Humans' earliest known relative was likely an egg-shaped creature that ate and expelled from the same gaping orifice some 540 million years ago, scientists reported Monday.
Startlingly well-preserved fossils of the tiny beast, dubbed Saccorhytus, were discovered in central China's Shaanxi province, they reported in the journal Nature.
Several major branches of evolution -- one of them eventually leading to humans -- began from this inconspicuous, sea-dwelling organism, they speculated.
"This may represent the primitive beginnings of a very diverse range of species, including ourselves," said co-author Simon Conway Morris, a professor at Britain's University of Cambridge.
Saccorhytus belongs to a broad category of organisms called deuterostomes, and is the most ancient specimen unearthed so far.
Indeed, all deuterostomes - vertebrates (animals with backbones), echinoderms (starfish and sea urchins) and other distinct groups - are thought to have derived from this common ancestor, the study concluded.
To the naked eye, the fossils look like black grains of sand.
"But under the microscope, the level of detail is jaw dropping," Morris said.
The sack-like animal's most distinctive feature is a large -- relative to the rest of its body -- mouth ringed by concentric circles of raised bumps.
It probably ate by engulfing food particles and microscopic creatures.
- No eyes -
Intriguingly, the researchers did not find anything corresponding to an anus, leading them to conclude that waste was expelled through the same hole.
The tiny beast also featured eight cone-like structures on its body that may have allowed the water it swallowed to escape - probably "precursors to gill slits," Morris told AFP.
"But we have no evidence for eyes."
The researchers also suspect Saccorhytus had thin, flexible skin, along with a primitive musculature that allowed it to move around by wriggling.
Finding the creatures was not easy.
"We had to process enormous volumes of limestone - about three tonnes - to get to the fossils," said lead author Jian Han, a professor at Northwestern University in the city of Xian who made the discovery.
Once isolated, the samples were analysed with an electron microscope and a CT scan, allowing the team to build up an image of how the animal looked and lived.
The fossils date from the beginning of the 53-million year Cambrian period, which witnessed a dramatic burst of evolution and biological diversity known as the "Cambrian Explosion".
The period -- during which all life existed in the oceans -- ended with the first of five major extinction events over the next half billion years.
Scientists say that Earth is now experiencing a sixth mass die-off, caused by human impacts such as climate change.