Before the widespread use of DNA data, hippopotami had been thought to be closely related to pigs, but new genetic analysis show that they are more linked to whales. Consequently, the hippo family tree could be different than suggested by previous studies.
In 2007 J. G. M. Thewissen from Northeastern Ohio Universities College of Medicine and colleagues published a paper on Nature where they stated that whales are more closely linked to an extinct pig-like animal (often known as India's pig or Indohyus) while hippos are closely related to living pigs. But a recent paper judged Thewissen’s theory as ‘innacurate’.
Jessica Theodor and Jonathan Geisler, respectively from University of Calgary and Georgia Southern University, agree with Thewissen that Indohyus is the closest relative of whales, but argue on the assumption that ‘hippos are more closely related to true pigs than they are to whales’. The new theory relies on DNA data that show a strong relationship between whales and hippos and place hippos as the closest living relative to whales
The main difference between the two theories is the kind of analysis. Thewissen based his work on fossil evidences while Theodor on genetic. Geisler and Theodor argue that leaving out the DNA data not only ignores important information, it implies that the evolution of swimming evolved independently in hippos and whales, when it may have evolved only once in a common ancestor.
Photo: A hippopotamus in Okavango, Botswana; by iStockphoto/Peter Malsbury
For more information:
Geisler J.H. & Theodor J.M. 2009. Hippopotamus and whale phylogeny. Nature 458: E1-E4. doi:10.1038/nature07776 (abstract only)
Thewissen J.G.M., Cooper L.N., Clementz M.T., Bajpai S., Tiwari B.N. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450:1190-1194. (full article)
Thewissen et al. reply to Geisler J.H. & Theodor J.M. 2009.