This Prehistoric Human Ancestor Was All Mouth
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.
An artist’s impression of Saccorhytus, the oldest known members of an ancient group called deuterostomes.
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.
这种生物是已知最古老的一种后口动物，该团队成员、剑桥大学古生物学家西蒙·康威·莫里斯(Simon Conway Morris)说，在所有动物的生命树中，后口动物距离树的底部非常近，它们不仅是人类的祖先，也是从海胆、海星到脊椎动物大家庭等大批动物的祖先。
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.
这些化石是在岩层中发现的。据康威·莫里斯博士所任职的剑桥大学圣约翰学院(St. John's College)发布的新闻稿称，它们有大约5.4亿年的历史。新闻稿称这些化石是“人类已知的最早的史前祖先”。这里指的是它们作为已知的最早的后口动物。尽管已知的动物化石中有历史略微久远一些的，但在生命之树上，它们所处的分枝和人类的进化无关。