Astrobiology Science Conference 2010 (2010) 5197.pdf

THE ASTROBIOLOGICAL IMPLICATIONS OF THE DEBATE BETWEEN “DEEP ”, AND “REMARKABLE CONVERGENCE”. C. H. Lineweaver1, A. Chopra2 and J. J. Brocks2, 1Planetary Sci- ence Institute, Research School of Astronomy and Astrophysics and Research School of Earth Sciences, Australian National University, ACT, Australia, [email protected], 2Research School of Earth Sciences, Australian Na- tional University, ACT, Australia

Introduction: We would like to know if there are We argue that almost all examples of “independent universal features of biological -- features evolution” and “convergence” are more plausibly ex- that would evolve again if we could turn back time and plained as examples of deep homology [4,5,6]. We play the tape of life again. Identifying such features in explain how deep homology undermines the usefulness terrestrial biological evolution would give us our best of terrestrial biological evolution to predict the out- guesses about the range of outcomes from extraterres- comes of extraterrestrial biological evolution. trial evolution. Until his death in 2002, Stephen J. References: [1] Gould, S. J. (2002) The Structure Gould parried with other paleontologists, notably of Evolutionary Theory, Harvard Univ. Press, Cam- Simon Conway-Morris, about what the fossil record bridge, MA p1056 [2] Conway-Morris, S. (2003) could tell us about the outcome of such a thought ex- Life’s Solution: Inevitable Humans in the Lonely Uni- periment. Gould’s reading of the fossil record [1] em- verse, CUP, Cambridge, UK [3] Conway-Morris, S. phasized contingency and Dollo’s Law: are (2010) Evolution like any other science it is predict- unique -- once extinct, species do not evolve again. able, Phil. Trans. Roy. Soc. B, 365, 133-145. [4] Shu- Reading much the same fossil record Conway-Morris bin, N. Tabin, C. & Carroll, S. (1997) Fossis, genes [2,3] came to the opposite conclusion: body and the evolution of animal limbs, Nature, 388, 639 plans in general and humans specifically, are inevita- [5], Shubin, N., Tabin, C. & Carroll, S. (2009) Deep ble. For astrobiology, the importance of answering Homology and the origins of evolutionary novelty, this question is hard to over-estimate. We would like Nature 457, 818. [6] Rutishauser, R. & Moline. P. to know whether we are alone in the universe. We (2005) “Evo-devo and the search for homology would like to know how life elsewhere in the universe (“sameness”) in biological systems”, Theory in Bio- has evolved. If we know what to expect (hyperther- sciences, 124, 213-241 mophiles? anerobic microbial mats? aerobic verte- brates? humans?), we will be able to do a better job searching. If some feature of evolution is inevitable, one would expect it to have evolved multiple times inde- pendently on Earth. Conway-Morris [3] has compiled an extensive list of features that have evolved “inde- pendently” and have been referred to as examples of “remarkable convergences”. On the other hand, evo- devo biologists [4,5] have suggested that many and possibly most examples of convergence are more cor- rectly understood as examples of deep homology -- that is, with 4 billion years of evolution of life, there are many shared features, HOX genes, master genes, transcription factors that have evolved during the 3.5 billion years during which currently “independent” species were identical. Gould [1] defines the advent of deep homology [4] as “…the discovery that major phyla, separated by more than 500 million years of independent evolution- ary history, still share substantial (if not predominant) channels of development based on levels of genetic retention that proponents of the Modern Synthesis had specifically declared inconceivable….”