The riddle of the river dolphins

River dolphins are some of the world's least-known and endangered species of mammals. Although better-known marine dolphins are found in estuaries, river dolphins represent four distinct genera each inhabiting different river basins such as the Amazon, Ganges, Indus and Yangtze. But their relationship to each other and other cetaceans has been puzzling and disputed. One of the key problems is the lack of fossil evidence. Recently some researchers have turned to molecular analyses of existing species to try to help to build possible phylogenetic relationships and a recent study by Healy Hamilton at the Museum of Palaeontology at the University of California, Berkeley, and colleagues reports one of the most wide-ranging molecular studies yet carried out (Proc R Soc Lond B 2001, 2 26 68 8: :549-556). Hamilton and his team have examined sequence data from three mitochondrial genes from a total of 29 cetacean species. Analysis of the results led the team to infer phylogenetic relationships so far impossible from the fossil record. One of their clearest conclusions is that the river dolphins do not represent a group of modern, closely related species. They appear to represent three ancient branches in cetacean evolution. But how might such evolutionary events have take place? The team looked at ancient biogeographical conditions and at the raised sea levels present in the middle of the Miocene period 15 million years ago when most of the major river basins would have been shallow seas. As sea levels subsequently fell, populations of cetaceans may have been stranded from their marine counterparts and evolved independently: a scenario that fits well with the new molecular data. It seems likely therefore that river dolphins are not a modern-day cetacean group but animals with direct links to a long-lost marine and estuarine world.