On embroyos and ancestors: Fossils of tiny embryos 570 million years old may well be the greatest paleontological discovery of our time

Natural History, July-August, 1998 by Stephen Jay Gould

This crucial distinction has been recognized by most commentators on the work of Wray et al. Geerat J. Vermeij, in his direct evaluation (Science, 1996, page 526), wrote that "this new work in no way diminishes the significance of the Vendian-Cambrian revolution." Fortey, Briggs, and Wills added that "there is, of course, no necessary correspondence between morphology and genomic change." (See BioEssays, 1997, vol. 19, p. 433.) In any case, a recent publication by Ayala, Rzhetsky, and Ayala (Proceedings of the National Academy of Sciences, vol. 95, 1998, pp. 606-11) presents a powerful rebuttal to Wray et al.'s conclusions. By correcting statistical errors and unwarranted assumptions, and by adding data for twelve additional genes, these authors provide a very different estimate for initial diversification in late Precambrian times: about 670 million years ago for the split of chordates from the line of arthropods, annelids, and mollusks; and 600 million years ago for the later divergence of chordates from echinoderms.

We are left, of course, with a key mystery (among many others): where are Precambrian adult triploblasts "hiding" now that we have discovered their embryos? An old suggestion, dating from the 1870s and devised by the bombastic German theorist Ernst Haeckel (who was, nonetheless, outstandingly right far more often than random guesswork would allow) held that Precambrian animals had evolved as tiny forms not much larger than, or very different from, modern embryos--and would therefore be very hard to find as fossils. (The similarity between Haeckel's speculative ancestors and Xiao, Zhang, and Knoll's actual embryos is almost eerie.) Recently, in a brilliant paper, E. H. Davidson, K.J. Peterson, and R. A. Cameron (Science, 1995, vol. 270, pp. 1319-25) have made a powerful case, based on genetic and developmental arguments, that Precambrian animals did originate at tiny sizes, and that the subsequent Cambrian explosion depended upon the evolution of novel embryologicai mechanisms for greatly increasing cell number and body size, accompanied by consequent potential for greatly enhanced anatomical innovation. If Haeckel's old argument, buttressed by Davidson's new concepts and data, has validity, we then gain genuine hope, even realistic expectation, that Precambrian adult triploblasts may soon be discovered, for such animals will be small enough to be preserved by phosphatization.

As a final point, this developing scenario for the early history of animals might foster humility and generate respect for the complexity of evolutionary pathways. To make the obvious analogy, we used to regard the triumph of "superior" mammals over "antediluvian" dinosaurs as an inevitable consequence of progressive evolution. We now realize that mammals originated at the same time as dinosaurs and then lived for more than 100 million years as marginal, small-bodied creatures in the nooks and crannies of a dinosaur's world. Moreover, mammals would never have expanded to dominate terrestrial ecosystems (and humans would surely never have evolved) without the supreme good fortune (for us) of a catastrophic extraterrestrial impact that, for some set of unknown reasons, eliminated dinosaurs and gave mammals an unanticipated opportunity.