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Saturday, May 28, 2011

The Value of Panspermia

Chapter 6I

The Value of Panspermia

We have seen that Hoover’s claim to have found fossils of extraterrestrial cyanobacteria in meteorites does not really show that terrestrial life did not originate in our own planet. Even if those structures are indeed extraterrestrial fossils, such an admittedly extraordinary finding would only support the view that alien life has existed. The reasons are simple:

1. Fossils are dead things. Those meteorites did not bring living things to this planet.

2. Even if they had brought living things to this planet, and no one suggests they did, Hoover went to pains to argue that those structures were too different from terrestrial life, so they had to be extraterrestrial (e.g. only 8 amino acids in common out of 20, although for some reason some of the comments in the Journal of Cosmology referred to 8 out of 22). That is, Hoover gives no evidence that terrestrial life came from elsewhere.

We have also seen that the value of panspermia as an explanation for the origin of terrestrial life is quite low.

1. The motivation for the “impossibility proofs” against the idea that life could originate on Earth is often based on misunderstandings of biology, either of evolutionary biology, or genetics, or both.

2. By pointing to panspermia we have not explained the conditions under which life originate at all. That is, by saying that life must have started somewhere else we still have not explained the origins of life.

3. When this problem has been addressed by the advocates of panspermia, they have ultimately grounded their views on absurdities, either non-scientific fictions (e.g. Hoyle’s “superior beings”) or patently false claims (e.g. Klyce’s notion that life has always existed).

This is not to say that the idea of panspermia should not be entertained at all. We do not know yet how life originated. By elaborating different scenarios we challenge our imagination and our understanding. And we prepare ourselves for the fruitful exploration of other worlds where life might exist, or at least might have existed. Having trained our biological thinking on a variety of scenarios, for example, we will be better able to deal with the surprises such exploration is likely to offer.

Even the idea that living things might hitch rides inside meteorites or comets is not completely implausible. Spores can survive for long times. Extremophiles can withstand, it seems, extreme temperatures or radiation, and in addition to bacteria and lichens, even a very small animal, the Tardigrade, has been shown to survive in the vacuum of space! [1] Of course, as we have also seen in this blog, that life can survive in extreme conditions does not imply that life can originate in those extreme conditions. Nor should we infer either that those extreme conditions may persist for millions, let alone billions of years, as the interstellar journeys envisioned by some panspermia advocates would require, without destroying the extremophile voyagers.

A plurality of views may also help us determine what new types of observations and experimental investigations might be worth undertaking. All in all, the idea of panspermia might still prove useful as we search for life in the universe.



[1]J├Ânsson, K. Ingemar; Rabbow, Elke; Schill, Ralph O.; Harms-Ringdahl, Mats; Rettberg, Petra (2008-09-09). "Tardigrades survive exposure to space in low Earth orbit". Current Biology 18 (17): R729–R731.

Monday, May 23, 2011

The Cradle of Life

Chapter 6H

The Cradle of Life

Those who take Panspermia seriously, and particularly the ones who claim that Earth life must have originated elsewhere, owe the rest of us an explanation of how life begins. If the conditions were not right on this planet, what are then the right conditions? And where and how were those conditions met instead?

Unfortunately the most notable proponents of this view offer sheer nonsense for an answer. Hoyle and Wickramasinghe speak of “superior beings” as the creators of life (silicon beings, for example, who could have created bacteria that led to creatures like us that in turn create computers, i.e. more silicon beings). And who created those “superior beings”? Their answer is: Even more superior beings, of course, and so on, climbing the ladder of superiority of being all the way to God Himself. “It would be ridiculous to suppose otherwise,” they write.[1] Thus we move from science to science fiction, and then to a very idiosyncratic theology.

Even more absurd is the answer provided by Brig Klyce, in Cosmic Ancestry: The modern version of panspermia: “In science God never emerges from behind a curtain and performs an act of supernatural intervention. This principle is maintained if life has no beginning. We think this conclusion is the scientific one.” [2] Klyce’s “scientific answer” is reminiscent of the argument that since the universe may have had no beginning, it requires no creator. That might be a perfectly good argument, but the analogous move is not open to Klyce, at least not while claiming to be supported by the findings of experimental science. The reason is simple: living things are made of atoms such as H, O, C, and a host of others. But according to the best evidence, atoms have not always existed. The early universe had to cool down enough for quarks to form protons and neutrons, and for those to come together to form nuclei around which electrons could orbit (the standard scientific story of the formation and evolution of the universe). If atoms, let alone molecules, have not always existed, life has not always existed.

