Whales: Can Evolution Account for Them?
Paleontologist E. H. Colbert has commented on cetaceans (whales and their kin) as follows:
Like the bats, the whales (using this term in a general and inclusive sense) appear suddenly in early Tertiary times, fully adapted by profound modifications of the basic mammalian structure for a highly specialized mode of life. Indeed, the whales are even more isolated with relation to other mammals than the bats; they stand quite alone.
Clearly the cetaceans can serve as ammunition for the creationists (Mayo). Just what makes us so sure that these beasts were not plunked down in the ocean as is?
Well, one of the nice things about science is that we can become more or less sure of our educated guesses as we test them. With regard to the evolution of extant groups of organisms, we can test our assumptions based on one set of characters, such as morphology, by looking at another set of characters. For example, if we decide that whales are more closely related to porpoises than to tuna, based on their comparative anatomy, and that we would like to convince our friends that we're right, the thing to do is search out comparative literature on each group's behavior, embryology, fossil record, biochemistry, and so on. If each time we use a different set of criteria the whale turns out to be more similar to the porpoise than to the tuna, we assume our initial theory is more likely to be correct than not. If two groups of organisms show many similarities, it is more likely that they shared a common ancestor with these characteristics than that the two groups evolved all the shared characteristics independently. We choose the theory which makes the fewer number of assumptions—that is, the most parsimonious choice.
We also choose the theory that explains or predicts the most facts. For example, creationism cannot explain the vestigial hind limbs found in whales, but evolution can. The evolutionary explanation is that whales evolved from land mammals that had four legs. Other structures not explained by creationism, such as the teeth of baleen whale embryos which are resorbed before birth (Peyer), are also explained by evolution: baleen whales evolved from mammals having teeth.
It will be the purpose of this article to further reveal the parsimonious nature and explanatory ability of evolution as it applies to whales. The primary focus will be the evidence of whale evolution found in the fossil record and the relationship of whales with other mammals as seen through biochemistry. The conclusions drawn from both sets of data will hopefully suggest a parsimonious and explanatory solution to the question of why whales are the way they are.
Romer, Slijper, and Colbert have briefly reviewed our knowledge of extinct cetaceans. Zeuglodon (an elongate giant which reached seventy feet in length), Dorudon, Protocetus, and their relatives belonged to a primitive group of cetaceans, the archaeocetes. These mammals are best known from Eocene and Oligocene deposits (twenty-five to fifty million years ago) and give us some clues as to how the modern cetaceans evolved. A number of transitional features are shown in the archaeocetes whales which link the modern cetaceans with their ancestors on land. These features include: (1) independent neck vertebrae; (2) a forelimb which showed less fusion than extant whales; (3) a pelvic girdle which was somewhat reduced and unattached to the vertebral column but still more like their terrestrial relatives than today's whales (a ball and socket joint to articulate the femur and pelvis is especially telling); (4) facial bones of the skull that were not telescoped as in modern whales; and (5) nostrils placed in a forward position (not on the top of the skull to form the "blowhole" of extant species), typical of land mammals.
By the Miocene, or perhaps earlier, the two main groups of extant cetaceans, the toothed whales (odontocetes) and the whalebone whales (mysticetes) were already well established. Early toothed whales, such as Prosqualodon, showed dentition similar to the primitive Archaeocerta but had an advanced skull structure—another good "transitional fossil."
Van Valen reviewed the characters of extant and extinct whales. He concluded that extinct whales such as Protocetus may have given rise to both major suborders of recent whales. Speaking of the Protocetidae, he said, "It is beautifully intermediate between primitive mesonychids and recent whales, although in most respects more similar to the latter." The mesonychids were a group of very primitive ungulates (hoofed animals), some of which were rather large carnivores, that are known largely from the Paleocene and Eocene.
Recently Gingerich and Russell described a new archaeocete, Pakicetus, which was found in Pakistan. These and other fossil whales found in that area belong to the Protecetidae family. However, the specimen of Pakicetus found had a very well preserved skull and appears to have several cranial characteristics in common with other archaeocetes, but it "exhibits few of the other specializations
of this group required for hearing under water." (Olson has pointed out that toothed whales have developed remarkable mechanisms for reception and transmission of sound. The mysticetes have a much lesser degree of modification, only moderately different from the archeocetes.) They go on to say, "The primitive nature of the dentition in Ichthyolestes, Pakicetus, and Gandakasia, the fact that all three genera are found in association with land mammals, and the primitive nature of the basicranium in Pakicetus combine to suggest that whales made the transition from land to sea as late as the early or early-middle Eocene." Gingerich had previously identified what seemed to be part of the mandible (jaw) of a juvenile artiodactyl (an even-toed, hoofed mammal in the same order as modern pigs, hippopotamuses, deer, cattle, and camels), but later Gingerich and Russell reinterpreted it as belonging to an archaeocete. Evidently the jaw's structure hints at an original version common to the two groups.
