Blob-like fossils dating back about 600 million years may indicate that complex life evolved much earlier on our planet than had been thought, scientists say.
The animals may even have possessed sensory organs
The animals are less than a fifth of a millimetre long and have a two-sided body plan previously thought to have existed much later in Earth's history.
These "bilaterians" have what look like mouths and guts, as well as internal and external layers of body tissue.
The findings are reported by a US-Chinese team in Science magazine.
If the analysis of the fossils by Jun-Yuan Chen and colleagues is accepted by the scientific community, the specimens would represent the most ancient evidence yet of animals complex enough to have the symmetrical two-sided body plan found in modern creatures, such as humans.
The animals - their scientific name is given as Vernanimalcula guizhouena - even have what look like pits in their outer surface that might have contained sensory organs.
They were discovered in rocks from the Doushantuo Formation, dated to a time researchers now refer to as the Ediacaran Period.
This was a period when scientists think our planet was emerging from a super-glacial event - the so-called "Snowball Earth" which saw giant ice caps stretch almost to the equator.
And it is at least 50 million years before the Cambrian Period, when the fossil record shows there was an explosion of life - a multiplicity of different shapes and sizes.
But the age of V. guizhouena suggests the roots of this complexity are much older than had been thought and the Cambrian explosion may not have been quite the sudden burst scientists had believed.
Professor Chen, from Nanjing University, and his team write in Science: "The organisation of these fossils, taken together with their provenance, indicates that the genetic toolkit and pattern formation mechanisms required for bilaterian development had already evolved by Doushantuo times, long before the Cambrian."
However some scientists are sceptical of the claims, suspecting that what have been characterised as fossils may in fact be natural mineral formations.
Stefan Bengtson, from the Swedish Museum of Natural History in Stockholm, said: "These may well have started out as fossils, but we can't say much about their morphology."
He told Science magazine the presumed tissue layers may be just thin, banded mineral crusts.
Traces of simple bacteria-like organisms in the fossil record date back more than three billion years.