the first it also frees amounts of argon, krypton and xenon in relative ratios that are pretty much the same as those found in terrestrial planets. Other carbon in some meteorites, however, encases a completely different mix of rare gases (of neon, xenon, and krypton) that cannot be accounted for by any process known in the solar system. Further investigation reveals that such is the mixture that can be expected in the process by which stars become red giants. This presumably leads to the conclusion that the stellar cloud from which the solar system formed was already seeded with carbon from the death of another star.
This interplay between biology and astrophysics takes place on many fronts. As another example, we may consider the matter of extinction, which has been described earlier. The 26 million-year period for large extinctions was calculated by David Raup from the fossil records of marine animals. It was this periodicity together with the asteroid hypothesis of Luis Alvarez that led to the further astrophysical hypothesis that the sun had a companion star. Incidentally, the computer program used to estimate the effects of the Alvarez asteroid was based on a program originally developed by Brian Toon and others to study the dust clouds of Mars, and in turn served as the basis for the so-called "nuclear winter" study — which some observers still consider a reliable model of what would happen to the Earth in case of nuclear war.[2]
If such cycles of extinction are indeed determined by astrophysical events, posterity will have much reason to thank the day these combined biological and astrophysical studies were undertaken; even if the astrophysical causes turn out to be very different from those now proposed. But quite apart from preventing great disasters, the study of extinction cycles will contribute greatly to our understanding of the forces that affect the environment of the Earth.
These and other investigations underscore the intimate connections between astrobiology and those aspects of space science that deal with the formation and evolution of planets. Since the role of life has been of crucial importance for the Earth, and since we need to know specifically how terrestrial life may not only survive but also prosper, this study of origins is highly justified. To put the point differently, the biota and many of the other elements of the global environment co-evolved. Thus to understand the evolution of one of those elements we need to understand it in its relationship to the evolution of the others. Moreover, the very role of the imagination in trying to determine the range within which life can be born and the possible forms life may take provides a fruitful context in which to discuss questions of origin and evolution. For by the consideration of likely scenarios for life, and by the comparative examination of the planets in our solar system and of other planetary systems, we will be better able to understand not only how life came about but also why it took the paths that it did when it apparently had others available.
[1]. Although the emphasis has been mine all along, in making these remarks I find myself paraphrasing Harold P. Kline's many comments on earlier drafts of this essay.
[2] R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, C. Sagan, "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions," Science, 23 December 1983, Vol.222, P.1293. See also O.B. Toon et al, "Evolution of an Impact-Generated Dust Cloud and its Effects on the Atmosphere," Geological Society of America, Special Paper 190, 1982, p.187.
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