Our most ancient ancestor is a little monster
It is called Saccorhytus and represents a new piece of the puzzle of the evolution of life on Earth: one of the oldest fossils that can be linked to human descent.
Monstrous and infinitely small, with a huge mouth accompanied by several rows of small growths that resemble primordial teeth and no anus! It is not the terrifying protagonist of a science fiction film, but the Saccorhytus, probably the most ancient progenitor of man.
The curious ancestor was identified in a fossil discovered by an international team of paleontologists from China, Germany and Great Britain, in Shaanxi province (central China). Paleontologists have unearthed 45 fossils smaller than a grain of rice, buried under three tons of limestone.
It is a marine microorganism that lived 540 million years ago, with a rather particular physiology: it had the shape of a bag just one millimeter long and had no anus. Precisely because of its bizarre shape it was called Saccorhytus coronarius.
The discovery was described in an article published in the Nature's journal (https://www.nature.com/articles/nature21072).
The microorganism, long sought after by paleontologists, is the most primitive example of a broader biological category, that of the so-called deuterostomes, from which vertebrates developed. During the Cambrian, the Deuterostomes diversified into a large number of groups. By studying the genetic differences of current populations, biologists have reconstructed their history and understood when the group split into different organisms.
Until now, the mother, i.e. the origin of diversification, had not been identified because the original organism was very small, so few have managed to fossilize it. It was even thought that they could never be found: this is why the discovery of these small organisms is of great importance. However, the discovery of these specimens was not at all simple. To isolate them from the surrounding rock, researchers had to process enormous volumes of limestone and then subject them to the electron microscope and CT scan. But eventually researchers were able to reconstruct what this ancient microorganism looked like and how it lived.
In that corner of the world, at the beginning of the Cambrian period, the sea was shallow, and Saccorhytus was so small that it probably lived among the grains of sand on the sea floor.
According to the 2017's study, its body was bilaterally symmetrical, a characteristic inherited by many of its descendants, including humans, and was covered with thin, relatively flexible skin. This suggests that the microorganism was equipped with a sort of musculature that allowed it to move by contracting in a similar way to that of earthworms.
Perhaps its most striking feature was the way it fed itself.
Saccorhytus had a huge mouth compared to the rest of its body, and probably ate by swallowing even very large pieces of food or whole other creatures. Curiously, scholars have found no evidence of an anus, and probably in this species the waste was expelled from the mouth. The small conical structures noted in the reconstruction could instead have been the precursors of the gills, of which our species also keeps traces during embryonic development.
If the conclusions of the study are correct, then Saccorhytus was the common ancestor of many species and at the same time the first step on the evolutionary path that hundreds of millions of years later led to man.
