The 2in-long worm with a place on the mantelpiece

The extinct Pikaia gracilens has been confirmed as the oldest known member of the chordate family, which includes all modern vertebrates including humans.

These gave rise to the panoply of vertebrate animals alive today – fish, amphibians, birds, reptiles and mammals.

When Pikaia was first discovered in 191, scientists assumed it was related to leeches and earthworms. But a new study has now confirmed what many experts suspected, that the creature was a primitive ancestor of animals with spinal cords.

At some point in their development from embryo to adult, all chordates have a notocord, a flexible rod supporting a nerve cord running down the back. The notocord becomes part of the backbone in more evolved vertebrates.

The new analysis is published in the journal Biological Reviews.

Lead author Professor Simon Conway Morris, from Cambridge University, said: “The discovery of myomeres is the smoking gun that we have long been seeking.

“So, next time we put the family photograph on the mantel-piece, there in the background will be Pikaia.”

Pikaia had a sideways- flattened body divided into a series of segmented muscle blocks that lay on either side of its notocord. It is thought to have swum above the sea floor by bending its body from side to side.

Every specimen of Pikaia discovered so far has come from the Burgess Shale fossil beds in Canada’s Yoho National Park, which date back 505 million years.

Dr Jean-Bernard Caron, from the University of Toronto, who took part in the research, said: “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.

“We hope that, with continuing explorations and field work studies there, other species will be discovered allowing us to refine our understanding of the early history of our own group.”