CASE TWO: Discovery of the first complete hind limb in a fossil whale. In the most famous mistake of early American paleontology, Thomas Jefferson, while not engaged in other pursuits usually judged more important, misidentified the claw of a fossil ground sloth as a lion. My prize for second worst error must go to R. Harlan, who, in 1834, named a marine fossil vertebrate Basilosaurus in the Transactions of the American Philosophical Society. Basilosaurus means “king lizard,” but Harlan’s creature is an early whale. Richard Owen, England’s greatest anatomist, corrected Mr. Harlan before the decade’s end, but the name sticks — and must be retained by the official rules of zoological nomenclature. (The Linnaean naming system is a device for information retrieval, not a guarantor of appropriateness. The rules require that each species have a distinctive name, so that data can be obtained unambiguously with a stable tag. Often, and inevitably, the names originally given become literally inappropriate for the unsurprising reason that scientists make frequent mistakes, and that new discoveries modify old conceptions. If we had to change names every time our ideas about a species altered, taxonomy would devolve into chaos. So Basilosaurus will always be Basilosaurus because Harlan followed the rules when he gave the name. And we do not change ourselves to Homo horribilis after Auschwitz, or to Homo ridiculosis after Tonya Harding — but remain, however dubiously, Homo sapiens, now and into whatever forever we allow ourselves.)
Basilosaurus, represented by two species, one from the United States and the other from Egypt, is the “standard” and best-known early whale. A few fragments of pelvic and leg bones had been found before, but not enough to know whether Basilosaurus bore working hind legs — the crucial feature for our usual concept of a satisfying intermediate form in both anatomical and functional senses.
In 1990, Phil Gingerich, B. H. Smith, and E. L. Simons reported their excavation and study of several hundred partial skeletons of the Egyptian species Basilosaurus isis, which lived some 5 to 10 million years after Pakicetus. In an exiting discovery, they reported the first complete hind limb skeleton found in any whale — a lovely and elegant structure (put together from several partial specimens), including all pelvic bones, all leg bones (femur, tibia, fibula, and even the patella, or kneecap), and nearly all foot and finger bones, right down to the phalanges (finger bones) of the three preserved digits.
This remarkable find might seem to clinch our proof of intermediacy, but for one problem. The limbs are elegant but tiny (see the accompanying illustration), a mere 3 percent of the animal’s total length. They are anatomically complete, and they did project from the body wall (unlike the truly vestigial hind limbs of modern whales), but these miniature legs could not have made any important contribution to locomotion — the real functional test of intermediacy. Gingerich et al. write: “Hind limbs of Basilosaurus appear to have been too small relative to body size to have assisted in swimming, and they could not possibly have supported the body on land.” The authors strive bravely to invent some potential function for these minuscule limbs, and up speculating that they may have served as “guides during copulation, which may otherwise have been difficult in a serpentine aquatic mammal.” (I regard such guesswork as unnecessary, if not ill-conceived. We need not justify the existence of a structure by inventing some putative Darwinian function. All bodies contain vestigial features of little, if any, utility. Structures of lost usefulness in genealogical transitions do not disappear in a evolutionary overnight.)
Verdict: Terrific and exiting, but no cigar, and no bag-packer for creationists. The limbs, though complete, are too small to work as true intermediates must (if these particular limbs worked at all) — that is, for locomotion on both land and sea. I intend no criticism of Basilosaurus, but merely point out that this creature had already crossed the bridge (while retaining a most informative remnant of the other side). We must search for an earlier inhabitant for the bridge itself.
CASE THREE: Hind limb bones of appropriate size. Indocetus ramani is an early whale, found in shallow-water marine deposits of India and Pakistan, and intermediate in age between the Pakicetus skull and the Basilosaurus hind legs (cases one and two above). In 1993, P. D. Gingerich, S. M. Raza, M. Arif, M. Anwar, and X. Zhou reported the discovery of leg bones of substantial size from this species.
