What can a massive African elephant possibly have in common with a tenrec, a hedgehog look-alike that can weigh less than a quarter? According to a controversial hypothesis, they're close kin. Researchers who base their evolutionary trees on comparative anatomy may not buy the theory, but DNA sequences supposedly link elephants and tenrecs to a diverse extended family that also includes manatees and aardvarks. Now, researchers say, new evidence from proteins bolsters this unorthodox grouping.
Molecules and morphology often clash when it comes to evolutionary relationships. For example, family trees based on anatomy unite pigs with hippos, while DNA sequences suggest that hippos are more closely related to whales. Likewise, study after study has identified DNA similarities among a diverse group called the Afrotherians. Besides elephants, aardvarks, and tenrecs, this mainly African bunch also contains such dissimilar creatures as elephant shrews (named for their long noses), golden moles, and vaguely rabbitlike creatures called hyraxes. Most morphologists insist that they see no family resemblance at all and scatter these animals around the mammalian family tree.
Looking for characteristics that might unite this group, a team led by evolutionary biologist Mark Pagel of Reading University in the United Kingdom and Wilfried de Jong, a protein chemist from Nijmegen University in the Netherlands, focused on the species' proteins. The scientists searched published protein sequences for so-called "signatures"--unique combinations of amino acids found only in members of the group. In three proteins, the Afrotherians shared an amino acid combination absent in other mammals examined, the researchers report in the 12 December online early edition of the Proceedings of the National Academy of Sciences. The researchers estimated the likelihood that these sequences had evolved independently in unrelated animals was virtually zero.
Other systematists aren't convinced. Although the study supports the hypothesis that Afrotheria is a cohesive group, "I don't think we've had the definitive answer," says molecular systematist C. William Kilpatrick of the University of Vermont in Burlington. He's waiting for a study that is both broader and deeper, including species from within and outside the group, before concluding that the signature protein sequences are found in all Afrotherians but not in other mammals.