This blog (first posted on June 28, 2010) is the third piece in a series by Darrel Falk and David Kerk. The previous entry is found here.
In our previous essay, we learned that a tree summarizing species relationships can be built using DNA information, and how we can use DNA as a “molecular clock” to date ancient events. Both of these methods have made specific predictions about the origin of whales. If evolution is true: whales are related to the even-toed hoofed mammals and should share common ancestors with them; transitional fossil forms dating from about 45 to 50 million years ago should be found which can be shown to be related to both the even-toed hoofed mammals and modern whales; whales are most closely related to modern hippos, and should share a common ancestor with them.
What other types of information might we be able to use to construct a phylogenetic tree (i.e. a family tree) of species relationships? It turns out that characteristics of body structure also can be used—for example, the presence or absence of certain bones, or the specific shapes of those bones. An advantage of using bony features is that they can be recovered from fossils, whereas DNA (with only certain limited exceptions) must come from living organisms.
We can also derive functional information from an examination of bony features. The various protrusions, bumps and knobs found on bones usually have important implications. For example, smooth rounded areas at the ends of bones allow them to fit together and move easily. The shapes of such surfaces determine which bone motions are “allowed” or “disallowed.” Consider, for example, the motion of the forearm against the upper arm at the elbow. This is a “hinge” joint, whose normal motion is defined by the shapes of the upper arm bone and one of the forearm bones, where they meet each other. You might normally exercise the action of this hinge joint when you pick up a cup of coffee, bring it to your mouth, then set it back down again. Let’s try to imagine another motion. For this exercise we first need to get our arm into the proper starting position. Place your arm at your side, bent at the elbow at a ninety degree angle, with your palm up. Now, while keeping your palm up, let’s attempt to move your arm only at the elbow (no shoulder motion – that’s cheating!). Now swing your forearm out to the side and attempt to end up with your fingers pointed directly away from your side. Most of you will not be able to do this. If you can, it’s because your shoulder is rotating in spite of yourself. This motion at the elbow is normally not allowed. Hence a careful analysis of bone shapes can allow us to infer how the bones were used. This in turn can assist us in the task of phylogenetic (evolutionary) classification of organisms. That is, we will have more confidence in the grouping together of animals in our tree diagram if corresponding bones are used functionally in the same way.
Therefore we would expect that we could use various bony features to help us examine the predictions generated by our previous look at different types of DNA data. Are there any bony features that are particularly relevant to the even-toed hoofed mammals? Well, it turns out that there are. These are mainly running animals, and there are several features of their ankle bones, which taken together define the “allowed” motions which make them efficient runners. If one takes the various ankle bones of a large group of mammals, examines them carefully to note their shapes, scores that information into a table, then uses a computer program to build a phylogenetic tree, it turns out that all the even-toed hoofed mammals are placed together. So far, so good. But what about whales? Well, now we have an obvious problem. Modern whales are very specialized—they have flippers which correspond to the forelimbs, and they have almost no hind limbs! I say “almost” because they do have small pelvic bones, which are not attached to the rest of their skeletons. But they certainly have no ankles. This is where the fossils should come in—if evolution is true, we should expect to be able to identify transitional fossils which are ancestral to whales which contain the characteristic ankle bony features of the even-toed hoofed mammals.
Now let’s look at bony features from the whale perspective. We have already mentioned the almost complete loss of hind limbs, and the presence of forelimbs modified into flippers. In addition, as air breathers, whales have a blowhole at the top of their skull. And as powerful swimmers, which use a large tail fluke in vertical motions, whales have enormous sets of muscles which attach to enlarged projections from their vertebral column. So if evolution is true, we should begin to see fossil forms which manifest changes in bony features which correspond to the gradual accumulation of these whale-like characteristics. However, we still need more, because these various bony features all would be expected to occur in largely or exclusively aquatic forms. We might expect this to correspond to the later stages of a transition from terrestrial even-toed hoofed mammals. But what about the earlier stages? It would be very helpful if we had some “defining” characteristic of whales, similar to the ankle structure of even-toed hoofed mammals.
It turns out that the structure of the bones of the skull and ear apparatus of whales are highly modified to allow efficient hearing underwater. The mechanical aspects of efficiently receiving sound through water are somewhat different than receiving sound traveling through air. If evolution is true, we should expect to be able to find key transitional fossil forms with a progressive series of modifications of the skull and ear bones, features which would not be found in any other mammals.
Now that we know what we should expect to see, if evolution is true, let’s look at what has actually been found in the fossil record. Over the last fifteen years or so, a series of fossils, many of them discovered in the Indian subcontinent, have fulfilled nearly all of our predictions.1,2 The entire fossil progression illustrated occurs from a little over 50 million years ago to about 40 million years ago. So a remarkable alteration in general body form occurred in a little over 10 million years. This time frame agrees well with the previous prediction from the DNA “clock” that we discussed in our previous essay. Second, the general change in body shape corresponds to what we predicted in our discussion of whale bony features above. That is, there is a gradual elongation and streamlining, there is a modification of the forelimb into flippers and progressive reduction of the hindlimb, the nostrils for breathing move toward the top of the skull to form a blowhole, and the vertebrae develop enlarged projections to support the attachment of swimming muscles.
