Meet mankind's newest relative: a mudworm in a Swedish fjord

At just over an inch long, the mudworm may not look like much but scientists have discovered it has the same evolutionary lineage as humans.

At just over an inch long, the mudworm may not look like much but scientists have discovered it has the same evolutionary lineage as humans.

A DNA study has revealed that a worm found living at the bottom of a Swedish fjord shared an ancestor with Homo sapiens 500 million years ago. In fact, the creature, known by its Latin name Xenoturbella bocki,is more closely related to human beings than shellfish.

Xenoturbella, or "strange flatworm", is not immediately recognisable as a relative. It has a primitive body plan with no proper sex organs, gut, excretory system or brain. Scientists previously thought it belonged to the same group as bivalve molluscs because it carried mollusc eggs, but in fact it had eaten them.

Max Telford, from Cambridge University, who led the British and Dutch team, said: "We have now been able to show that, amongst all of the invertebrates that exist, Xenoturbella is one of our very closest relatives. It is fascinating to think that whatever long-dead animal this simple worm evolved from, so did we."

The animal kingdom is divided into three main groups of species: ecdysozoa, animals with a sheddable outer layer; trochozoa, comprising molluscs and earthworms; and deuterostomes, including vertebrates, such as humans, and starfish, sea urchins and certain marine worms.

The research, published in the journal Nature, categorises the mudworm as a deuterostome. "Xenoturbella has joined a pretty exclusive group of species," Dr Telford said.

Scientists hope the worm will give them a better understanding of vertebrate evolution. It may help researchers to characterise the common ancestor, thought to have lived 500 million years ago.