Humans’ oldest ancestor discovered

Saccorhytus coronariusm reconstruction (Image courtesy Jian Han)
Saccorhytus coronariusm reconstruction (Image courtesy Jian Han)

An article published in the journal “Nature” describes the identification of what is considered humans’ oldest known ancestor. Called Saccorhytus coronariusm, 45 specimens were discovered during excavations carried out in today’s Shaanxi Province, China. This animal had a length around a millimeter, lived 540 million years ago and is the ancestor of deuterostomes, which include vertebrates, echinoderms and other animals.

Deuterostomes (superphylum Deuterostomia) are also called enterocoelomates. These are names that derive from certain features in common among animals that are very diverse in many ways such as mammals and sea stars. The taxonomic classification changed over time with the increase in biological knowledge that excluded other animals from this large group that has common ancestors. An international team of researchers believe they have found the oldest on in Saccorhytus coronariusm.

A big problem in paleontological research is due to the fact that primitive deuterostomes were very small and fossilizes only in exceptional cases. They started diversifying between 510 and 520 million years ago into the various phylum existing today making even harder to imagine what their common ancestors looked like. The discovery of 45 tiny fossils initially forced the researchers to wonder what they were but after a series of tests they concluded that those were really the ones they were looking for.

Probably Saccorhytus coronariusm lived between the grains of shallow sea sediments that formed the Shaanxi province at the beginning of the Permian period, about 540 million years ago. To examine the 45 fossils they used an electron microscope and they were subjected to a CAT-scan. This allowed to study their details and features, consistent with the theories about early deuterostomes.

Probably Saccorhytus coronariusm had a bilateral symmetry, which was inherited by many of its descendants. Its body was covered with a thin skin and had some kind of muscles so it could also make some movement, although clumsy, wiggling. It had a primitive way of eating through a mouth very big compared to its body and to get rid of waste.

Some conical structures on Saccorhytus coronariusm’s body perhaps allowed it to get rid of the water it swallowed so they could be the precursors of gills. It seems that this tiny animal lacked the anus, a very primitive characteristic when compared with subsequent deuterostomes so it would seem that the anus evolved later.

The discovery of Saccorhytus coronariusm is important because it helps to better understand the evolution of a vast group of animals in a key period in the history of life on Earth. In a period relatively short from a biological point of view the various phylum that form the deuterostome group developed. Today thousands of very diverse species exist because 540 million years ago there was this humble creature.

Saccorhytus coronariusm fossil (Image courtesy Jian Han)
Saccorhytus coronariusm fossil (Image courtesy Jian Han)

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