Earliest Known Relative

Saccorhytus coronarius, a newly-discovered fossilized sea creature (shown at left, with an artist’s rendering of it at right), may be the earliest known ancestor of humans.

Jian Han

During the Cambrian Period (543-490 million years ago), life on Earth rapidly diversified, and the ancestors of many current animal lineages appeared. Some of these organisms—both their soft bodies and hard shells—are well-preserved in such fossilrich areas as the Lower Cambrian strata in the mountainous area of Shaanxi Province, in central China. It is here that scientists may have found humankind’s oldest known relative.

Paleobiologist Simon Conway Morris of the University of Cambridge and colleagues from Germany and China have discovered several microfossils of a possibly deuterostome organism that pre-dates all other known deuterostomes–a group of animals that includes numerous modern life forms, such as vertebrates and echinoderms. The researchers first dissolved tons of limestone surrounding the fossils in acetic acid, laboriously sifted the product, and finally examined the fossils using an electron microscope.

They named the 540-millionyear-old organism Saccorhytus because its millimeter-sized body is shaped like a small, wrinkled sac. It has a large mouth ringed with teeth-like bumps, and four volcano-shaped body cones on each side. The scientists were not able to find evidence for an anus, which sets this creature apart from other deuterostomes and makes their classification somewhat controversial.

However, Conway Morris says that deuterostomes might be related to another group of organisms that have no anus: the acoel flatworms. Perhaps Saccorhytus shared this trait with its flatworm cousins. Conway Morris also suggests that the anus may be absent because Saccorhytus is so tiny, obviating the need for an elongate digestive system. The body cones might also have played a role in passing waste, releasing water taken in from the mouth. These cones may represent an evolutionary step toward gills in fish.

While Saccorhytus seems to be the most primitive deuterostome discovered to date, the researchers have collected an abundance of material that remains to be examined. Combined with the Cambrian strata around the world that have yet to be explored, “It’s fair to say the surface has just been scratched,” says Conway Morris. (Nature)

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