Just a few 1.56 billion year old fossils from southern China.
Maoyan Zhu
The Cambrian “explosion” of life about 540 million years ago is an amazing story, in which a wide variety of animal body shapes appear for the first time in the fossil record. But the closer we look, the more interesting and incredible the Cambrian prequels become. Now there is a report of organisms large enough to be easily visible, but dating back to more than 1.5 billion years ago.
The fuse of the Cambrian bomb was quite long and at least fireworks were attached to it. Unicellular eukaryotes, organisms with a nucleus and other complex internal structures, joined the bacteria and archaea around 1.5 years old. billion years before the Cambrian. A significant array of complex organisms appeared about 60 million years before the start of the Cambrian, although their relationship to Cambrian life is disputed.
The history of multicellular eukaryotes in between is difficult to summarize, as it takes extraordinary luck to preserve evidence of their soft cell bodies for us. We have a few examples of small multicellular organisms that be able to may have been eukaryotes, but a new discovery from a team led by Shixing Zhu of the China Geological Survey adds a big one to the family. The long, flat fossils they found in 1.56 billion-year-old rocks measured a whopping 30 centimeters long and 8 centimeters wide.
The fossils were found in shale rock in southern China. The rock is made of mud deposited just offshore in an ancient ocean. The researchers found 53 specimens that could be grouped into four consistent shapes — all pretty much variations on a tie. Some were shaped like long tongues, some had square ends, and some looked more like skinny wedges. We’re often unsure whether some of the puzzling shapes in rocks so old are fossil imprints or just shredded flakes of mud, but the consistency of the shapes avoids that ambiguity in this case.
By dissolving part of the rock, the researchers recovered pieces of cells connected in sheets that allow closer inspection. As you would hope, organic carbon was definitely present. There was no sign of any organization of cells in specialized types, although the tongue-shaped fossils showed visible lines running from one end to the other. Some fossils also showed hints of what may have been “holdfasts” — anchors that seafloor organisms use to attach to the bottom.
Maoyan Zhu
Based on similarities to algae, the researchers suspect these organisms were probably photosynthetic. That idea would fit some estimates of when red and green algae evolved photosynthesis, but other estimates disagree, so the situation is murky. The fossils could be within the algae family tree, or it could be something else that simply disappeared. Whatever they were, they seem to have been large eukaryotes, pushing back the timeline for the appearance of that kind of life.
The researchers write, “Continued research promises new insights into marine ecosystems in the oxygen-poor world, misleadingly caricatured as a ‘boring billions’ year interval of both evolutionary and ecological stability.”
Nature communication2016. DOI: 10.1038/ncomms11500 (About DOIs).