The history of the first animal life remains somewhat confused. Ediacaran fossils are clearly multicellular, but lack many of the features shared by all modern animals. In the ensuing period, the Cambrian, all of these organisms are gone, and most of the groups we're familiar with—along with a few unfamiliar ones—are present. The transition between the two is murky.

Spectacular fossils from Doushantuo in China appeared to resolve this issue. The tiny remains date from the Ediacaran, but appeared to share features with animal (more properly, metazoan) embryos, suggesting that metazoans were around for many millions of years, even though we've been unable to identify any fossils of their adult forms. Since their initial announcement, however, this interpretation has been challenged, with some even suggesting that the fossils were little more than clusters of bacteria.

Now, a group has performed X-ray imaging of the internal structure of the fossils and concluded that they most certainly aren't bacteria... but it's not actually clear what they are. The most likely identity, according to the researchers, is a relative of the first metazoans.

First, the new data. The authors have essentially performed a CAT scan of these tiny fossils, except they use energies high enough to fry human tissues. This is sufficient to reveal subtle internal features, including what appears to be a nucleus inside many of the cells, as shown above. The authors argue that this isn't just a matter of similar appearance. The object has a consistent location within the cells, takes up a consistent fraction of the cell's volume, and only appears once per cell, all of which are features of a nucleus.

That clearly eliminates bacteria as a source of these fossils, and would appear to put metazoans back on the table, since these fossils are eukaryotes with multiple cells. But there are still some problems with this interpretation. For example, all the cells appear identical, while metazoans are notable for having multiple specialized cell types. In addition, other fossils from the same deposit appear to lack internal nuclei. Instead, they appear to contain large numbers of individual cells.

The authors interpret this as an indication of a sort of sporulation. A number of organisms reproduce by forming a spore and then divide internally within it, making a large number of identical cells that eventually burst out when the spore germinates and then live independently as single cells. In fact, the authors identify a number of fossils that can be interpreted as in the process of germination. Metazoans, at least existing species of metazoans, don't do this. So, animals remain off the table.

But that doesn't mean these things don't tell us anything about the evolution of metazoans. Metazoans are a subgroup of what are called Holozoans, which is a grab-bag of everything that evolved once we parted ways with the fungi (which went multicellular separately). Some of the holozoans that survive display precisely the mode of reproduction that the authors describe in these fossils, and the clustering of cells in these spores certainly could provide an opportunity for specialized cells to evolve. So, if these fossils don't represent actual metazoans, there's a good chance that the organisms here set the stage for them.

That paints an appealing picture, but there are still a few issues here. For one, there clearly were multicellular organisms in the Ediacaran, and (as far as I'm aware) we don't know what their embryonic stages looked like. Since we're not quite sure of what existing organisms they'd group with, we don't even have a strong idea of what their embryos should look like.

Plus the first metazoans we see in the Cambrian are generally tiny, and heavily biased towards things with skeletons or shells. If they were around earlier, but lacked shells, it's possible we could miss them. And then there's the molecular evidence, which suggests that many of the major groups of organisms that appear in the Cambrian had probably been separated for a significant amount of time in order to evolve their distinct body plans. All of that would suggest that the metazoans probably predate the Cambrian.

So, even if this turns out to be the last word on the Doushantuo fossils, it probably won't be the last word on the origins of animal life.

Science, 2011. DOI: 10.1126/science.1209537 (About DOIs).