Chapter 6E
The Serendipity of Astrobiology
Two remarkable developments in biology are worth mentioning in connection with the serendipity of astrobiology. Let us remember that a key objection to the possibility of Martian fossils in meteorite AL84001 was that the worm-like features could not be bacteria because they were one hundred times smaller than real (terrestrial) bacteria. The controversy, however, spurred interest in the possibility that the Earth itself may contain bacteria that small. The interest increased when it was realized that the methods for looking for bacteria would not have detected such terrestrial “nano-bacteria” even if they existed. Lo and behold: biologists soon claimed to have found many such varieties of bacteria, even smaller than the presumed Martian bugs, right here on our own planet! This discovery, however, seems to have been short lived. More recent investigations revealed that some candidates to the title of nanobacteria are non-living mineral structures, e.g. calcium carbonate crystals, that do mimic bacteria in some respects and even reproduce.[1] Although not as exciting perhaps, this finding is nevertheless quite interesting in its own right. Moreover, it has some practical importance, since those nano structures are apparently in the formation of kidney and other stones.
This serendipitous result of astrobiology, as valuable as it has been in giving us a new understanding of life on Earth, may pale in comparison with the creation in the laboratory of life forms that incorporate a 21st amino acid and others with non-standard DNA codes![2]
It is clear that much needs to be done in this field, and that space science is particularly well poised to nourish its advance. At the same time we should beware of placing unfair demands upon the field, particularly where it concerns the search for the origins of life. We should beware especially of the carefree use of probabilities in trying to settle this important issue. The most notorious is the estimate of the probability that all the constituent atoms of a cell may come together to form the cell. Even for a strand of DNA the probability would be extremely low. And so it would be for any complex arrangement of matter, as long as we assume that it started from scratch. As Fred Hoyle put it, what is the probability that a Boing 747 will arise spontaneously from a tornado-swept junkyard? Of course the probability is nil. But cells are not formed from scratch. Some elements combine together more easily than others, and if they are abundant then we will find many of their compounds. Such is the case with carbon and hydrogen. Once those compounds are formed, more complex compounds can form using them, and so on. The rising complexity of molecules can give rise to very complex molecules indeed -- and then the very long process of organic evolution can begin.
Robert Shapiro, a critic of the field, tries to impose two requirements that deserve special comment. He claims that the thesis that the origin of life was an accident is not scientific. Apparently he feels that a truly scientific approach would explain why life was inevitable, given the Earth’s early environment. And he also objects to laboratory simulations of hypothetical early terrestrial environments in which the experimenters manipulate the environment to determine whether certain complex molecules can be produced from certain others. He wants the experiments left alone, to see whether the molecule so produced is capable of evolving on its own (otherwise we are not really dealing with organic evolution, I presume). His suggestion is that much of the work in the field fails to meet these two requirements and he concludes that the field is in disarray.[3]
The first thesis is rather strange coming from a biologist. If organic evolution is evolution at all, it is subject to the vagaries of natural history. The evolution of mammals, for example, may have well depended on extraordinary accidents (such as the Alvarez asteroid, which made available to our ancestors the niches previously ruled by dinosaurs).
It seems that Shapiro is unhappy because the search for the origin of life does not demand the sense of inevitability that we expect from physics. But that is one of the differences between physics and historical sciences like geology or biology. But even if we wish to use physics as a model, it seems that either chaos theory or Prigogine's dissipative structures would serve us better. A very small change in initial conditions may lead to radically different outcomes. A tiny amount of a catalyst can produce an oscillating reaction (say where the color of the solution keeps changing from red to blue). At the time of the Cambrian Revolution (about 650 million years ago) there was a great explosion in the forms of life that began to populate the planet. An observer could not have predicted then that human beings were sure to come along millions of years hence, unless he had knowledge of all the accidents that would take place in the ensuing years, and of all the ways in which complex environmental relations were going to change. Nevertheless, this particular outcome of evolution (humans) is an accident, and so is any other particular outcome. If life is the outcome of organic evolution, life itself could be said to be an accident too.
A compromise position may be defended. We need claim neither that life (as we know it) is an accident, nor that such life was inevitable. For example, we may hope for an explanation of origins that makes it look as if some accident of this sort (life) was likely to happen (e.g., a self-reproducing molecule that can protect itself from most typical, short range, environmental dangers, even though its genetic code is very different from ours).
As for Shapiro's second requirement, it seems to me that we should want to create in the laboratory a molecule that can reproduce in the sorts of environments that we think may have existed long ago. It would be unreasonable to demand that such a molecule should reproduce in any environment that might develop if we leave the apparatus unattended. We must remember that most species that ever lived are now extinct. As the environment changed, only those organisms to which the change was not unfavorable were able to leave progeny. Thus, by a similar reasoning, a molecule may be of the right sort and still fail to reproduce under the conditions required by Shapiro instead of conditions similar to those that an evolving Earth made available to its complex organic molecules.
In its own ways, astrobiology thus illustrates how space science preserves the dynamic character of science in general. Its vigorous pursuit would inevitably lead to the profound transformation of our views of the living world. And since those views are linked to our understanding of the global environment, the resulting theoretical adjustment would be of great magnitude — and so eventually would be the change in the way we may interact with the universe. The justification of astrobiology is, then, ultimately much like that of the other space sciences, and in line with the general philosophical position of this essay, whether or not we ever find a single extraterrestrial specimen!
[1] Martel, Jan, and Ding-E Young, John. “Purported nanobacteria in human blood as calcium carbonate nanoparticles.” Proceedings of the National Academy of Sciences. April 8, 2008. vol. 105, no. 14, 5549-5554.
[2] See, for example, R. F. Service, “Researchers Create First Autonomous Synthetic Life Form,” Science, Vol. 299, 31 January, 2003, p. 640.
[3] R. Shapiro, Origins: A Skeptic’s Guide to the Creation of Life on Earth, Bantam Books, 1987.
Posts
No comments:
Post a Comment