PALEONTOLOGY:

Fossils Give Glimpse of Old Mother Lamprey

Carl Zimmer*

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. But this week our genealogy took a giant leap back in time. Chinese and British paleontologists reported in Nature that they have found the fossils of 530-million-year-old vertebrates--fossils that have other paleontologists in awe. "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.

You might expect that such ancient creatures would be primitive, transitional forms linking us to our pre-vertebrate past. Yet surprisingly, the fossils are actually full-fledged vertebrates--more advanced, in fact, than some vertebrates alive today. As a result, paleontologists think fossils of even older vertebrates must be waiting to be discovered, perhaps in rocks dating from well before the Cambrian explosion.

The two fossils come from a site in southern China called Chengjiang, already famous for its Cambrian treasures, where the fine-grained rock retains impressions of muscles and other soft tissues. "Chengjiang really takes your breath away," says Simon Conway Morris, a paleontologist at the University of Cambridge. After learning that two different teams of paleontologists, one led by Degan Shu of Northwest University in Xian, had unearthed the vertebrate fossils, Conway Morris traveled to China this April to analyze them with Shu and other Chinese colleagues.

They found that the two fossils represented different species, and although the fossils measured only a couple of centimeters long, the researchers could recognize 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. "They were presumably filter feeders, but they have these muscular bodies and things which we cautiously interpret as an eye," says Conway Morris. "And so presumably they could go along at a fair pace if they had to, and they might have grabbed prey."

The researchers then tried to find a place for the fossils in vertebrate evolution. A number of researchers believe that vertebrates evolved from an ancestor something like Amphioxus, otherwise known as the lancelet. Amphioxus, which lacks eyes or fins and looks rather like a miniature anchovy fillet, has a notochord--a primitive backbone. The first vertebrates added new traits to that body plan, such as a skull with a brain; later vertebrates acquired jaws and fins. The most primitive vertebrate alive today is the hagfish, a jawless fish, and the second-most primitive is the lamprey.

Conway Morris and his colleagues concluded that the fossils fall into a surprisingly advanced position. One of the species, which the researchers named Haikouichthys, is most closely related to the lamprey. 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.

Features seen on both fossils may help answer the controversial question of how early vertebrates evolved the paired fins that later gave rise to arms and legs (Science, 23 April, p. 575). The new fossils show what look like two long folds of tissue running along their underside--exactly what some theories of fin evolution predicted. "We think there's a reasonable case for a double arrangement," says Conway Morris.

Janvier, who has argued that the paired fins came much later, has his doubts. "From what I could see of the fossils, it's not 100% certain." He is also uncertain about the fossils' placement on the vertebrate family tree, because many details of the creatures' anatomy have been lost. He has no doubt that they are vertebrates, but says, "I wouldn't put my money on the exact positions."

If Conway Morris is right about the creatures' sophistication, however, 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. According to a new study by Blair Hedges of Pennsylvania State University in University Park, for example, vertebrates got their start 750 million years ago. "Some of my colleagues who take molecular clocks seriously will be skipping for joy" over the new finds, Conway Morris acknowledges ruefully.

He himself doesn't think vertebrates got their start so long ago. He suspects the first ones arose just before the Cambrian Period, about 565 million years ago. The traces of these ancestral creatures, he thinks, may be waiting, still unrecognized, among the fossils known as the Ediacaran fauna. "These stem groups are all lurking down there," Conway Morris maintains, "but we're just too dim to see them."

Carl Zimmer is the author of the book At the Water's Edge.