When you think of our evolutionary ancestors, hairy ape-like beings are likely to come to mind. But a bag-like blob that is mostly mouth, has no anus and resembles a monster from 20,000 Leagues Under the Sea? Probably not. Yet, according to a team of researchers from multiple universities, just such a creature, traces of which were found in the fossil record, could very well be a common ancestor of both people and a large number of other species.

The creature in question is known as Saccorhytus coronarius and traces of it were found through highly detailed examination of microfossils found in China. And while it might look like something that would haunt your dreams, the bag-shaped animal was likely no bigger than a millimeter in size and lived among the grains of sand on the sea floor.

The creature is known as a deuterostome and it's far from the first one ever discovered. But it is a bit older. Most deuterostomes that we know of were from between 510 to 520 million years ago. Saccorhytus coronarius, though has been found to have lived earlier – about 540 million years ago – leading the researchers to believe that it could be the common ancestor to the other deuterostomes that evolved into vetebrates including starfish, sea urchins and fish and eventually – millions of years later – humans.

"We think that as an early deuterostome this may represent the primitive beginnings of a very diverse range of species, including ourselves," said Simon Conway Morris, professor of evolutionary palaeobiology and a Fellow of St John's College, University of Cambridge, said. "To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail is jaw-dropping. All deuterostomes had a common ancestor, and we think that is what we are looking at here."

Another researcher involved in the project, Dr Jian Han, of Northwest University, said that finding the fossils was a mighty task.

"We had to process enormous volumes of limestone – about three tonnes – to get to the fossils, but a steady stream of new finds allowed us to tackle some key questions: was this a very early echinoderm, or something even more primitive?," he asked. "The latter now seems to be the correct answer." Once the rock was processed, electron microscopes and CT scans were employed to create a picture of how the mini animal used to live.

One of the clues to the creature's future evolution into fish might be small conical structures that could have allowed Saccorhytus coronarius to swallow water and then escape, much like the gills on modern-day fish. Even more intriguing was the fact that the researchers failed to find anything like an anus represented in the fossil; in later deuterostomes, the mouth typically evolves into this feature. "If that was the case, then any waste material would simply have been taken out back through the mouth, which from our perspective sounds rather unappealing," Conway Morris said.

Saccorhytus coronarius also seems to have had bilateral symmetry, meaning it basically looked the same on both sides of a horizontal line drawn down its front, a trait we humans also share.

The researchers have reported their findings in the journal Nature.