HUMANS have evolved from this two-inch worm, scientists claim. The extinct Pikaia gracilens lived in the sea more than 500million years ago. Now scientists have linked it to humans, saying that it is a primitive ancestor of animals with spinal cords.
It also gave rise to fish, birds, reptiles and other mammals.
Although Pikaia was first discovered in 1911, scientists assumed it was only related to leeches and earthworms.
But the spinal cord, paired with zig-zag patterened blocks of muscle tissue known as myomeres, relates it to humans.
Lead author Professor Simon Conway Morris, from Cambridge University, said: "The discovery of myomeres is the smoking gun that we have long been seeking.
"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."
Using cutting-edge microscopes and imagery techniques, scientists revealed fine details in the Pikaia fossils.
Every specimen of Pikaia discovered so far has come from the Burgess Shale fossil beds in Canada's Yoho National Park.
It is thought to have swum above the sea floor by bending its body from side to side.
Dr Jean-Bernard Caron, from the University of Toronto in Canada, took part in the research.
He 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."