scientists explain their discovery

Hans Thewissen, PhD, Professor of the Department of Anatomy, Northeastern Ohio Universities Colleges of Medicine and Pharmacy (NEOUCOM), holding skull of Indohyus. Photo

Scientists track down the whale’s walking ancestor

small deer-like mammal about the size of a modern fox or racoon was the ancestor of whales and dolphins, according to research that fills a missing link in their evolutionary history.

The creature, indohyus, which lived in what is now India around 48 million years ago, may have been the land animal that first took to the water to escape predators, leading ultimately to the evolution of the cetaceans - the order that includes whales, dolphins and porpoises.

While it had long been understood that cetaceans are mammals that had forerunners that lived on land, and a series of intermediate fossils have been found, scientists had not previously identified a species from the era in which cetacean ancestors took up an aquatic lifestyle.
A new analysis of fossils belonging to indohyus has now filled this important evolutionary gap. Details of its anatomy have shown that while it belongs to a terrestrial group called the raoellids, it spent much of its time in water.

Further similarities with other intermediate cetacean fossils suggest that either indohyus or something like it was probably the ancestor of modern whales and dolphins, scientists said.

As indohyus was a herbivore, the new research also challenges a standard view that as whales are carnivorous, they are most likely to be descended from predatory land ancestors which moved to the water to feed on fish.

“Clearly, this is not the case,” said Hans Thewissen, of Northeastern Ohio Universities, who led the research. “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.”

The small mammal's aquatic lifestyle has been inferred from an examination of its bones, which have a much thicker outer layer than is usual for land animals. It is, however, found in modern mammals that are largely aquatic, such as the hippopotamus.

The hippo is also known from DNA evidence to be the closest relative of cetaceans still living today, though as it is found in the fossil record only from 35 million years ago it is not a candidate for being a direct ancestor.

Further evidence for aquatic adaptation comes from indohyus's teeth, which have a similar ratio of oxygen isotopes to modern animals that live principally in water. Details of the discovery are published in the journal Nature.

Dr Thewissen said that while it seems odd that a mammal similar to a deer might have lived mainly in water, there is a modern herbivore that follows just such a lifestyle. The African mouse-deer, also known as the chevotain, is known to jump into water when in danger from predators.

While the chevotain is not closely related to whales, its behaviour suggests that cetacean ancestors might first have taken to the water to escape predators, before adapting to a mainly aquatic lifestyle.

Dr Thewissen's previous research has established that whales and dolphins are evolved from a group of land mammals known as the artiodactyls - the even-toed ungulates that include cattle, pigs and camels.

In 1994, his team discovered Ambulocetus natans, an amphibious whale that was a later ancestor of cetaceans, and in 2001 it identified Pakicetus attocki, the oldest known true whale. Nothing, however, was previously known about the first artiodactyls to take to the water.

Walt Horton, vice-president for research at Northeastern Ohio Universities, said: “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."

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.

The Northeastern Ohio Universities Colleges of Medicine and Pharmacy are community-based, public institutions focused on the interdisciplinary training of health professionals, offering both a doctor of medicine (M.D.) and a doctor of pharmacy (Pharm.D.) degree. The colleges’ educational partners include teaching hospitals, community pharmacies, two boards of health and four northeast Ohio public universities. Through these and other collaborative arrangements, the colleges participate in the Consortium of Eastern Ohio Master of Public Health program and offer graduate-level coursework and research opportunities leading to master’s and doctoral degrees in biomedical sciences and biomedical engineering.