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It's believed that this new species is the most primitive example of a deuterostome, a category in biology that includes many sub-groups, including vertebrates such as ourselves.
"Frankly, it doesn't look like anything very exciting; it looks like a tiny dot, a bit smaller than a grain of rice. But in the electron microscope, it's just eye-popping," Simon Conway Morris, a professor and fellow of St. John's College, Cambridge, who authored the paper, told CBC News.
The fossils, which were exquisitely preserved, were found in limestone — three tonnes of it — that was broken down into very small pieces allowing researchers to study them under electron microscopes.
Saccorhytus — which means "wrinkled bag" — was just about one millimetre in size with an elliptical body and large mouth.
One of the things that deuterostomes share is gill slits, and the researchers found that the Saccorhytus had small conical features believed to be the precursors to gills. That's where excess water — and possibly waste — was expelled. The researchers couldn't find any sign the creature had an anus.
Morris said this discovery doesn't mean they've found our earliest human ancestor, but it is still exciting.
“Saccorhytus now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us,” palaeontologist Degan Shu of China’s Northwest University said in a statement provided by the University of Cambridge.
Fossils of about 40 Saccorhytus individuals, including a few superbly preserved specimens, were unearthed in Shaanxi Province in central China and look like minuscule black grains.
Saccorhytus probably lived between individual grains of sediment on the bottom of a shallow sea.
With its large mouth, it probably ate by simply engulfing food particles or smaller creatures whole. The researchers found no evidence that it had an anus, meaning that its waste material may have been expelled out of the mouth. Conway Morris said that process “from our perspective sounds rather unappealing.”
It also boasted small conical structures on its body, which was covered with a thin, flexible skin. These structures may have allowed water it swallowed to escape, possibly the evolutionary forerunner of fish gills.
The name deuterostome means "mouth second", and refers to one important developmental feature unique to this group. To understand this feature, a little animal embryology is necessary. In the earliest stages of embryo development, when there are only a few cells and the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo. This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer. At this stage, the embryo is known as a gastrula. In the Protostomia, which is the other major group of the Bilateria, the mouth develops from the edge of this pocket, where the inner and outer layer of cells meet; the anal opening develops later. In the Deuterostomia, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later. Hence the byline at the top of this page: "Your mouth comes second."