Our oldest known living ancestor was called a deuterostome, which gave rise to a huge swathe of life including present-day starfish and sea squirts, has been discovered fossilised in 540-million-year-old limestone in central China.

The find is the earliest known deuterostome, according to a paper published in the journal Nature.

"Deuterostomes gave rise to very diverse animals, from ourselves to starfish and sea urchins. That degree of diversity begs the question: What did the common ancestor look like?" study author Simon Conway Morris of the University of Cambridge told IBTimes UK.

The answer is: quite bizarre. It is tiny, measuring about 1.3mm long, 0.8mm wide and 0.9mm high, and had mouth very large compared with the rest of its body, measuring about 0.3-0.5mm across. It also had four conical holes in either side.

"These are a hallmark of deuterostomes, and the precursors of gills in fish. Water passes through the gills and expelled from the side of the animals," Conway Morris said.

This was the primary way the animal got rid of waste, as the researchers have not been able to identify an anus in any of the 45 specimens of the early deuterostome species they found.

Finding such a primitive deuterostome provides a step in the fossil record between very small, simple organisms – so small that they gained their oxygen by diffusion – to more complex animals such as fish that breathe through gills. "It's part of the answer to how very complex things evolve," said Conway Morris.

The 45 specimens of this previously unknown species were found in ancient limestone rock in Shaanxi Province, China. Researchers had to sift through about three tonnes of rock to find and identify the tiny fossils.

This is what the earliest known human ancestor looks like. Jian Han, Northwest University, China

Primitive deuterostomes have been identified before, but none as early as this, which lived about 540 million years ago in the early years of the Cambrian explosion. The organisms were fossilised in a process called phosphatic fossilisation, in which phosphate molecules quickly replace parts of the organism before it can be broken down.

"The level of the fidelity of preservation of the original tissues is jaw dropping, it's absolutely astonishing. It gives us confidence that we can really see the original details of the deuterostome."

It's not clear for how long this species lived, Conway Morris said. "But my guess is that the species was probably fairly short-lived. Here they are, they appear from nowhere, disappear to nowhere. But that's normal in the fossil record."