The explosion of "biochemical systematics" during the past three decades is a direct function of the power of science.
Boyden and Gemeroy used an immunological technique called the precipitin test to speculate about the relationship of Cetacea with other mammal groups. To perform this test, an antiserum is manufactured by rabbits after they are injected with an antigen, such as serum. The various antisera taken from the blood of the sensitized rabbits are reacted with different antigens. The relative turbidity, a measure of the interaction, is used as a measure of similarities between the groups. They found that the degree of similarity between Cetacea and other groups of animals was small except for Artiodactyla.
Constructing a phylogenetic tree (Figure 1) in the form of a cladogram (for an excellent discussion of the theory and use of cladograms, see Eldredge and Cracraft) on the basis of cytochrome sequences (matrix 1 in Dayhoff), we can independently confirm the work of Boyden and Gemeroy. Cytochromes are relatively stable proteins, compounds composed of long chains of smaller molecules called amino acids which are linked together in a very specific order. Using computers we can statistically estimate the similarity of several sequences to each other; the greater the similarity two sequences share, the more recently their common ancestors diverged.
However, the relationships are not always that clear cut. For example, myoglobin (modified from Dayhoff) gives us a slightly different cladogram (Figure 2) than do the cytochromes. While not identical to the cytochrome tree, we see that the horses, whales, and Artiodactyla make up a collective group whose common ancestor branched off after the common ancestors of the kangaroo or man.
Fossil whales show a number of characteristics that are intermediate between extinct land mammals and living cetaceans. Many of these morphological features allude to hoofed organisms in particular. Concurrently, several biochemical assay techniques point to the same conclusion. The rule of parsimony therefore dictates that evolutionary biologists are well within the bounds of reason when they speculate about a common ancestor shared by the modern ungulates and living whales.
However, the whale does present those not familiar with natural history with a problem, since, based on external anatomy, the cetacean certainly doesn't resemble other mammals, especially those with hoofs. This is just the sort of thing creationists delight to use in debate since it is much more economical to show a humorous slide which depicts a cow or pig trying to become a whale than it does to discuss protein sequences and cranial specializations. Nothing can really be done about this except to point out that, regardless of first impressions, a diamond is much more similar to a lump of coal than to a quartz crystal; sometimes we have to go beyond a first impression and study a subject in an orderly and objective manner. In the long run, a scientist must have a lot more than cute slides or he'll find himself laughed into a less-demanding field.
The author would like to thank Ms. B. J. Tylka of the University of Texas, Dr. Walter Nelson of Florida Institute of Technology, and Dr. Ken Miller of Brown University for their helpful suggestions.
Boyden, A., and Gemeroy, D. 1950. "The Relative Position of the Cetacea Among the Orders of Mammalia as Indicated by Preciptin Tests." Zoologica 35:145-151.
Colbert, E. H. 1980. Evolution of the Vertebrates, 3rd edition. Wiley-lnterscience.
Dayhoff, M. O. 1972. Atlas of Protein Sequence and Structure, Vol. 5. National Biomedical Research Foundation.
Eldredge, N., and Cracraft, J. 1980. Phylogentic Patterns and the Evolutionary Process. Columbia University Press.
Gingerich, P. D. 1977. "A Small Collection of Fossil Vertebrates from the Middle Eocene Kuldana and Kohat Formations of Punjab (Pakistan)." Contributions to the Museum of Paleontology. University of Michigan, 24:190-203.
Gingerich, P. D., and Russell, D. E. 1980. "Pakicetus inachus (Mammalia, Cetacea) from the Early-Middle Eocene Kuldana Formation of Kohat (Pakistan)." Contributions to the Museum of Paleontology. University of Michigan, 25:235-246.
Mayo, A. 1981. "The Creationist Campaign to Convert America." Village Voice (October 14-20), pp. 30-33.
Olson, E. C. 1971. Vertebrate Paleozoology. Wiley-lnterscience.
Peyer, B. 1968. Comparative Odontology. University of Chicago Press.
Romer, A. S. 1966. Vertebrate Paleontology, 3rd edition. University of Chicago Press.
Slijper, E. J. 1979. Whales, 2nd edition. Cornell University Press.
van Valen, L. 1968. "Monophyly or Diphyly in the Origin of Whales." Evolution,