Velvet Worm Creeps Back up Evolutionary Tree

27 July 2006 (All day)

Lynn Kimsey / Bohart Museum of Entomology / University of California Davis

Not so ancient after all?
A comparison of their brain anatomy suggests that velvet worms are ancestors of crabs and spiders rather than the missing link of all arthropods.

Crawling about like a silky crimson caterpillar and capturing prey by spitting goo, the velvet worm hardly seems like an evolutionary milestone. But for decades, many scientists have pegged it as such, claiming it as the only surviving example of a group of invertebrates that gave rise to the majority of today's animal species. Now, a new analysis of the velvet worm's brain suggests this "living fossil" may not be so ancient after all.

DNA evidence suggests velvet worms are closely related to crabs and spiders, possibly as a very early member of the group that gave rise to both. But fossil analysis seems to push the worm's origins much farther back, relating it to a look-alike in 540-million-year-old rocks.

DNA and fossils aren't the only evidence for teasing apart evolutionary relationships, however. While a lot of body parts can look wildly different between even closely related arthropods, the brains change little between close relatives. Like a fingerprint for each taxonomic group, the arachnids, insects, and other arthropods have their own characteristic arrangement of brain structures.

To see if the velvet worm's brain layout is unique, a team led by Nicholas Strausfeld, a neurobiologist at the University of Arizona in Tucson, compared 118 brain features between velvet worms and 26 arthropod groups, from horseshoe crabs to bees. They then produced a family tree based on this data to see how it matched up with the known relationships between these species.

Just like comparisons based on DNA sequence, the neuroanatomical family tree pegs velvet worms as distant relatives of crabs and spiders, but that's about as far back as they go. The worms aren't old enough to have predated all arthropods, the team reports 7 August in Proceedings of the Royal Society B.

Not everyone is convinced. While the study "provides excellent data on [velvet worm] brain anatomy," says Georg Mayer, a zoologist at the Free University of Berlin, "the interpretation of these similarities seems premature." Rather than considering the neuroanatomical data alone, Mayer would like to see a family tree based on all the known data before deciding.

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