Simple marine worm redraws evolutionary tree

Since its discovery just over 50 years ago, the marine worm Xenoturbella bocki, found on the beds of Swedish fjords, has posed a classification problem to biologists. This is because its name appears to be the most complex thing about it: the worm has no centralised nervous system, no organised gonads and, possibly, a combined mouth and anus. It was classed as a belonging to the mollusc family in 1997, after mollusc-like eggs were found in its gut.

In 2003, Drs Bourlat and Telford showed that this classification was wrong, proving that the marine worm had simply dined on molluscs. Now, by sequencing the worm's entire mitochondrial genome (which Dr Bourlat describes as "the powerhouse of cells") and studying the largest DNA datasets available to date, the researchers have shown that Xenoturbella deserves its own phylum, or branch, of the evolutionary tree.

This means that the humble marine worm is a cousin to humans and other vertebrates, closer to us in evolutionary terms than insects, and many other animals with more complex structures. It is also the simplest creature still alive that belongs to the 500 million year-old 'deuterostome' grouping.

"The finding is very surprising," says Dr Bourlat. "But using DNA rather than morphology (studying creatures' forms) to determine how animals evolved from their ancestors is taking us back to the drawing board all the time."

The result also throws up some interesting questions. The term 'deuterostome' - the evolutionary grandparents that humans and the marine worm share - literally means 'anus first, mouth second' in development terms. The marine worm, however, has only a mouth (though this may possess properties of an anus). It also lacks other physical characteristics that its ancestors must have had.

This means that the marine worm, like parasites, has shed its more sophisticated features over millions of years, because they were surplus to their lifestyle requirements. This conclusion serves as a reminder that evolution - successful adaptation to one's environment - does not always equal increasing complexity.

The research, published as a letter in 'Nature' on 18 October, also confirms that the sand-dwelling eel-like lancelet and other members of the cephalochordate group belong with humans, sea otters and other chordates rather than with the echinoderms, which count starfish among their members.

"The results have important consequences for our understanding of the evolution of several animals, including humans. Further progress in understanding our origins - and that of our animal cousins - is likely to come from the integration of palaeontological, comparative embryological and genomic approaches," said Dr Bourlat, who is planning further research into Xenoturbella.

To read the research findings in full, follow the link at the bottom of this article.

• Link:
• Letter in 'Nature'