Humans evolved from a worm
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Humans evolved from a two-inch long worm-like creature that wriggled in the sea more than 500 million years ago, scientists have learned.
The extinct Pikaia gracilens has been confirmed as the oldest known member of the chordate family, which includes all modern vertebrates including humans.
It gave rise to the panoply of vertebrate animals alive today - fish, amphibians, birds, reptiles and mammals.
When Pikaia was first discovered in 1911 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.
Another characteristic of chordates are myomeres, blocks of muscle tissue often arranged in a zig-zag pattern.
Pikaia had both features, according to the new analysis of 114 specimens 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.
"Now with myomeres, a nerve cord, a notocord and a vascular (blood vessel) 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."
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.
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The fossil beds have yielded a number of unusual marine organisms and provided valuable clues about the "Cambrian explosion". This was a period just over 500 million years ago when animals evolved into a vast array of different forms over a relatively short time.
Scientists used a range of imagery techniques, including cutting edge scanning electron microscopes, to reveal fine details in the Pikaia fossils.
Dr Jean-Bernard Caron, from the University of Toronto in Canada, 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.
"Fossils of primitive chordates are incredibly rare. With no backbones or other mineralised elements, Pikaia would stand no chance of preservation in normal conditions outside exceptional sites like the Burgess Shale.
"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."
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