Researchers have discovered the earliest known ancestor of humans - along with a vast range of other species.
They said that fossilised traces of the 540-million-year-old creature were "exquisitely well preserved".
The microscopic sea animal is the earliest known step on the evolutionary path that led to fish and - eventually - to humans.
Details of the discovery from central China appear in Nature journal.
The research team says that Saccorhytus is the most primitive example of a category of animals called "deuterostomes" which are common ancestors of a broad range of species, including vertebrates (backboned animals).
Saccorhytus was about 1mm in size, and is thought to have lived between grains of sand on the sea bed.
The researchers were unable to find any evidence that the animal had an anus.
"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," research team member and University of Cambridge professor Simon Conway Morris said.
The study was carried out by an international team of researchers, from the UK, China and Germany.
"To the naked eye, the fossils we studied look like tiny black grains, but under the microscope the level of detail was jaw-dropping," Dr Conway Morris said.
"We think that as an early deuterostome this may represent the primitive beginnings of a very diverse range of species, including ourselves. All deuterostomes had a common ancestor, and we think that is what we are looking at here."
Degan Shu, from Northwest University in Xi'An, Shaanxi Province, where the fossils were found, said: "Saccorhytus now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us."
Until now, the deuterostome groups discovered were from between 510 to 520 million years ago. These had already begun to diversify into vertebrates, the group to which we and our ancestors belong, as well as invertibrates such as starfish and sea urchins.
Because they looked so different from one another, it was difficult for the scientists to determine what an earlier, common ancestor might have looked like.
The study suggests the animal's body was symmetrical, a characteristic inherited by many of its evolutionary descendants including humans.
Saccorhytus was also covered with a thin, relatively flexible skin and muscles, leading the researchers to conclude that it moved by contracting its muscles and got around by wriggling.
The researchers said its most striking feature was its large mouth relative to the rest of its body. They said it probably ate by engulfing food particles or even other creatures.
Also interesting are the conical structures on its body. These, the scientists suggested, might have allowed the water it swallowed to escape and so might have been a very early version of gills.