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Our oldest known ancestor had a huge mouth and no anus

London: A microscopic, bag-like sea creature with a large mouth and no anus, which lived about 540 million years ago, could be our earliest known ancestor, say researchers.

Named Saccorhytus, after the sack-like features created by its elliptical body and large mouth, the species is new to science and was identified from microfossils found in China.

According to a study, published in the journal Nature, Saccorhytus was the common ancestor of a huge range of species, and the earliest step yet discovered on the evolutionary path that eventually led to humans, hundreds of millions of years later.

Saccorhytus was about a millimetre in size, and probably lived between grains of sand on the seabed.

It is thought to be the most primitive example of a so-called “deuterostome” — a broad biological category that encompasses a number of sub-groups, including the vertebrates.

“We think that as an early deuterostome, this may represent the primitive beginnings of a very diverse range of species, including ourselves,” said one of the researchers Simon Conway Morris, Professor of Evolutionary Palaeobiology at University of Cambridge.

“To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail is jaw-dropping. All deuterostomes had a common ancestor, and we think that is what we are looking at here,” Conway Morris added.

Most other early deuterostome groups are from about 510 to 520 million years ago, when they had already begun to diversify into not just the vertebrates, but the sea squirts, echinoderms (animals such as starfish and sea urchins) and hemichordates (a group including things like acorn worms).

The Saccorhytus microfossils were found in Shaanxi Province, in central China, and pre-date all other known deuterostomes.

By isolating the fossils from the surrounding rock, and then studying them both under an electron microscope and using a CT scan, the team were able to build up a picture of how Saccorhytus might have looked and lived.

This revealed features and characteristics consistent with current assumptions about primitive deuterostomes.

The study suggests that its body was bilaterally symmetrical — a characteristic inherited by many of its descendants, including humans — and was covered with a thin, relatively flexible skin.

This in turn suggests that it had some sort of musculature, leading the researchers to conclude that it could have made contractile movements, and got around by wriggling.

Perhaps its most striking feature, however, was its rather primitive means of eating food and then dispensing with the resulting waste.

Saccorhytus had a large mouth, relative to the rest of its body, and probably ate by engulfing food particles, or even other creatures.

Its features were spectacularly preserved in the fossil record — and intriguingly, the researchers were unable to find any evidence that the animal had an anus.

“If that was the case, then any waste material would simply have been taken out back through the mouth, which from our perspective sounds rather unappealing,” Conway Morris said.