August 19, 2022
Victoria Gill BBC Science Correspondent
Scientists claim to have solved an evolutionary mystery involving a 500 million year old microscopic spiny creature with a mouth but no anus.
When discovered in 2017, it was claimed that this tiny fossil of a bag-like sea animal may be the earliest known ancestor of humans.
Saccorhyntys coronarius, as this ancestral creature is called, has been tentatively classified in the group of deuterostomes, which are characterized by the formation of their anus before the mouth during embryonic development.
They are the primitive ancestors of vertebrates, including man.
Now, a new study suggests that Saccorhytus should be classified in a completely different group of animals.
A team of researchers in China and the UK carried out a very detailed X-ray analysis of the creature and concluded that it belongs to a group called ecdysozoa, which are the ancestors of spiders and insects.
One of the origins of this evolutionary confusion is the absence of an anus in this animal.
Emily Carlisle, a researcher who has studied Saccorhytus in detail, explained this on BBC Radio 4's Inside Science program.
"It's a little confusing, (most) ecdysozoa have an anus, so why not this one?".
An "intriguing option" to explain this, he says, would be that an ancestor even older than this whole group had no anus and that Saccorhytus evolved after that.
"It's possible that it lost it during its own evolution, that it didn't need it, or that it was content with a fixed place with one opening for everything", says- he.
However, the main reason for the "repositioning" of Saccorhytus on the Cambrian tree of life - the division of the geologic time scale which belongs to the Paleozoic era - is that, upon first examination, the holes around its mouth have been interpreted as gill pores, a primitive feature of deuterostomes.
When scientists took a closer look, using powerful X-rays to examine the creature from a millimeter close, they realized it was actually the base of the spines that had snapped off.
Scientists studying these fossils attempt to place each animal on a tree of life, much like a family tree, which allows them to build the picture to understand where they came from and how they evolved.
"Saccorhytus would have lived in the oceans, in the sediments, with its spines holding it in place," says Carlisle, who works at the University of Bristol in the UK.
"We think it just sat there (where it was found), in a very strange environment with a lot of animals that would look like some living creatures there, but a lot of totally weird ones."
The rocks containing these Cambrian fossils are still being studied.
"We still have a lot to learn about their environment," Carlisle added.
"The more I study paleontology, the more I realize how much is missing. When it comes to this creature and the world it lived in, we are only scratching the surface."