SCIENCE, Vol. 144, p. 614
8 May, 1964
. . . The following diversion in numerology, which is no more naive than calculations of the number of earthlike planets in the universe, is merely intended to temper the widely held faith that humanoids must necessarily arise on an appreciable proportion of as yet hypothetical earthlike planets:
Numbers associated with the magnitude of the universe, for example the estimates of from 108 to 1020 earthlike planets among up to 1021 stars within the observable universe (radius, 1010 light years = 6 x 1021 miles) overpower many people. In a diploid cell 10-12 gram of DNA can code for a sequence of about 108 of the 20 amino acids. Such an "information source" contains 20(108) = 10(1.3x108) = 2(0.43x109) possible genetic "messages" (organisms?). The improbabilities associated with such a 0.43-billion-bit information source make even Eddington's "cosmical number," the supposed total number of protons and neutrons in the observable universe (approximately 2.4 x 1079), pale into insignificance.
If we consider each section of the DNA which codes each polypeptide of about 15,000 molecular weight as a "gene" or cistron, then there are 106 genes for each such organism. If we allow ten variants ("alleles" representing single amino acid substitutions along a polypeptide) with approximately equal adaptive value for each gene, and consider all colinear sequences of these as again representing adaptively equivalent phenotypes, then if this amount of diversity were the maximum that is likely to occur within a Mendelian population the equal-likelihood probability of producing any member of any Mendelian population (species) is l06 x 10-(1.3x108) = 10-(1.29x108). Even if we suppose that all possible "chromosomal rearrangements" of such million kinds of nonallelic genes are adaptively equal and therefore represent members of other species with "equal evolulionary potential," the probability of producing any member of any such species, if all permutations are equally likely, is still only 106! x 10-(1.29x108) or approximalely 10-(1.23x108).
Since evolution is "opportunistic" and "deterministic," all permutations are not equally likely. Let's consider a hypothetical collection of 108 Mendelian populations, each of 106 sexual members at a time 3 x 109 years ago on earth. Let each organism have the coding for 106 polypeptides, and allow a mutation rate per gene per generation of 10-6. Let the generation time equal one year. If there were 2 x 106 offspring per mating pair, and if two mutations per 2 x 106 offspring confer a sufficiently large selective advantage in the given environnlent so that their carriers are the only survivors among their sibs, then 106 x 108 x 3 x 109 = 3 x 1023 possible different "progressive" viable genotypes would have been produced "deterministically" and "opportunistically" out of a total of 3 x 1029 "tested by evolution."
This represents a tempo of evolution far exceeding that which could have occurred on earth or on any earthlike planet, yet only an infinitesimal proportion of the 10(1.23x108) possible genotypes with "different evolutionary potential" would have been sampled.
Clearly, unless a large proportion of genotypes with "different evolutionary potential" nonetheless should prove to have "similar evolutionary potential," only organisms related by descent from a common ancestor will have any appreciable chance of having "similar evolutionary potential."
Restated, only if observation of the evolutionary record strongly indicates that independent origin and "convergent evolution" have occurred in connection with important humanoid characteristics does it at present make sense to give serious consideration to the occurrence of extraterrestrial humanoids.
If a "humanoid" is an organism which has a sophisticated data-processing and information-retrieval system and which, for adaptive purposes, can communicate substantial amounts of the information used within its data-processing system to other oryanisms of the same kind, then what is the evidence for independent origin and convergent evolution of important elements of such systems?
I believe we can discount the significance of such primitive "reflex arcs" among the higher plants as that of the Venus's-Flytrap. The relationship of nervous tissues and systems among animals is almost certainly the result of parallel evolution based on descent from a common ancestor.
Sophisticated data processing has apparently depended upon the evolution of a complex visual apparatus, and it is therefore particularly relevant to ask whether the arthropod, molluscan, and chordate eyes are examples of independent origin, and whether the cephalopod and vertebrate eyes are samples of convergent evolution.
Cytological evidence indicates that visual receptors have a common cellular origin, representing the realization of thc evolutionary potential indicated for the cilium among, for example, the euglenoids. Were there no living flagellated species with eye-spots, and were none of the electron-microscopic evidence on the structure of photoreceptors yet in, we might argue for independent origins of primary photoreceptors. Were there no living tupaioids ("tree shrews"), the marked similarity (almost identity) of the detailed sculpturing of the external ear (too soft to leave a fossil record) of some ceboids (New World monkeys) to that of some Old World monkeys and apes (and especially man), but dissimilarity to that of living lemuroids, lorisoids, tarsioids, and hapaloids (marmosets), might suggest convergent evolution determined by some obscure adaptive value special to the higher primates. Tupaia glis with its very human-like ears weakens that argumcnt and instead spotlights "evolutionary conservatism" as well as "opportunism."
