Spectacular new fossils from China have pushed back the origins of vertebrates. Paleontologists report in tomorrow's issue of Nature that they have found 530-million-year-old fossils that have backbones and other vertebrate features and appear more advanced than the most primitive vertebrates alive today. That suggests fossils of even older vertebrates must be waiting to be discovered.
Evolution went on a creative spree about 540 million years ago. Over the course of less than 20 million years during the Early Cambrian period, a huge diversity of animals appeared for the first time, including many of the major groups living today, such as arthropods, mollusks, and various sorts of worms. Notably missing from this party--known as the Cambrian explosion--was any member of our own lineage, the vertebrates. Until now the oldest unambiguous vertebrate fossils dated back 475 million years.
Now two teams of paleontologists, one led by Degan Shu of Northwest University in Xian, have unearthed the vertebrate fossils from a site in southern China called Chengjiang, already famous for its Cambrian treasures. "I was absolutely amazed the first time I saw these fossils. They're just unbelievable," says Phillippe Janvier, a paleontologist at the Museum d'Histoire Naturelle in Paris who is an expert on early vertebrates.
This April, Simon Conway Morris, a paleontologist at the University of Cambridge, United Kingdom, traveled to China to analyze the centimeters-long fossils with Shu and other Chinese colleagues. The two fossils represented different species and bore some key vertebrate traits. They had rows of gills, and their muscles were arranged in W-shaped blocks along their flanks, a pattern unique to vertebrates.
Conway Morris and his colleagues concluded that the fossils fall into a surprisingly advanced position in the vertebrate family tree. One species, which the researchers named Haikouichthys, is most closely related to lampreys. The other fossil--tortuously named Myllokunmingia--is more primitive (its gills are simpler), but Conway Morris says it is still a closer relative to us than to the hagfish, the most primitive vertebrate.
If Conway Morris is right about the creatures' sophistication, millions of years of vertebrate evolution must have preceded them, reaching back before the Cambrian explosion. Some researchers already suspected as much, based on the clocklike divergence of genes in different animal lineages.