A recently published paper describes some fantastic fossil finds from China that date to the earliest era of multicellular life. The fossil deposits date from the Ediacaran, a period in which the first multicellular life was evident. Most of the Ediacaran fossils we’re aware of come from a bizarre and extinct group called the rangeomorphs (PDF), The new fossils appear to be even older than the rangeomorphs, but include forms that could be mistaken for modern algae.
Many of the major groups of multicellular life first evolved during the Cambrian, which started about 540 million years ago. There is evidence of multicellular life before that, in fossils of the deep ocean in the Ediacaran, which started at the end of the last global glaciation. But the rangeomorphs seen then look bizarre and unfamiliar to modern eyes, consisting of a collection of similar segments, odd fronds, and few signs of complex, specialized cell types.
The new fossils come from the Lantian Formation in China, and the authors argue that they existed very early in that era. Lining up changes in the carbon isotope ratios with those of dated samples suggest that the fossils are over 580 million years old, and possibly over 600 million. That pushes them very close to the end of the last global glaciation of the Cryogenian period, and makes them older than the rangeomorphs.
The real surprise, however, is that some of the fossils look very modern. These include branched fronds emanating from a stalk that includes a base with a hold-fast for attaching to surfaces. Other examples are much more enigmatic and difficult to place; the authors compare some to modern bilateral animals and cnidarians (a group that includes corals and anemones), but these assignments are very iffy. Still, the fossils make it clear that more complex organisms predate the typical Ediacaran fossils.
What can explain this discrepancy? The Lantian fossils were laid down in what appear to be shallow waters, below the area that would be disrupted by storms, but still likely to have seen sunlight. The rangeomorphs originated in much deeper water. Although most of the organisms appear to only be a few dozen millimeters long, their size and complexity pose their own problem: the shallow waters of that period are thought to have been anoxic, and multicellular life does not thrive in anoxic conditions.
The authors propose that the organisms here thrived under brief periods when the waters contained sufficient oxygen, and what we are seeing are the remains left behind when anoxic conditions returned and killed them. This doesn’t explain where they came from, however; presumably, there must have been a region that had both sufficient sunlight and oxygen for these organisms to survive and populate the Lantian area when oxygen was more plentiful. As things now stand, however, we have no idea where, either geographically or in terms of depth, that area might be.