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Scientists are relieved to discover that the mysterious creature was not the first ancestor of humans

Side view of Saccorhytus.Credit: Philip Donoghue et al.

“Strange” creatures with no anus proved unrelated to humans.

An international team of researchers has discovered that the mysterious microscopic creatures believed to be the ancestors of humans actually belong to a different family.

Saccorhytus is a spiky, wrinkled sac with a huge mouth surrounded by spines and holes interpreted as gill pores.

However, a thorough examination of Chinese fossils dating back to 500 million years has shown that the holes around the mouth are actually the bases of spines that split during the fossil preservation process, eventually leading to microfossils. Saccorhytus evolutionary similarity revealed.


Researchers believe that Saccorhytus is actually a molting.Credit: Philip Donoghue et al.

“Some fossils are so perfectly preserved that they almost look alive,” says Yunhuan Liu, professor of paleontology at Chang’an University in Xi’an, China. “Saccorhytus was a curious beast with a mouth but no anus and an intricate ring of thorns around its mouth.”

Findings recently published in the journal Naturemaking important revisions to the early phylogenetic tree and our understanding of how life developed.

The true story of Saccorhytus’ ancestors lies in the microscopic internal and external features of this tiny fossil. With the help of powerful computers, they were able to reconstruct a detailed 3D digital model of the fossil by taking hundreds of X-ray images at slightly different angles.

Emily Carlyle, a researcher at the University of Bristol’s Department of Earth Sciences, explains: As a basis for the analysis of fossils, it was necessary to use a synchrotron, a type of particle accelerator. Synchrotrons provide extremely powerful X-rays that can be used to take detailed images of fossils. He took hundreds of X-ray images from slightly different angles and used a supercomputer to create his 3D digital model of the fossil. This revealed small features of internal and external structures. “

Digital models showed that the pores around the mouth were closed by another body layer, forming spines around the mouth. We believe it helped Saccorhytus to capture and process its prey.”

Researchers believe that Saccorhytus is actually a molting. That is, a group that includes arthropods and nematodes. “Corals, sea anemones, jellyfish, etc. have mouths but no anuses.”[{” attribute=””>University of Bristol’s School of Earth Sciences, who co-led the study. “To resolve the problem our computational analysis compared the anatomy of Saccorhytus with all other living groups of animals, concluding a relationship with the arthropods and their kin, the group to which insects, crabs, and roundworms belong.”

Saccorhytus’ lack of anus is an intriguing feature of this microscopic, ancient organism. Although the question that springs to mind is the alternative route of digestive waste (out of the mouth, rather undesirably), this feature is important for a fundamental reason in evolutionary biology. How the anus arose – and sometimes subsequently disappeared – contributes to the understanding of how animal body plans evolved. Moving Saccorhytus from deuterosome to ecdysozoan means striking a disappearing anus off the deuterosome case history, and adding it to the ecdysozoan one.

“This is a really unexpected result because the arthropod group has 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,” says Shuhai Xiao from Virginia Tech, USA, who co-led the study. “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.”

Reference: “Saccorhytus is an early ecdysozoan and not the earliest deuterostome” by Yunhuan Liu, Emily Carlisle, Huaqiao Zhang, Ben Yang, Michael Steiner, Tiequan Shao, Baichuan Duan, Federica Marone, Shuhai Xiao and Philip C. J. Donoghue, 17 August 2022, Nature.
DOI: 10.1038/s41586-022-05107-z