THE idea that all six-legged animals are insects has been turned on its head, along with the theory that all such creatures evolved from a common ancestor. Flightless springtails are actually more closely related to crustaceans such as the hermit crab and brine shrimp than to insects such as locusts.
An analysis of mitochondrial DNA from different members of the phylum Arthropoda implies that springtails, known as Collembola, and insects both evolved similar features, but in fact crawled out of the water separately, hundreds of millions of years ago, rather than stemming from a more recent common ancestor.
Insects, springtails and crustaceans are classified as arthropods, a phylum that also includes many-legged myriapods, and the Chelicerata 鈥� spiders, mites, scorpions and horseshoe crabs. Relationships between the arthropod groups are uncertain, because their highly specialised features make it hard to deduce links from anatomy alone.
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A few experts have recently questioned whether all six-legged 鈥渉exapods鈥� should be classified as true insects. But, the standard view is that wingless springtails and insects form one hexapod group, sharing a common ancestor, and that their closest relatives are the myriapods. Insects are supposedly only distantly related to crustaceans.
But Italian evolutionary biologists have come up with a different view. They examined well-known mitochondrial DNA sequences taken from insects and other arthropods and compared them with three little-studied species that they had sequenced themselves 鈥� two springtails and a bristletail, a wingless insect that, unlike the springtails, shares the same mouth structure as flying insects (Science, vol 299, p 1887).
The bristletail sequence was the closest to the flying insects. However, the brine shrimp and hermit crab sequences fell between those of the bristletail and springtails. That implies that springtails diverged from insects before crustaceans did, meaning that they do not form a single hexapod group with the 鈥渢rue鈥� insects (see Diagram).
Francesco Nardi of the University of Siena in Italy is one of the researchers who carried out the work. He says springtails probably split more than 500 million years ago during the Cambrian period. He believes they evolved first, followed by crustaceans and then insects.
The oldest known fossil springtail is 415 million years old. But this specimen was already quite advanced, and is placed in the same family as some modern relatives, says Nardi. So the group as a whole must have originated much earlier. In contrast, the earliest fossils of true insects are less than 400 million years old.
鈥淚鈥檓 not completely convinced, but they make a very strong case,鈥� says Conrad Labandeira, an expert in insect fossils at the Smithsonian Institution鈥檚 National Museum of Natural History in Washington DC. He agrees that insects probably evolved from a crustacean, but says 鈥渢he big question is which one鈥�. Nardi鈥檚 analysis could not hope to answer that question, as it only looked at two crustaceans.
It is possible that groups such as myriapods and spiders also evolved from crustaceans, which display a variety of different body shapes. Major modifications to this body plan, such as growing a different number of legs, can be achieved simply by varying the expression of HOX genes, notes Labandeira.