Gingerich and colleagues found pelvic bones and the end of both femur and tibia, but no foot bones, and insufficient evidence for reconstructing the full limb and its articulations. The leg bones are large and presumably functional on both land and sea. (the tibia, in particular, differs little in size and complexity from the same bone in the related and fully terrestrial mesonychid Pachyaena ossifraga). The authors conclude: “The pelvis has a large and deep acetabulum [the socket for articulation of the femur, or thighbone], the proximal femur is robust, the tibia is long … All these features, taken together, indicate the Indocetus was probably able to support its weight on land, and it was almost certainly amphibious, as early Eocene Pakicetus is interpreted to have been … We speculate that Indocetus, like Pakicetus, entered the sea to feed on fish, but returned to land to rest and to birth and raise its young.”
Verdict: Almost there, but not quite. We need better material. All the right features are now in place — primarily leg bones of sufficient size and complexity — but we need more and better-preserved fossils.
CASE FOUR: Large, complete, and functional hind legs for land and sea — finding the smoking gun. The first three cases, all discovered within ten years, surely indicate an increasingly successful paleontological assault upon an old and classic problem. Once you know where to look, and once high interest spurs great attention, full satisfaction often follows in short order. I was therefore delighted to read in the January 14, 1994, issue of Science, an article by J. G. M. Thewissen, S. T. Hussain, and M. Arif, titled “Fossil evidence for the origin of aquatic locomotion in archaeocete whales.”
In Pakistan, in sediments 120 meters above the beds that yielded Pakicetus (and therefore a bit younger in age), Thewissen and colleagues collected a remarkable skeleton of a new whale — not complete, but far better preserved than anything previously found of this age, and with crucial parts in place to illustrate a truly transitional status between land and sea. The chosen name, Ambulocetus natans (literally, the swimming walking-whale) advertises the excitement of this discovery.
Ambulocetus natans weighed some 650 pounds, the size of a hefty sea lion. The preserved tail vertebra is elongated, indicating that Ambulocetus still retained the long, thin mammalian tail, and had not yet transmuted this structure to a locomotory blade (as modern whales do in shortening the tail and evolving a prominent horizontal fluke as the animal’s major means of propulsion). Unfortunately, no pelvic bones have been found, but most elements of a large and powerful hind leg were recovered — including a complete femur, parts of the tibia and fibula, an astragalus (ankle bone), three metatarsals (foot bones), and several phalanges (finger bones). To quote the authors: “The feet are enormous.” The fourth metatarsal, for example, is nearly six inches long, and the associated toe almost seven inches in length. Interestingly, the last phalanx of each toe ends in a small hoof, as in terrestrial mesonychid ancestors.
Moreover, this new bounty of information allows us to infer not only the form of this transitional whale, but also, with good confidence, an intermediary style of locomotion and mode of life (an impossibility with the first cases, for Pakicetus is only a skull, Basilosaurus had already crossed the bridge, and Indocetus is too fragmentary). The forelimbs were smaller than the hind, and limited in motion; these front legs were, to quote the authors, “probably used in maneuvering and steering while swimming, as in extant cetaceans [“modern whales” in ordinary language], and they lacked a major propulsive force in water.”
Modern whales move through the water by powerful beats of their horizontal tail flukes — a motion made possible by strong undulation of a flexible rear spinal column. Ambulocetus had not yet evolved a tail fluke, but the spine had requis
ite flexibility. Thewissen et al. write: “Ambulocetus swam by means of dorsoventral [back-to-belly] undulations of its vertebral column, as evidenced by the shape of the lumbar [lower back] vertebra.” These undulations then functioned with (and powered) the paddling of Ambulocetus’s large feet — and these feet provided the major propulsive force for swimming. Thewissen et al. conclude their article by writing: “Like modern cetaceans — it swam by moving its spine up and down, but like seals, the main propulsive surface was provided by its feet. As such, Ambulocetus represents a critical intermediate between land mammals and marine cetaceans.”
Ambulocetus was no ballet dancer on land, but we have no reason to judge this creature as any less efficient than modern sea lions, which do manage, however inelegantly. Forelimbs may have extended out to the sides, largely for stability, with forward motion mostly supplied by extension of the back and consequent flexing of the hind limbs — again, rather like sea lions.