In the absence of knowledge concerning the structure of the "eyes" (if any) of possible extinct ancestral stocks of the arthropods, molluscs, annelids, echinoderms, and chordates, embryological, anatomical, and neurological evidence strongly favors independent origin and, in the case of the cephalopods and vertebrates, convergence. However, until a comprehensive "molecular evolutionary" study of living membcrs of these groups has been made, or "missing links," living or fossil, are discovered, one would not want to hinge important arguments for support of Project Ozma or any substantial part of NASA's program on such evidence. (Click here for the subsequent molecular-biological confirmation that the 20 or so cases of apparent independent convergent evolution of eyes are, almost certainly, cases of divergent evolution from one common ancestor with eyes!)
Extensive communication among members of a species seems to exist only among some social hymenoptera and some few vertebrates. Here the case for independent origin of the mechanisms in the two lines and of convergence growing out of the "deterministic potential" of the visual data-processing equipment seems quite convincing. Yet the scarcity of instances of such communication among the many species with such potential emphasizes how largely uncertain is the "determinism" of evolution. Since ours appears to be the only sophisticated communicating species on earth, it seems reasonable to favor Simpson's view that "humanoids are, to say the least, nonprevalent."
Division of Cell Biology, Mount Sinai Hospital, New York 10029
Since 1982; and especially since 1964, an enormous number of molecular evolutionary studies has accumulated. To the surprise of many biologists, the record indicates that a large fraction of processes and structures present in modern organisms have been inherited from remote common ancestors with remarkably small changes from their original genetic coding demonstrating extreme evolutionary conservatism.. And in particular, since 1994, the laboratory of Walter J. Gehring, in Basel, Switzerland, has provided increasingly persuasive evidence in support of my suggestion in 1964 (above) and in my "Extraterrestrial intelligence: the debate continues; A biologist looks at the numbers" Physics Today, March 1982: 27 - 31, that it might turn out that "eyes" (if any) of possible extinct ancestral stocks of the arthropods, molluscs (including cephalopods), annelids, echinoderms, and chordates (including vertebrates) may account for the apparent convergence from multiple independent origins.
From: Gehring, W.J. "Eye Evoluition" Science 272: 468 - 469 (1996). "On the basis of these experiments, we are proposing that the prototype eye arose only once in evolution and that subsequent convergent evolution gave rise to the image-forming eyes of vertebrates and cephalopods, whereas the compound eyes of insects resulted from divergent evolution. The main difference from the 'traditional' view is the assumption of a single, rather than more than 40 prototype eyes." (Italics, mine) See also Halder, G., Callaerts, P., and Gehring, W.J. Science 267: 1788 (1995) and Callaerts, P., Halder, G. and Gehring, W.J. "Pax-6 in Development and Evolution" Ann. Rev. Neurosci. 20: April (1997).,
Gehring's Biennial Report and
from Science 296: 1010 (2002):
EVOLUTION OF DEVELOPMENTAL DIVERSITY:
Evo-Devo Devotees Eye Ocular Origins and More
COLD SPRING HARBOR LABORATORY, NEW YORK--From 17 to 21 April, evo-devo researchers met herefor the Evolution of Developmental Diversity meeting to discuss how environment and quirks in development prompted the branching of the tree of life.
Did Eyes Come From Microbes?
In the mid-1990s, developmental biologist Walter Gehring came up with a heretical proposition: Eyes evolved only once. The idea was a hard sell. Because eyes are ubiquitous and come in vastly different varieties, most evolutionary biologists assumed that they arose independently many times. Now, Gehring has come up with an even more eye-popping suggestion: The original eye belonged to a microbe that later became a chloroplast, a subcellular center of photosynthesis best known for fueling growth in plants.
"It's a wild idea," says Richard Behringer, a developmental biologist at the University of Texas M. D. Anderson Cancer Center in Houston. If true, "it would be fantastic," adds Nipam Patel, an evo-devo researcher at the University of Chicago. But he and others have their doubts.
At the meeting, Gehring of the University of Basel, Switzerland, described the thinking behind his new view of eye evolution. His proposal that eyes evolved only once rests on his 1995 discovery of a gene called Pax-6 that is involved in eye formation in fruit flies, mice, and humans. Since then, he and others have been shaking the tree of life in search of more versions of this gene.
