Precambrian Cell Division Imaged
Embryos frozen in stone in the act of cell division were reported in Science.1 According to a press release from Virginia Tech, there are millions of fossilized embryos in the Doushantuo formation in south China, estimated to be 551 million years old, but “later stages of these animals are rare.” The EurekAlert version of this press release contains images of the embryos. A press release from Indiana University says some of the embryos have 1000 cells or more.
With X-ray computed tomography, the researchers were able to get past taphonomic artifacts and image the actual cells. The embryos show asynchronous cell division, which means that the embryos were differentiating into more complex organisms than bacteria in strata said to be 10 million years prior to the Cambrian explosion. The original paper in Science puts the find into an evolutionary context: “Asynchronous cell division is common in modern embryos, implying that sophisticated mechanisms for differential cell division timing and embryonic cell lineage differentiation evolved before 551 million years ago.” None of the larger embryos in the 162-sample set showed differentiation into epithelial tissues, however, an observation they call “striking.” “Many of these features are compatible with metazoans, but the absence of epithelialization is consistent only with a stem-metazoan affinity for Doushantuo embryos…. Epithelialization, by whatever mechanism of gastrulation, should be underway in modern embryos with >100 cells.” Thus, they imply these represent pre-animal experiments in cell division. “The absence of this 3D hallmark of sponge- and higher-grade metazoans may indicate that they did not yet exist… the combined observations suggest that the Doushantuo embryos are probably stem-group metazoans”; i.e., organisms on the way to evolving into full-fledged multicellular animals.
It’s hard to be sure, though, because specimens in later stages of development are lacking. Even so, these embryos have characteristics of the embryos of advanced Cambrian animals:
Despite hypotheses that Doushantuo embryos are unusual in comparison to most known metazoans, the patterns of cleavage and cell topology are compatible with a range of animal groups. For instance, in embryos composed of eight or more cells, the offset arrangement of successive tiers of cells, strong cell cohesion, and a stereoblastic cell topology are comparable to early cleavage embryos of many arthropod groups. Stereoblastulae are also particularly common among sponges and scyphozoan cnidarians. Doushantuo embryos composed of many hundreds of cells resemble the purported gastrulae of demosponges, before the development of parenchymella larvae, although at this stage demosponges exhibit evidence of gastrulation, with a differentiated superficial layer of micromeres surrounding a core of macromeres.
If juvenile and adult forms of these organisms appeared in the strata, would they resemble the Cambrian animals? Or do these embryos represent experiments in cell division that would later explode into the diversity of Cambrian forms? Take your pick: the Indiana U press release says, “Either these embryos are primitive and don’t have a clear blastocoel, or a blastocoel existed but didn’t survive the preservation process.” See also a story posted on the UK Telegraph.
1Hagadorn et al, “Cellular and Subcellular Structure of Neoproterozoic Animal Embryos,” Science, 13 October 2006: Vol. 314. no. 5797, pp. 291-294, DOI: 10.1126/science.1133129.
If all the Darwinian assumptions (biological, paleontological and geological) were purged out of this story, you would only have some advanced-looking embryos in a certain layer of Chinese rocks. They are incapable of interpreting themselves. The simplest explanation is that complexity exists from the very beginning of the fossil record, whenever that was.