Original Kin: Six-legged bugs may have evolved twice

Six legs, new; more legs, old. That could be an adage for biologists who hold that all six-legged terrestrial bugs evolved from a single relatively recent branch of the ancient lineage of arthropods. Earlier, they say, that tree had sprouted branches of crustaceans, spiders, millipedes, and other organisms with more legs.

New genetic data may invalidate that view. In the scenario that these findings support, insects and other six-legged arthropods parted evolutionary paths before insects split off from crustaceans. This would mean that six-leggedness arose twice–once within the lineage of insects and once on a lower branch of the tree. The new theory intrigues arthropod biologists, but many are skeptical.

The matter focuses on an order of minuscule hexapods called collembolans, or springtails, which includes thousands of species. Springtails turn up in nearly every habitat on Earth. Like true insects, adult springtails have hard, segmented bodies and six legs, but they display some unique biological traits that set them apart.

Springtails’ proper place among arthropods has long vexed scientists, says Nipam H. Patel of the University of Chicago, who studies arthropod development.

To place springtails on the arthropod family tree, molecular biologist Francesco Nardi of the University of Siena in Italy and his colleagues analyzed the genetics of cell structures called mitochondria in two springtail species and an insect.

For 27 other arthropods, they examined corresponding information from a scientific database.

Previous studies had suggested that insects and springtails lie close together on the evolutionary tree of arthropods and come from a common six-legged ancestor.

Crustaceans are less closely related to insects than springtails are, in this view.

However, the new tree described in the March 21 Science shows springtails arising before insects and crustaceans split. So, despite the resemblance of the six-legged arthropods, insects are more closely related to crustaceans than to springtails, it implies.

If the new tree is accurate, insects and springtails went along their separate evolutionary ways when their ancestors were still aquatic and had more than six legs. That means that “today’s terrestrial hexapods are products of at least two independent invasions of land,” Richard H. Thomas of the Natural History Museum in London says in the same issue of Science. Six-leggedness and some other features that insects and springtails share would reflect the convergence of two independent evolutionary paths toward the same anatomical form, he says.

To rigorously test that scenario, scientists will need to examine more than just mitochondrial genes, says Gonzalo Giribet of Harvard University. They’ll also need to look at four other orders of hexapods that, like springtails, aren’t true insects.

Most comparative studies of arthropods’ nonmitochondrial genes, anatomy, and developmental biology don’t support the newly proposed tree, says Michael F. Whiting of Brigham Young University in Provo, Utah. Whiting, Giribet, and other researchers who spoke with Science News suggest that deceptive information contained in some mitochondrial genes may have led Nardi and his colleagues astray.

Nevertheless, says Whiting, “they could be correct. It’s a very interesting hypothesis.”

The new tree is a plausible alternative to existing ones, says Patel. With the arthropods’ evolutionary tree still in flux, however, “there are plenty of changes to be seen,” he predicts.

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