While the Cambrian Explosion marks the appearance of most modern animal lineages, our oldest common ancestor may well be even older, having lived more than 545 million years ago, during the Ediacaran.
If we could go back in time and observe the Earth’s environment around 600 million years ago, we would discover a world totally different from the one we know today. No animal or plant life then occupies the desert and ice-covered land masses. But in the waters of the gigantic ocean which occupies a very large part of the Globe, things are however very different.
The strange creatures of the Ediacaran
The Ediacaran period (635 to 538.8 million years ago) is indeed characterized by the appearance of the first complex multicellular organisms. This fauna could however seem particularly strange to our eyes. On the bottom of the sea evolve soft-bodied creatures, in the shape of leaves, discs, tubes or bags filled with mud, some of which can measure several meters.
Skeletons do not yet exist and only a few organisms show an outline of a shell. This absence of hard parts is also certainly responsible for the small quantity of fossils found dating from this period, the tissues being very difficult to preserve. Nevertheless, many footprints of these organisms have made it possible to trace, at least partially, the characteristics of this strange fauna. It appears that while some species seemed to have a mouth and a digestive tract, others were totally devoid of internal organs, which raises the question of their way of life and reproduction. This is particularly the case with vendozoans, these flat organisms segmented by fractal patterns, which had to absorb the nutrients present in the water by ion exchanges through their tissues (osmosis).
Almost all of these life forms left no descendants
It is difficult to trace in detail the appearance and functioning of these organisms, because the great majority of the species constituting the fauna of the Ediacaran disappeared completely by the end of this period, leaving almost no descendants. Following a mass extinction, these organisms will indeed be replaced by the rapid development of new species during the Cambrian, a period during which the forms of animal organization existing today will appear. And yet, certain elements suggest that our common ancestor would take its roots not from the Cambrian, but from the Ediacaran.
Our oldest known ancestor looks like a slug grazing on seafloor algae
This is suggested by a new study published in Current Biology. The researchers of theAustralian National University analyzed ancient Ediacaran fossils inside which they found molecules of phytosterol, a chemical compound found in plants. These molecules are in a way the traces of the last meal of these organisms. It was by analyzing these molecules that the researchers were able to determine that the Kimberella species had a mouth and a digestive tract that allowed it to digest its food in a way that was certainly primitive, but quite similar to modern animals. This slug-like organism would therefore be one of the most evolved creatures of the Ediacaran and perhaps one of the most distant common ancestors of current animal species, which is observable in the fossil series.
Molecular analyzes notably revealed that Kimberella fed on the films of algae covering the bottom of the sea. These algae were particularly rich in nutrients and energy, which could explain why the life forms of the Ediacaran could reach such imposing sizes for such primitive beings and direct descendants of simple single-celled organisms.