Cambridge scientists have recently discovered one of mankind's closest invertebrate relatives - a rare 3cm worm that resides in mud at the bottom of a Swedish fjord.

The research, published in Nature this week, is the first conclusive proof that humans and the Xenoturbella worm, whose Latin name means strange flatworm, derive from a common ancestor, thereby placing Xenoturbella in the same division of the animal kingdom as man.

With funding from the Wellcome Trust, a team led Dr Max Telford, from the Department of Zoology, investigated the genetic make-up of Xenoturbella. Telford's colleague, Dr Sarah Bourlat, was able to purify Xenoturbella's DNA to shed new light on the origins of this much-neglected species.

The confusion over the origins of Xenoturbella occurred when previous studies examined DNA that had been contaminated by the food in its gut. This food comprised molluscs and when Xenoturbella was ground-up for study the food in its gut was also included, therefore contaminating its DNA and leaving scientists with the wrong impression that it was a mollusc.

"Up until now Xenoturbella had been thought to be related to the bivalve molluscs - which include mussels and oysters," said Dr Telford. "We found this hard to believe as it looks nothing like a bivalve mollusc. 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 thought to be divided into three major groups of species: the first containing animals which have a moulted cuticle such as nematodes and insects (ecdysozoa); the second containing molluscs and earthworms (trochozoa); and the last containing vertebrates, such as fish, amphibians birds and mammals, as well as our distant relatives the starfish and the little known acorn worms (deuterostome). It is this latter division into which Xenoturbella falls.

"Xenoturbella has joined a pretty exclusive group of species," said Dr Telford. "Alongside vertebrates such as humans, there are only two other groups within this division: Xenoturbella is a significant addition."

This research may well assist scientists in their studies of vertebrate evolution. Further research will hopefully identify similarities between vertebrates and Xenoturbella and so provide data on the characteristics of our common ancestor, which is thought to have lived half a billion years ago. "One obvious next step for scientists wishing to pursue this line of research is to study the embryonic development of Xenoturbella," said Dr Telford. "The challenge now is to find these embryos because nobody knows how Xenoturbella mates!"

Photo: Graham Budd

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