Of course, if his hero Hoyle had been right about the hypothesis that the universe had always been the same as it is now (the Steady-State cosmological hypothesis), Klyce reasoning might be permissible as “the” scientific answer. But Hoyle was wrong. Incidentally, in these discussions about panspermia, Hoyle is credited with having coined the term “Big Bang.” What is left out is that he coined it as a term of derision. Ironically, his opponents really liked it and ran with it.

Nevertheless, returning to the first paragraph, why should we believe that the conditions were not right for life to emerge on this planet? Proponents of panspermia often ally themselves with creationists in pointing out that there was not enough time. It might take billions of years just to put a complex organic molecule together, for the chances that its thousands of atoms will assemble themselves together in the short time available before life appeared on the planet (as evidenced in ancient rocks) are practically nil. So you must have a large span of time, this objection claims, in order for such an assembly to take place eventually.

But that is not the way chemical evolution would work (that would be spontaneous generation instead). The probabilities in question are conditional probabilities. Given that the planet was already awash in organic compounds, some probably manufactured on Earth itself, perhaps, by Miller-Urey type of reactions, and many others brought here by comets and meteorites, then what would the probability be that such and such polymers would become prevalent? And then, on condition that you have those polymers available, what would the probability be that a replicator would emerge, and so on? I will return to this point below.

It is perhaps a sign of progress that creationists are now talking about the universe in terms of billions of years, but the argument that there was not enough time, normally employed against biological evolution as well, is completely misguided. Earth is about 4.6 thousand million years old (4.6 American billions). We have strong evidence of bacteria in rocks about 3.5 billion years old, and some evidence of fossils as old as 3.8 or perhaps 3.85 billion years old.[3] It is plausible that the Late Bombardment of comets and asteroids (around 3.9 B years ago) may have sterilized the surface of the Earth. If life already existed then, and thrived in deep ocean vents and under the young continents, it might have survived and then spread anew once conditions were favorable again.[4] Otherwise, if life had established itself only on or near the surface of the Earth, it would have been annihilated. In the first case, organic evolution could have taken place over many hundreds of millions of years. In the second case, organic evolution would have had available less than a hundred million years to produce the miracle of life.

Let us consider the worst case scenario.

A first the task seems daunting. Even if the Earth was awash in organic chemicals right after the Late Bombardment, those were monomers, i.e. small organic molecules, whereas very complex polymers are the basis of the chemistry of living things. Moreover, special circumstances are needed to turn monomers into polymers (e.g. a reaction that combines two monomers into a more complex molecule may require giving up a water molecule, which may be difficult to accomplish in a water environment), and, to make matters worse, in principle billions and billions of polymers could, given the proper catalytic conditions, be produced, but only a relatively few are biologically useful (e.g., capable of making copies of themselves). How did the right complex organic molecules, then, come about in spite of serious obstacles and in the face of nearly impossible odds? It may be tempting, then, to say, “The answer must be elsewhere: It is not plausible that life began here.”

But to yield to this temptation only moves the question to a different time and place, and may even lead to abandoning the search for a scientific explanation altogether, as we have seen. How can we proceed, then? In biological evolution we have an example of a process that may increase complexity while reducing the space of possible outcomes. Since the first characteristic is obvious, let me concentrate on the second. In general, evolutionary processes that have mechanisms for variation and selection can quickly reduce the space of possibilities. In his book Why We Feel, Victor S. Johnston describes a face-recognition program that he invented to “evolve” a criminal’s face (police composites are not very satisfactory). In writing his genetic algorithms, he created dimensions such as width of chin, with several values, from very pointy to double chin; size of ears; amount of hair, etc. Each “genotype” was made up of 14 computer “genes” specified, all together, in a 60-bit binary string. This “program can generate more than a billion billion different faces (260).”[5] If a witness viewed one face every second, it would take him longer than 36 billion years to go through all of them. But assisted by Johnston’s FacePrints program, a witness can come up with a close resemblance (far superior to a composite) to the criminal in less than one hour!