It has turned up in some 20 species; even primitive ones, such as sponges, have precursor versions of the gene. Each new find has convinced him that his proposition is correct. However, the notion remains controversial--indeed, it met with some opposition at the meeting--in part because it's hard to imagine how a primitive eye from a common ancestor could lead to the great diversity seen today. So Gehring has been looking at the simpler animals for clues to support his theory.
He has found Pax-6 in Planaria, primitive flatworms that can regenerate their bodies. And he has used a technique called RNA interference (RNAi) to demonstrate that the gene is involved in the animal's eye formation. He reported at the meeting that if he interferes with Pax-6 function when a planarian is growing back a severed head, "it can regenerate the brain, but it can't regenerate the eyes." Once the RNAi effect wore off, eyes appeared. The same proved true in eye regeneration in ribbonworms.
Eye to eye. This schematic of the eye of an obscure species of dinoflagellate reveals a complexity akin to that of our own eyes. CREDIT: MARIE-ODILE SOYER
Gehring hadn't really considered an even more primitive origin for eyes until a French colleague sent him a 40-year-old Ph.D. thesis that described an obscure dinoflagellate--a single-cell plankton--that has an eye "basically like a human eye and could focus light," Gehring said. This eyespot, which is presumably derived from the dinoflagellate's chloroplast, has a lens, protective pigments, and a stacked layer of membranes akin to a retina.
Over the past decade, many evolutionary biologists have become convinced that, like mitochondria, chloroplasts were once independent microbes. According to one scenario, early in life's history, these productive bugs were engulfed by a larger microbe and subsequently became part of that microbe's cellular machinery. Gehring suggests that the independent microbes may have developed a light-sensing mechanism: "There's a great selective advantage," he explained, "as sensing light could have enabled these early organisms to avoid damaging UV light and track down light for photosynthesis." These proto-chloroplasts, he suggests, were engulfed by dinoflagellates, which in turn became symbionts of more complex organisms. "It could be that eyes are coming from a symbiont within a symbiont," he says.
Some colleagues are skeptical, however. "It's hard to understand how you would take [a subcellular component] and have it become a multicellular structure" such as a modern eye, Patel notes. But others find the theory intriguing. "It's a neat idea, and [he] can really check into it," says Ronald Ellis, a developmental biologist at the University of Michigan, Ann Arbor.
To do that, Gehring wants to isolate DNA from the species of dinoflagellate with the eyespot to check for Pax-6 and other genes related to vision. "The dinoflagellate is very difficult to find," he notes. At the same time, he's planning to scour the genomes of other animals for chloroplast genes. "If my prediction is right, we should be able to find some chloroplast genes in multicellular animals and dinoflagellates." If so, that would be a real eye-opener.
And from PNAS (July 14, 2010):
Hiroshi Suga, Patrick Tschoppa, Daria F. Graziussia, Michael Stierwaldb, Volker Schmidb, and Walter J. Gehringa. "Flexibly deployed Pax genes in eye development at the early evolution of animals demonstrated by studies on a hydrozoan jellyfish"
Pax transcription factors are involved in a variety of developmental processes in bilaterians, including eye development, a role typically assigned to Pax-6. Although no true Pax-6 gene has been found in nonbilateral animals, some jellyfish have eyes with complex structures. In the cubozoan jellyfish Tripedalia, Pax-B, an ortholog of vertebrate Pax-2/5/8, had been proposed as a regulator of eye development. Here we have isolated three Pax genes (Pax- A, Pax-B, and Pax-E) from Cladonema radiatum, a hydrozoan jellyfish with elaborate eyes. Cladonema Pax-A is strongly expressed in the retina, whereas Pax-B and Pax-E are highly expressed in the manubrium, the feeding and reproductive organ. Misexpression of Cladonema Pax-A induces ectopic eyes in Drosophila imaginal discs, whereas Pax-B and Pax-E do not. Furthermore, Cladonema Pax-A paired domain protein directly binds to the 5? upstream region of eye-specific Cladonema opsin genes, whereas Pax-B does not. Our data suggest that Pax-A, but not Pax-B or Pax-E, is involved in eye development and/or maintenance in Cladonema. Phylogenetic analysis indicates that Pax-6, Pax-B, and Pax-A belong to different Pax subfamilies, which diverged at the latest before the Cnidaria&endash;Bilateria separation. We argue that our data, showing the involvement of Pax genes in hydrozoan eye development as in bilaterians, supports the monophyletic evolutionary origin of all animal eyes. We then propose that during the early evolution of animals, distinct classes of Pax genes, which may have played redundant roles at that time, were flexibly deployed for eye development in different animal lineages.
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