Ancient creature with no anus is not earliest human ancestor, research suggests

Saccorhytus , a spikey, wrinkly sack, with a large mouth surrounded by spines and holes, is part of a different family tree.

Scientists have discovered a curious creature with no anus is not the earliest human ancestor, as previously thought.

The mysterious microscopic creature is instead part of a different family tree, a new study suggests.

Resembling an angry purple Minion, the Saccorhytus  is a spikey, wrinkly sack, with a large mouth surrounded by spines and holes.

These were interpreted as pores for gills – a primitive feature of the deuterostome group – animals typically characterised by their anus forming before their mouth – from which human ancestors emerged. But analysis of 500 million-year-old fossils from China suggests the holes around the mouth are bases of spines that broke away during the preservation of the fossils.

Yunhuan Liu, professor in Palaeobiology at Chang’an University, China, said: “Some of the fossils are so perfectly preserved that they look almost alive.

“Saccorhytus was a curious beast, with a mouth but no anus, and rings of complex spines around its mouth.”

Professor Shuhai Xiao, a paleobiologist with the department of geosciences, part of the Virginia Tech College of Science, said: “We are back to square one in the search for the earliest animal with a secondary mouth.

“The next oldest deuterostome fossil is nearly 20 million years younger.”

The true story of Saccorhytus’ ancestry lies in the microscopic internal and external features of this tiny fossil.

Researchers took hundreds of X-ray images and used powerful computers to create a detailed 3D digital model of the fossil.

Researcher Emily Carlisle from the University of Bristol’s School of Earth Sciences said: “Fossils can be quite difficult to interpret and Saccorhytus is no exception.”

She added: “We took hundreds of X-ray images at slightly different angles and used a supercomputer to create a 3D digital model of the fossils, which reveals the tiny features of its internal and external structures.”

The digital models showed that pores once interpreted as gills are actually broken spines, challenging the only piece of evidence in support of a deuterostome interpretation, researchers say.

They now believe that Saccorhytus is in fact an ecdysoszoan: a group that contains arthropods and nematodes.

According to the scientists, the ancient creature’s lack of anus is an intriguing feature.

Although the question that springs to mind is the alternative route of digestive waste (out of the mouth), this feature is important for a fundamental reason of evolutionary biology.

How the anus arose – and sometimes subsequently disappeared – contributes to the understanding of how animal body plans evolved.

Prof Xiao, who co-led the study, said: “This is a really unexpected result because the arthropod group have a through-gut, extending from mouth to anus.

“Saccorhytus’s membership of the group indicates that it has regressed in evolutionary terms, dispensing with the anus its ancestors would have inherited.

“We still don’t know the precise position of Saccorhytus within the tree of life but it may reflect the ancestral condition from which all members of this diverse group evolved.”

The findings are published in Nature.