As the precursor of vertebrates the species is also believed to be the direct ancestor of all members of the chordate family, which includes fish, birds, reptiles, amphibians and mammals.
The finding means the 5cm long creatures, known as Pikaia gracilens, were the forerunners of animals as diverse as snakes, swans and humans, scientists said.
The fossils, preserved in shale beds in Canada, were first found 100 years ago by American paleontologist Charles Doolittle Walcott, who suggested they could have been an early type of leech or worm.
Scientists had since speculated that the creatures could have been chordates because they appeared to have a simple form of notochord, a flexible rod which makes up part of the backbone in vertebrates.
But because they lacked a fully developed backbone, there remained doubts about exactly which category of species the fossilised animals should fall into.
Now an analysis of 114 specimens by Cambridge University scientists and Canadian colleagues has identified lumps of skeletal muscle tissue known as myomeres, which appear to lay any doubts to rest.
Simon Conway Morris, lead author of the study published in the Biological Reviews journal, said: “The discovery of myomeres is the smoking gun that we have long been seeking.
“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."
Jean-Bernard Caron, a co-author on the paper, added: "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."
Read more at The Telegraph
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