Meet your earliest known ancestor: Saccorhytus

Newly discovered fossils of a tiny sea creature are the earliest known step in humans’ evolutionary history.

Time to update your family tree. Scientists have found a 535-million-year-old fossil belonging to a group from which all vertebrates—including humans—descended. Named Saccorhytus, the animal was about a millimeter in size and bilaterally symmetrical, a trait shared by many vertebrates today. The discovery marks the earliest known step in the evolution of humans. We speak with two of the study’s authors, Jian Han of Northwest University in Xi’an, China and Simon Conway Morris, a palaeobiologist at the University of Cambridge, to learn more.

Photograph of the Saccorhytus fossil and an artists' impression of the creature as it would have looked when it was alive. Credit: Jian Han (Northwest University, China)

ResearchGate: Where and how did you discover this fossil?

Jian Han: The specimen, which lived about 535 million years ago in the Early Cambrian Period, was found in China, in Shaanxi Province. The rocks around it were dissolved by acetic acid, and the surviving fossils were manually picked up under a microscope.

RG: What did it look like?

Simon Conway Morris: More or less globular to hemispherical. It had a prominent mouth and a series of openings on either side of the body. These are marked externally by so-called body cones. Additional structures include sets of pores, some of which are possibly sensory. There’s no evidence for eyes.

RG: What do you know about what its life was like?

Conway Morris: It would have lived in marine sediments, in the equivalent of a carbonate sand. The location where the fossil was found would have been tropical, shallow water in the in the Early Cambrian Period. The animal would have wriggled between the sediment grains, although short-term attachment to them was also likely. We also suggest that the mouth could expand quite significantly and potentially ingest relatively large prey for its size.

RG: How is Saccorhytus related to humans?

Conway Morris: Humans are vertebrates and so are one of the major groups that collectively define the deuterostomes, a group that also includes animals that are quite different from humans, such as sea-urchins and sea-squirts, all ultimately descended from an original deuterostome. Our argument is that Saccorhytus is close to this ancestral form, and so is the most primitive known deuterostome.

RG: Will you now start looking for even earlier ancestors? What do you expect they might be like?

Han: It is very difficult to find even earlier fossils of deuterosotmes in older rocks. But in current fossils, we expect to find new specimens and recognize more primitive conditions.

Conway Morris: There are suggestions based on molecular data that fairly close relatives of the deuterostomes can be found amongst the so-called acoel worms. So, if this hypothesis is correct, we'd be looking for some sort of worm-like ancestor.