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

Given our understandably greater interest in our own animal kingdom, however, most of the attention will be riveted upon some smaller and rarer globular fossils, averaging half a millimeter in diameter and found phosphatized in the same strata: an exquisite series of earliest embryonic stages, beginning with a single fertilized egg and proceeding through two-cell, four-cell, eight-cell, and sixteen-cell stages to small balls of cells representing slightly later phases of early development. These embryos cannot be assigned to any particular group (more distinctive, later stages have not yet been found) but their identification as earliest stages of triploblastic animals seems secure, both from characteristic features (especially the overall size of the embryo during these earliest stages, which remains unchanged as average cell size decreases to pack more cells into a constant space) and from their uncanny resemblance to particular traits of living groups (several embryologists have told Knoll and colleagues that they would have identified these specimens as embryos of living crustaceans had they not been informed of their truly ancient age).

Elso Barghoorn, Knoll's thesis advisor, opened up the world of earliest life by discovering that bacteria could be preserved in chert. Now, a full generation later, Knoll and colleagues have penetrated the world of the earliest known ancestors of triploblast animals by accessing a new domain where phosphatization preserves minute embryonic stages but no known process of fossilization can reliably render potentially larger phases of growth. When I consider the cascade of knowledge that proceeded from Barghoorn's first report of Precambrian bacteria to our current record spanning three billion Precambrian years and hundreds of recorded forms, I can only conclude that the discovery by Xiao, Zhang, and Knoll places us at a gateway of equal promise for reconstructing the earliest history of modern animals, before their overt evolutionary burst to large size and greatly increased anatomical variety in the subsequent Cambrian explosion. If we can, thereby, gain any insight into the greatest of all mysteries surrounding the early evolution of animals--the causes of both the anatomical explosion itself and the "turning off" of evolutionary fecundity to generate new phyla thereafter--then paleontology will shake hands with evolutionary theory in the finest merger of talents ever applied to the resolution of a historical enigma.

Two final comments might help to establish a context of both humility and excitement at the threshold of this new quest. First, we might be able to coordinate the growing direct evidence of fossils with a potentially powerful indirect method for judging the times of origin and branching for major animal groups: the measurement of relative degrees of detailed genetic similarity among living representatives of diverse animal phyla. Such measurements can be made with great precision upon large masses of data, but firm conclusions are hard to obtain because various genes evolve at different rates that also maintain no constancy over time, and most methods applied so far have made simplifying (and probably unjustified) assumptions about relatively even ticking of supposed molecular clocks.