Hippo to Whale: Missing Chain
Despite claims to the contrary,2 whale and hippo evolution are poorly understood. That’s the gist of a paper in PNAS this week1 that tries to connect the dots between hippopotami (artiodactyls) and whales (cetaceans) and other groups of mammals. There’s lots of missing dots:
The origin of late Neogene Hippopotamidae (Artiodactyla) involves one of the most serious conflicts between comparative anatomy and molecular biology: is Artiodactyla paraphyletic? [i.e., unrelated, but similar by “convergent evolution”]. Molecular comparisons indicate that Cetacea should be the modern sister group of hippos. This finding implies the existence of a fossil lineage linking cetaceans (first known in the early Eocene) to hippos (first known in the middle Miocene). The relationships of hippos within Artiodactyla are challenging, and the immediate affinities of Hippopotamidae have been studied by biologists for almost two centuries without resolution. (Emphasis added in all quotes.)
Their paper proposes a phylogenetic tree based on morphology; it rejects hippo relationship to pigs, and puts them as a sister group to whales, but puzzles remain: for example, “the position of Ruminantia [cows] is a central question, still to be solved.” Clearly, they need more fossils: “Further progress in this debate is likely to come from morphological studies of paleontological data, whether known or still to be discovered” (emphasis added).
1Boisserie et al., “The position of Hippopotamidae within Cetartiodactyla,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0409518102, published online before print January 26, 2005.
2See, for example, 11/21/2003 and 11/18/2003 headlines. National Geographic News, reporting on this paper, calls whales and hippos “close cousins,” which is an odd statement, considering they conclude, “There is a 40 million-year gap between fossils of early cetaceans and early hippos.”
And you thought the animators on PBS and the Discovery Channel had whale and hippo evolution all figured out. From what missing links did hippos get their antibiotic sunscreen sweat? (see 05/25/2004 entry).