The Whale That Walked In Kashmir

From DNAIndia.com this headline:

Whales once walked in Kashmir

Whales evolved from land mammals that did walk. Scientists have found quite a remarkable series of fossils documented through morphological changes this transition from land to sea. The earliest known whale is the 47 million year old Ambulocetus natans, a small otter size creature with short stubby legs. Scientists reckon that whales ultimately evolved from land mammals called artiodactycls, even toed ungulates whose living representatives include camels, pigs and hippos. But before Ambulocetus the fossil record is poor which means we don't know much about the earliest ungulate ancestors of whales and their ecology. Now a new article reports on the discovery of a 48 million year old raccoon size fossil artiodactycl named Indohyus, which would make it one of the earliest known relatives of whales and should throw light on the early environmental conditions in which ancestors of whales lived. This fossil was found in Kashmir by a team of researchers, Sunil Bajpai of IIT Roorkee, B N Tiwari of the Wadia Institute of Himalayan Geology, Dehradun and US-based researcher Carl Buell Hans Thewissen.The article appeared in the journal Nature. Based on isotope analysis of teeth that indicates the type of diet (vegetarian) and morphological characters, Indohyus has been interpreted to have been a creature that spent a lot of time wading through and underwater, munching on aquatic vegetation. Its leg bones are particularly heavy, probably to prevent it from floating up to the surface of water. Similar bone structure can be seen in hippos. Below is an artists reconstruction of Indohyus.

Source: BBC News

The DNA article calls Indohyus the ancestors of whales.Yet if you look at the evolutionary relationships of Indohyus with the Cetaceans (the group that includes whales) which scientists have constructed by comparing the morphological characters of Indohyus with living and fossil Cetaceans and other mammals this is what you find:

Source: Nature

Indohyus is not an ancestor of the Cetaceans but a member of a sister group, a close relative. Why does the article refer it as an ancestor? This goes to the very heart of a common misconception of how evolution works. When Indohyus was interpreted to be an ancient even toed ungulate related to whales and sharing some morphological similarities with whales, the report automatically assumed that it must be an ancient ancestor of the whales. This happens because of the long discredited but still popular notion of evolution as a linear process whereby an entire species gets transformed over time into a new species. This view leads to interpreting evolutionary relationships as direct ancestor descendant lineages. But evolution is a branching process. One species can give rise to just one, or two or several new species and continue to coexist with its descendants. Moreover, the rate of morphological change in these different branches may vary. One branch may experience rapid morphological change while a closely related branch may not change too much. The above evolutionary tree also known as a cladogram is basically a hypothesis about the evolutionary relationships between the different organisms being studied. Based on this cladogram, Indohyus and Cetaceans shared a common ancestor in the past. This common ancestor gave rise to two daughter species. One species became the founding ancestor of the Raoellidae, the group that includes Indohyus and the other species became the founding ancestor of the Cetaceans. The branch of artiodactycls that evolved into Indohyus retained many morphological characters of its ancestors. The other branch also retained some ancestral traits such as the middle-ear space called the involucrum. But they also evolved many new characters including carnivory, shorter legs and a more hydrodynamic body shape. Eventually the short legs became modified into the Cetacean fins. The Cetacean branch has undergone dramatic morphological changes. At first the founding species of the two branches would have been morphologically very similar. But since morphological evolution in the branch that lead to Indohyus was limited, it resembles the earliest ancestors of the Cetaceans more than living Cetaceans do. This leads to the common misperception when interpreting evolutionary relationships that a creature with more ancestral traits is actually the ancestor. The common remark that we evolved from chimpanzees is based on just such a misunderstanding. We shared a common ancestor with the chimps 6-8 million years ago. The branch that lead to chimps did not change much morphologically as against the changes that took place in the human lineage. So a chimp likely resembles our very early ancestors more than we do. That doesn't make the chimp our ancestor.

The report also says that

three-member team of researchers, including two from India, claim to have found the missing link between whales and land-based mammals.

I doubt if the researchers made any such claim. Indohyus is a close relative of the Cetaceans. Its morphology and the interpreted ecology in which it lived can tell us something about how the ancient even toed ungulates from which the Cetaceans evolved looked like and the environment in which they lived. The term missing link is now outdated. It used to refer to a species with transitional or intermediate form between any two end-member morphologies thought to be forming an ancestor descendant series. Discovery of such forms is always useful since it allows paleontologists to understand just how morphological evolution took place as one form evolved into another, but one missing link cannot be expected to explain all the evolutionary changes between one suite of morphological characters into another. In modern evolutionary thinking a missing link is really a reference to an intermediate morphology and not to any particular fossil species. Because of the branching nature of evolution, the discovered fossil species with an intermediate morphology may be an evolutionary cousin and not a direct ancestor. There is another reason why Indohyus should not be thought of as a missing link or a transitional form. It was not in transition into becoming more aquatic and whale-like. The terms missing link and transitional fossil are retrospective labels. But evolution has no foresight. It did not build Indohyus and related semi-aquatic ungulates as a stop gap measure towards more full fledged aquatic Cetaceans. Evolution is simply change in response to the environment on a generation by generation basis. Indohyus like any other creature evolved characteristics that were well adapted to its environment. It was a representative of its time and place. Modified descendants of this "transitional" morphology of Indohyus are still with us. We call them hippos.