An enigmatic worm that resides in the mud at the bottom of a Swedish fjord is today identified as mankind’s long-lost relative, after being written off for years as an obscure mollusc.

Scientists have discovered one of mankind’s closest relatives, among those that lack a backbone, in the shape of a rare 3-cm worm called Xenoturbella, they reported in the journal Nature on Thursday.

The research offers proof that humans and Xenoturbella, whose Latin name means strange flatworm, derive from a common ancestor that lived half a billion years ago, thereby placing Xenoturbella in the same division of the animal kingdom as man and offering new insights into how we evolved.

With funding from the Wellcome Trust, a team led by Max Telford of Cambridge University made the discovery by investigating the genetic make-up of Xenoturbella. “We have now been able to show that among all of the invertebrates that exist, Xenoturbella is one of our very closest relatives,” he said.