The origins of humans and other vertebrates have been traced to a worm that swam in the oceans half a billion years ago.
A new analysis of fossils unearthed in the Canadian Rockies determined that the extinct Pikaia gracilens is the most primitive known member of the chordate family, which today includes fish, amphibians, birds, reptiles and mammals.
The research, published in the British scientific journal Biological Reviews this week, identified a notochord (or rod) that would become part of the backbone in vertebrates, and skeletal muscle tissue called myomeres in 114 fossil specimens of the creature. They also found a vascular system.
"The discovery of myomeres is the smoking gun that we have long been seeking," said the study's lead author, Simon Conway Morris of the Cambridge University in England. "Now with myomeres, a nerve chord, a notochord and a vascular system all identified, this study clearly places Pikaia as the planet's most primitive chordate. So, next time we put the family photograph on the mantle-piece, there in the background will be Pikaia."
Humans' humbling origin
The first specimens of Pikaia were collected by early explorers of the Burgess Shale in 1911. But the animals were overlooked as an ancestor of earthworms or eels.
It was not until the 1970s that Morris suggested the 5-cm-long, sideways-flattened, somewhat eel-like animal that likely swam by moving its body in a series of side-to-side curves could be the earliest known member of the chordate family.
"In particular, it was our use of an electron microscope that allowed us to see very fine details of its anatomy," said co-author Jean-Bernard Caron, an assistant professor of ecology and evolutionary biology at the University of Toronto in Canada. "It's very humbling to know that swans, snakes, bears, zebras and, incredibly, humans all share a deep history with this tiny creature no longer than my thumb."