There is probably little question that the fossil species Dorodon is well on the way to becoming a modern whale. However, it might be argued by a skeptic that the earlier species (like Rhodocetus, Ambulocetus, or Pakicetus), despite the “cetus” (whale) part of their names, are not so obviously “whale-like” that they deserve to be considered as fossil whale ancestors. However, remember the characteristic whale skull modifications for hearing? It has been shown very clearly that throughout this series of fossil species, the various bony changes necessary to support efficient hearing in water were being acquired in a stepwise fashion. Organisms earlier in the sequence had skeletal characteristics consistent with them being able to hear well in both air (using the “classic” mammalian hearing apparatus), and newly acquired changes to also allow better hearing in water. Later organisms in the sequence become increasingly specialized for hearing in water only.4
What of the earliest fossil shown in this diagram—Pakicetus? Careful examination shows that it has the features we would predict for an early whale ancestor. It has the ankle bone characteristics of the even-toed hoofed mammals (in fact these features are also found in several of the later fossil forms as well, ensuring their continuity). This confirms one of the predictions made by the DNA evidence we discussed earlier. Furthermore, it has some of the modifications of the skull bones necessary for more efficient underwater hearing, which were previously documented only for modern whales and their later (more obvious) ancestors.4 These features are also shared with the “whale cousin” Indohyus.5 Preservation of more of the skeleton of this latter species has allowed detailed analysis indicating characteristics likely shared with whale ancestors. Indohyus was probably a wading animal, which spent much of its time in the water. It appears to have fed mostly on land, so it is suggested that resort to the water was made to escape predators.5
Finally, we need to look back at the last prediction from our previous DNA evidence, namely that modern whales are most closely related to hippos. If evolution is true, we should expect to find fossil forms linking these two modern groups. This has proven to be a tougher nut to crack, mainly because the ancestral whales first appear about 50 million years ago in what is now south Asia, and the hippo family first appears about 15 million years ago, in Africa. The most recent tree diagram, produced by using a combination of skeletal features and DNA data, still supports this family connection, as shown by the following figure (Figure 2).6
Figure 2: Phylogenetic Tree Showing the Relationship of Modern Whales to Living and Extinct Even-Toed Hoofed Mammals
This tree is based on both bony features and DNA data. The organisms presented in blue are semi-aquatic or aquatic forms. Organisms shown in green are terrestrial even-toed hoofed mammals (Artiodactyls). In black is shown a member of the odd-toed hoofed mammals. In red is an extinct fossil ancestor group. (This figure is adapted from Fig 1a in Reference 6).
The blue lines in the diagram show species in which the skeleton is specially thickened, and the bone structure more dense. This is an adaptation which allows wading animals (like modern hippos and the fossil Indohyus) to be good “bottom-walkers” (it prevents them from floating due to lighter body tissues), and allows fully marine organisms (like modern whales) to have “neutral buoyancy” (so they don’t always tend to pop up to the water surface, like a cork). There has also been progress in clarifying the relationships between fossil ancestors of hippos and those of modern whales. A recent study of hippo evolution, based only on skeletal characteristics, has conclusively shown that the hippo family are descended from an extinct group of fossil Artiodactyls, known to go back more than 40 million years, and whose fossils are from southern Asia. Furthermore, this study produced a phylogenetic tree predicting that this extinct hippo ancestor group also shared a common ancestor with the fossil whales.7 Thus the investigation of hippo origins is independently leading us back toward the origin of whales. However, in this study the statistical support for predicted common ancestor of the ancient hippo group and the ancient whale group is not as strong as scientists would like to consider this “case closed.” What is necessary is more fossils, of the appropriate age in order to complete the story of hippo evolution. We still need that to fill in the details of the predicted relationship of hippos to modern whales.
Thus the “Whales’ Tale” is not yet complete. It is a story of scientific discovery in progress. As we finish, let’s briefly summarize what we have found out. Different types of DNA evidence agree that modern whales are most closely related to the even-toed hoofed mammals, despite the obvious great changes in limb anatomy of the modern whales. This prediction has been amply confirmed by the fossil record. The DNA sequence evidence predicted a time frame during which critical early events in evolution of whale ancestors should occur. This prediction has also been amply confirmed. Finally, DNA evidence predicts that modern whales are most closely related to hippos. There is some fossil evidence supporting a predicted common ancestor, but more data is needed. A final caution to possible sceptics—this state of “unfinished business” is precisely how the scientific process works. There is no “crisis.” There is no indication that evolution is not true. There is simply the ongoing work of mapping out of various lines of evidence. A scientific conclusion is considered well supported if “all roads lead to Rome.” In the case of whale evolution it might be prudent to say that the evidence has not quite converged in Rome yet, but that we are now in the suburbs. That is precisely what makes science interesting and fun. Stay tuned!
The next blog in this series can be found here.