About 540 million years ago, our ancestors were insignificant creatures no more than a millimeter in size. They wriggled around in the sediments of shallow seas, gulped prey into their minuscule, baglike bodies and expelled the water through cone-shaped spouts around their mouths.
Animals this small do not fossilize well, which is why this stage of the distant evolutionary past is so little known. A cache of 45 individuals has now been unearthed in Shaanxi Province, in central China. They are described in the Monday issue of the journal Nature by a team led by Jian Han of Northwest University in Xi’an, China.
The creatures are the oldest known members of an ancient group called deuterostomes, said Simon Conway Morris, a paleontologist at Cambridge University in England and a member of the team. Deuterostomes, which lie pretty close to the base of the family tree of all animals, are ancestral not just to humans but to a wide array of animals ranging from sea urchins and starfish to the vast family of vertebrates.
The deuterostomes, a name that means “mouth second” in Greek, were so called by anatomists to distinguish them from the protostomes, or “mouth firsters,” the other members of a vast group of animals with bilateral symmetry. In the early embryo — a sphere of cells formed shortly after the egg is fertilized — the protostomes form the mouth first, anus second. The deuterostomes do it the other way round.
But strangely, the new deuterostome fossils seem to have no anus, presumably using the mouth for evacuation. At least the researchers cannot see one. “Our material is generally crushed, and despite the superb preservation we might have overlooked the evidence,” Dr. Conway Morris said.
The fossils were found in rock strata, stated to be some 540 million years old in a news release issued by St. John’s College, Cambridge, of which Dr. Conway Morris is a fellow. The release describes the fossils as “the earliest known prehistoric ancestor of humans,” referring to their status as the earliest known deuterostomes. Although slightly older animal fossils are known, they lie on branches of the tree of life that do not lead to humans.