ENHS Nature Trails January 2000 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Origins of Cetaceans: A Whale of an Evolutionary Tale

Cetaceans (whales, dolphins, and porpoises) are, for good reason, classified as mammals: like all mammals, cetaceans are air breathing, possess mammary glands, sweat glands and hair, give birth to live young, possess a four chambered heart, and have eggs that are fertilized internally. However, unlike their mammalian brethren, cetaceans are so completely adapted for aquatic life that they are unable to survive on land, a characteristic that distinguishes them from other mammals, even pinnapeds (seals, sea lions, and walruses) which live a significant part of their lives on land. The success of cetaceans in developing a fully aquatic lifestyle raises the question, from which group of land dwelling mammals did cetaceans evolve? A related question is, which mammals are closest relatives of cetaceans?

The evolution of cetaceans from their land dwelling ancestors is a fascinating tale, rich in intermediate fossil forms spanning the transition from land to water. What is equally remarkable is that the traditional morphological data is at odds with more recently obtained molecular data, with each data set providing different answers to the above two questions.

Biologists determine the "relatedness" between organisms on the basis of shared characteristics; the more traits two organisms have in common, the more closely related they are to each other. For example, although obvious, identical twins have more common traits between each other than they do with their parents, thus they are more related to each other than to their parents. The principle is the same whether the comparison is between individuals, species or larger taxonomic units. Bats, sheep and dogs, as mammals, have more traits in common with each other than each share with birds, hence they are all more closely related to each other than to birds.

In the case of pinnapeds, their shared morphological characteristics have long suggested that they evolved from the carnivore mammalian group which includes bears, dogs and cats. Recent molecular data, obtained by determining the amino acid and DNA sequences of common proteins and chromosomes, respectively, also support the hypothesis that pinnapeds descended from terrestrial carnivores. Both types of analyses, morphological and molecular, generated the same conclusion.

Unfortunately this is not the case for the cetacean data. A recent study by O'Leary and Geisler in Systematic Biology (48:455-490, 1999) analyzed morphological and molecular evidence in an attempt to provide answers to the origins of cetaceans. They showed that the two types of data, morphological and molecular, agree on only one point: that cetaceans have a close affinity to the artiodactyl mammals, also known as ungulates, which include such even-toed animals as the cow, camel and hippopotamus. But unequivocal answers to the questions posed above proved more elusive.

In one camp are the paleontologists, traditional morphologists who have for most of the past 100 years provided strong evidence for the view that the cetaceans' closest relatives are the mesonychids, an extinct mammalian group that lived 60-30 million years ago. Mesonychid feet were even-toed and adapted for running like those of modern ungulates, while their teeth more closely resembled those of the earliest whales. In this scheme, all living ungulates are considered the closest living relatives to cetaceans.

The paleontological view of cetacean evolution predominated until the advent of molecular evolutionary techniques. Molecular studies concluded that hippopotamuses and cetaceans are more closely related than either group is to any other living ungulate. These data suggest that whales and other cetaceans evolved from an ancestral form of the hippo rather than some prehistoric ancestor of the modern ungulates as deduced from the morphological data.

These two different conclusions cannot be easily rectified. The morphological studies have an advantage because of the abundance of well- preserved ungulate and cetacean fossils. Moreover because 90% of ungulates and 86% of cetaceans are extinct, it is unlikely that there will ever be a significant increase in the molecular data. Because of this, O'Leary and Geisler come down on the side of the morphologists and believe that cetaceans likely evolved from mesonychids and that their closest living relatives are the entire group of ungulates.

Although it appears that the morphologists have prevailed on the origin of cetaceans, it would be foolhardy to discount the impact of molecular data on these evolutionary studies. In a very few years there will be more genes identified in ungulates and cetaceans than the current number of morphological traits and evolutionary analyses using molecular data will become more reliable. It is not inconceivable that in the future, the molecular biologists will prove conclusively that whales evolved from hippos. But don't hold your breath (at least not as long as a cetacean).

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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