The program creates 30 random faces and the witness ranks them (0 to 9) on how closely they “fit” the criminal’s face. The program then creates a new generation of faces in which the “fittest” are represented in greater numbers (in proportion to their rank). This and future generations are produced by taking two selected genotypes at a time and mixing them, using crossover and a small number of mutations (in analogy to biological evolution). Closer to our topic, Lazcano and Miller estimated that the evolution from a primitive heterotroph with a small genome into a far more sophisticated cyanobacterium would take no more than 7 million years.[6]

Organic evolution, if it is to parallel biological evolution, should then have mechanisms for variation and selection, apart from the specific requirements specified above. In a rich environment with a great many sources of energy, with many mechanisms for concentrating organic molecules, and so on that the early Earth would have offered, it would not all be surprising that some characteristics would favor some molecules more than others, particularly in the presence of certain other molecules. Indeed, enzymes can make certain reactions millions of times faster than they would otherwise be. And as Christian de Duve argues, the chemical reactions of life have to happen quickly or the reagents may breakdown or dissipate, as they would in deep space, where the densities of the molecular clouds are low and the time scales are long. [7]

For a while, probably into the early 1990s, most of the mechanisms proposed for organic evolution-- nucleic acids first, proteins first, RNA first (DNA long afterwards), metabolism first, clays as molecular templates, etc. – had some serious difficulties. Since then, very ingenious views have been proposed that both criticize assumptions made in the objections to each of the mechanisms, and suggest hypotheses that in certain scenarios would yield the transition from a non-living to a living world. A mere summary of all that work would take me too far afield in this posting, although I may return to the subject in the future. But just to mention one example, the likely high intensity of ultraviolet radiation of the early Earth would substitute for the possible initial absence of some enzymes in the operation of DNA and RNA. The reason we cannot decide which, if any, of those mechanisms is correct is that we do not know what the prevailing conditions were: the geological record is too scant. Eventually, when human geologists visit other worlds, or our telescopic observations improve considerably, we might be able to fill in the gaps and serious testing of the various alternatives can begin. For now they are good plausibility arguments.

The problems with the motivation for the advocacy of panspermia was that instead of merely offering a possible alternative, in some more moderate versions perhaps even a plausible alternative, its proponents claimed the impossibility of the terrestrial origin of life. That claim was based mostly on misunderstandings of the theory of evolution (Hoyle, in an interview with Klyce declared that from the time he was a child he thought that evolution was “bunk”). This is not the place to argue for the scientific merits of evolutionary biology, but an abundant literature is within the readers’ reach. For a recent account that is both erudite and accessible I recommend David N. Reznick’s The Origin Then and Now:An Interpretative Guide to the Origin of Species.[8] Incidentally, Reznick, who is an experimental evolutionary biologist, has done work on fish that suggest that the rate of natural selection may be 40 to 70 times faster than that suggested by the fossil record (which, for obvious reasons, emphasizes morphology). It does seem, however, that when panspermia advocates dismiss evolution in a rather ignorant way they make a bad impression on the scientific community. The same can be said for claims that bacteria and viruses brought here the genes that determine the traits terrestrial beings, including men and women, exhibit. Geneticists long ago gave up on the “one-gene-one trait” view, and would hardly pay much attention to such a bizarre view as this. Human beings, for instance, have crucial genes which are mutations of genes that our closest cousins, the chimps, have. Many new genes have arisen throughout the history of all lineages. Space bacteria did not bring them here. And eukaryotes are the result of symbiosis between different kinds of bacteria. For these and many other reasons, scientists with training in biology are going to find absurd the notion advanced by R. Joseph that "Just as apple seeds contain the genetic instructions for the growth of apple trees, these genetic seeds of life contained the DNA-instructions for the Tree of Life, and the metamorphosis of all life, including woman and man: the replication of creatures which long ago lived on other planets."



[1] As quoted in Robert Shapiro’s Origins: A Skeptic’s Guide to the Creation of Life on Earth, Bantam Books,1987, p. 244. Hoyle and Wickramasinghe’s views can be found in such books as their Evolution from Space, Simon and Schuster, 1981, or in Hoyle’s The Intelligent Universe, Holt, Rinehart and Wisnton, 1984.

[2] www.panspermia.org/

[3] Mojzis, S. J.; et al. (1996). "Evidence for life on earth before 3,800 million years ago". Nature 384 (6604): 55–9.

[4] Maher, Kevin A.; Stevenson, David J. (1988). "Impact frustration of the origin of life". Nature 331 (6157): 612–4.

[5][5] V.S. Johnston, Why We Feel, Perseus Books, 1999, p. 44.

[6] Lazcano, A.; Miller, S. L. (1994). "How long did it take for life to begin and evolve to cyanobacteria?" Journal of Molecular Evolution 39 (6): 546–54.

[7] de Duve, C., 2002, Life Evolving, Oxford University Press, and 2005, Singularities, Cambridge University Press.

[8] Reznick, D. N., The Origin Then and Now, Princeton University Press, 2010.

Saturday, May 14, 2011

Intermission

The end of the academic year has kept me from the blog, and from everything else, but I will be posting in the next few days on the very subject of Aimee's comment.