Science Literature

In their paper in Nature, Thewissen et al explain that the whale evolution story lacks a significant anchor point: "the link to the ancestor of cetaceans has been missing". It is not just a case of not knowing, but the focus of fundamental differences between researchers. Some emphasise morphology and some molecular evidence - and it should not be surprising to learn of a lack of harmony about the meaning of these data. Now, Thewissen and colleagues think they have an answer:

"It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth."

They use cladistic techniques to show the closeness of the raoellids and the cetaceans (although we should note that the cetaceans are represented by Pakicetus, Ambulocetus, Rodhocetus and Artiocetus - all supposedly transitional walking whales). Cladistics is not a straightforward tool to use for the purposes of tracing ancestries. This is because there are many shared characters to consider and it is necessary for researchers to select the character sets they use in the cladograms. Great stress is placed on finding the most parsimonious trees: the one with the minimum number of evolutionary changes needed to explain the data (a criterion that strikes me as unrealistic with the evolution of cetaceans, a process which necessarily requires an extraordinary number of changes). These shared characters do not always fit neatly into the cladistic framework of analysis: many animals (living and extinct) exhibit a puzzling mosaic of characters. The researchers identified four characters which they deem significant: the structure of ears and molars, the density of its limb bones, and the stable-oxygen-isotope composition of its teeth. From this they conclude:

"Raoellids are the sister group to cetaceans, and this implies that aquatic habitats originated before the Order Cetacea. The great evolutionary change that occurred at the origin of cetaceans is thus not the adoption of an aquatic lifestyle."

Whilst these particular characters may be used to defend the idea that the animals were aquatic waders, the claim that they represent the transition from artiodactyls to whales belongs to the realm of speculation. Hypotheses must be tested, and the researchers should evaluate their own hypothesis against alternatives. As a suggestion, consider the hypothesis that Indohyus was a specialised artiodactyl gathering its food in aqueous environments. Furthermore, the researchers must guard against the possibility that they are cherry-picking characters that allow them to reach a favoured decision. A report by Stokstad put a question mark against the claim that the long-lost relative of whales has been found. He concluded:

"Not everyone is convinced that Indohyus is the closest cetacean relative, however. Another analysis, in press at Cladistics, suggests that an extinct group of carnivorous mammals, called mesonychids, were more closely related to cetaceans."

The reference to this paper is given below.
Thewissen et al provide an adaptationist scenario for the pathway to cetaceans. The nearest analogy is suggested to be the African mousedeer which lives near streams, feeds on land, but flees into the water when danger occurs.

"Our working hypothesis for the origin of whales is that raoellid ancestors, although herbivores or omnivores on land, took to fresh water in times of danger. Aquatic habits were increased in Indohyus (as suggested by osteosclerosis and oxygen isotopes), although it did not necessarily have an aquatic diet (as suggested by carbon isotopes). Cetaceans originated from an Indohyus-like ancestor and switched to a diet of aquatic prey."

One newspaper report continues the tale:
"However, no one knows why some mammals returned to the water. Some may have started to wade in rivers and lakes to avoid predators or in search of better food. As they spent longer in the water, their legs evolved into flippers - while their noses developed into the blow holes found in the top of whales' heads."

This sounds like a good lead for a Rudyard Kipling story!

Whales originated from aquatic artiodactyls in the Eocene epoch of India
J. G. M. Thewissen, Lisa Noelle Cooper, Mark T. Clementz, Sunil Bajpai & B. N. Tiwari
Nature 450, 1190-1194 (20 December 2007) | doi:10.1038/nature06343

Abstract: Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.

See also:

Stokstad. E. Long-Lost Relative of Whales Found? ScienceNOW Daily News, 19 December 2007

O'Leary, M.A. and Gatesy, J. Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils, Cladistics (OnlineEarly Articles). | doi:10.1111/j.1096-0031.2007.00187.x

Derbyshire, D. Revealed: The deer that grew into a whale, Daily Mail, 19th December 2007