The missing link between whales and their four-footed ancestors discovered

Hans Thewissen, PhD, Professor of the Department of Anatomy, Northeastern Ohio Universities Colleges of Medicine and Pharmacy (NEOUCOM), has announced the discovery of the missing link between whales and their four-footed ancestors in the prestigious British journal Nature.

Scientists since Darwin have known that whales are mammals whose ancestors walked on land, and in the past 15 years, researchers led by Dr Thewissen have identified a series of intermediate fossils documenting whale's dramatic evolutionary transition from land to sea. But one step was missing: The identity of the land ancestors of whales.

Now Dr Thewissen and colleagues discovered of the skeleton of Indohyus, an approximately 48-million-year-old even-toed ungulate from the Kashmir region of India, as the closest known fossil relative of whales. Dr Thewissen's team studied a layer of mudstone with hundreds of bones of Indohyus, a fox-sized mammal that looked something like a miniature deer. In work published in the current issue of Nature, Dr Thewissen and colleagues report key similarities between whales and Indohyus in the skull and ear that show their close family relationship. The research by Dr Thewissen and his team was funded by the National Science Foundation (NSF).

Thewissen and colleagues also explored how Indohyus lived, and came up with some surprising results. They determined that the bones of the skeleton of Indohyus had a thick outside layer, much thicker than in other mammals of this size. This characteristic is often seen in mammals that are slow aquatic waders, such as the hippopotamus today. Indohyus' aquatic habits are further confirmed by the chemical composition of their teeth, which revealed oxygen isotope ratios similar to those of aquatic animals. All this implies that Indohyus spent much of its time in water.

'The work done by Dr Thewissen and his team highlights the exciting research going on in skeletal biology at NEOUCOM,' says Dr Walt Horton, Vice-President for Research at NEOUCOM. 'This remarkable research demonstrates that the study of the structure and composition of fossil bones can tell us about how the skeleton of whales and, by extension, other mammals like humans, interacts with the environment and changes over time.'

Before, it was often assumed that whales descended from carnivorous terrestrial ancestors, and some researchers speculated that whales became aquatic to feed on ocean-dwelling fish. According to Dr Thewissen, 'Clearly, this is not the case, Indohyus is a plant-eater, and already is aquatic. Apparently the dietary shift to hunting animals (as modern whales do) came later than the habitat shift to the water.'

Although it may seem strange to think of a tiny, deer-like animal living in water, one modern mousedeer offers something of an analogue to the ancient Indohyus, even though it is not closely related to whales: The African Mousedeer (also called Chevrotain) is known to jump in water when in danger, and move around at the bottom (for a movie showing this go to YouTube and watch 'Eagle vs. Water Chevrotain').

Whale evolution is one of the best documented examples of mammal evolution, and Dr Thewissen's discovery adds a significant new piece to the puzzle.

'Not much was known about the earliest whales, until the early nineties,' Dr Thewissen said. 'But then, a number of discoveries came in quick succession.'

The discovery of the first, and at that point only, amphibious whale, Ambulocetus natans, which was published in Science by the Dr Thewissen's team in 1994. The 'whale origins' discovery was considered one of the 100 Top Science Stories of that year by Discover Magazine.

In 2001, Dr Thewissen's team discovered the skeleton of Pakicetus attocki, the oldest known whale, and published it as a cover-story in Nature. Pakicetus and Ambulocetus represent the two earliest stages of whales, and Indohyus complements this by showing it what the ancestors of whales looked like.

The article documenting Dr Thewissen's latest discovery, titled Whales originated from Aquatic Artiodactyls in the Eocene of India, was published in the November issue of the journal Nature. It's authors (in order of appearance on the paper) include: J. G. M. 'Hans' Thewissen, PhD, Professor of the Department of Anatomy, Northeastern Ohio Universities Colleges of Medicine and Pharmacy (NEOUCOM); Lisa Noelle Cooper, Doctoral Student, Graduate Program in Biomedical Sciences, Kent State University and NEOUCOM; Mark T. Clementz, PhD, Department of Geology and Geophysics, University of Wyoming, Laramie WY; Sunil Bajpai, PhD, from the Department of Earth Sciences, Indian Institute of Technology, Roorkee, Uttarkhand in India; and B. N. Tiwari, PhD, from the Wadia Institute of Himalayan Geology, Dehra Dun, Uttarkhand, India.