Sac with a mouth and no anus wasn't our earliest ancestor after all

Carissa Wong is a person.

Our earliest ancestors might not have been a sac with a big mouth. The creature, which lived around 530 million years ago during the early Cambrian period, was thought to be an early descendant of crabs and spiders.

Fossils from the Kuanchuanpu Formation in South China show a sac-like animal with a mouth that was half a millimeter wide. It is believed to have been on the sea floor.

Philip Donoghue is a professor at the University of Bristol in the UK.

A group of people, led by Simon Morris at the University of Cambridge, analysed 45 poorly preserved S. coronarius specimen and suggested that a set of small openings around its mouth were the beginning of gills in fish. They proposed that the creature was a common ancestor of deuterostomes, a broad group in the animal kingdom to which humans belong, making it our earliest known ancestor and that of all other animals.

There is evidence that this isn't the case. The team was able to reveal the fossils of S. coronarius by dissolving hundreds of kilograms of rock from the same site in South China.

By using a high-resolution technique called synchrotron tomography, the team found three pointed spikes that were not seen in the previous fossils.

Donoghue says that there was a layer of tissue preserved that extended up through the holes. The lack of openings in the animal's body makes it difficult to interpret it as a deuterostome.

The ecdysozoans include insects, crustaceans and roundworms. It's not clear what it would have eaten, but the spikes probably aided it in catching prey.

Penis worms have the same arrangement of spines and they use them as part of capturing prey. We don't know what it ate because of the spines.

The findings show that there is a gap in the fossil record that can't be filled before the appearance of a large diversity of animals. The common ancestors of deuterostomes are so small that they are not visible in the fossil record. Donoghue thinks that S. coronarius would have supported that idea.

When contacted by New Scientist, Morris refused to comment.

The study is comprehensive, rigorous and compelling. There is no longer a strong case for interpreting S. coronarius as an early deuterostome.

He says the hunt for other early deuterostomes is still going on. Some of the fossils from the Cambrian period may find a home in the deuterostome tree. We will be able to piece together the earliest steps in deuterostome evolution based on fossils.

There is a journal reference called Nature.

Wild Wild Life is a monthly newsletter that celebrates the diversity and science of animals and plants.