While the Cambrian explosion marks the emergence of most modern animal lineages, our oldest common ancestor may be even older, having lived more than 545 million years ago, during the Ediacaran.
If we could go back in time and observe the terrestrial environment of about 600 million years ago, we would discover a totally different world from the one we know today. No animal or plant life therefore occupies the desert and ice-covered land masses. But in the waters of the gigantic ocean that occupies a large part of the globe, however, things are very different.
The strange creatures of the Ediacaran
The Ediacaran period (from 635 to 538.8 million years ago) is in fact characterized by the appearance of the first complex multicellular organisms. However, this fauna may seem particularly strange to our eyes. On the seabed, soft-bodied creatures evolve in the shape of leaves, discs, tubes, or mud-filled sacks, some of which can measure several meters.
Skeletons do not exist yet and only a few organisms show the outline of a shell. This absence of hard parts is certainly also responsible for the small amount of fossils found dating back to this period, as the tissues are very difficult to preserve. However, many footprints of these organisms have made it possible to trace, at least in part, the characteristics of this strange fauna. It seems that while some species appeared to have a mouth and digestive system, others were totally devoid of internal organs, which raises the question of their way of living and reproducing. This is especially true for vendozoans, these flat organisms segmented by fractal patterns, which had to absorb nutrients present in the water by ion exchange through their tissues (osmosis).
Most of these life forms left no descendants
It is difficult to trace the appearance and functioning of these organisms in detail, because the vast majority of species that make up the Ediacaran fauna disappeared completely by the end of this period, leaving almost no descendants. Following a mass extinction, these organisms will in fact 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. Yet, some elements suggest that our common ancestor would have its roots not from the Cambrian, but from the Ediacaran.
Our oldest known ancestor looks like a snail grazing on algae on the seabed
This is suggested by a new study published in Current biology. The researchers of theAustralian National University they analyzed ancient Ediacaran fossils within which they found molecules of phytosterol, a chemical compound present in plants. These molecules are in a sense the traces of the last meal of these organisms. It is precisely by analyzing these molecules that the researchers were able to establish that the Kimberella species had a mouth and a digestive system that allowed it to digest 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 highly evolved creatures of the Ediacaran and possibly one of the most distant common ancestors of the current animal species, which is observable in the fossil series.
Molecular analyzes have revealed in particular that Kimberella fed on the algae films that cover the seabed, particularly rich in nutrients and energy, which could explain why the life forms of the Ediacaran could reach such impressive dimensions for such primitive beings and direct descendants of simple single-celled organisms.