Verdict: Greedy Paleontologists, used to working with fragments in reconstructing wholes, always want more (some pelvic bones would be nice, for starters), but if you have had given me both a blank sheet of paper and a blank check, I could not have drawn you a theoretical intermediate any better or more convincing than Ambulocetus. Those dogmatists who can make white black, and black white, by verbal trickery will never be convinced by anything, but Ambulocetus is the very animal that creationists proclaimed impossible in theory.
Some discoveries in science are exiting because they revise or reverse previous expectations, others because they affirm with elegance something well suspected, but previously undocumented. Our four-case story, culminating in Ambulocetus, falls into the second category. This sequential discovery of picture-perfect intermediacy in the evolution of whales stands as a triumph in the history of paleontology. I cannot imagine a better tale for popular presentation of science, or a more satisfying, and intellectually based, political victory over lingering creationist opposition. As such, I present the story in this series of essays with both delight and relish.
Still, I must confess that this part of the tale does not intrigue me most as a scientist and evolutionary biologist. I don’t mean to sound jaded or dogmatic, but Ambulocetus is so close to our expectation for a transitional form that its discovery could not provide a professional paleontologist with the greatest of all pleasures in science — surprise. As a public illustration and sociopolitical victory, transitional whales may provide the story of the decade, but paleontologist didn’t doubt their existence or feel that a central theory would collapse if their absence continued. We love to place flesh upon our expectations (or put bones under them, to be more precise), but this kind of delight takes second place to the intellectual jolting of surprise.
I therefore find myself far more intrigued by another aspect of Ambulocetus that has not received much attention, either in technical or popular reports. For the anatomy of this transitional form illustrates a vital principle in evolutionary theory — one rarely discussed, or even explicitly formulated, but central to any understanding of nature’s fascinating historical complexity.
In our Darwinian traditions, we focus too narrowly on the adaptive nature of organic form, and too little on the quirks and oddities encoded into every animal by history. We are so overwhelmed — as well we should be — by the intricacy of aerodynamic optimality of a bird’s wing, or by the uncannily precise mimicry of a dead leaf by a butterfly. We do not ask often enough why natural selection had homed in upon this particular optimum—and not another among a set of unrealized alternatives. In other words, we are dazzled by good design and therefore stop our inquiry too soon when we have answered, “How does this feature work so well?”—when we should also be asking the historian’s questions: “Why this and not that?” or “Why this over here, and that in a related creature living elsewhere?”
To give the cardinal example from seagoing mammals: The two fully marine orders, Sirenia and Cetacea, both swim by beating horizontal tail flukes up and down. Since these two orders arose separately from terrestrial ancestors, the horizontal tail fluke evolved twice independently. Many hydrodynamic studies have documented both the mode and the excellence of such underwater locomotion, but researchers too often stop at an expression of engineering wonder, and do not ask the equally intriguing historian’s question. Fishes swim in a truly opposite manner — also by propulsion from the rear, but with vertical tail flukes that beat from side to side (seals also hold their rear feet vertically and move them from side to side while swimming).
Both systems work equally well; both may be “optimal.” But why should ancestral fishes favor one system, and returning mammals the orthogonal alternative? We do not wish to throw up our hands, and simply say “six of one, half a dozen of the other.” Either way will do, and the manner chosen by evolution is effectively random in any individual case. “Random” is a deep and profound concept of great positive utility and value, but some vernacular meanings amount to pure cop-out, as in this case. It may not matter in the “grand scheme of things” whether optimality be achieved vertically or horizontally, but one or the other solution occurs for a reason in any particular case. The reasons may be unique to an individual lineage, and historically bound — that is, not related to any grand concept of pattern or predictability in the overall history of life — but local reasons do exist and should be ascertainable.
This subject, when discussed at all in evolutionary theory, goes by the name of “multiple adaptive peaks.” We have developed some standard examples, but few with any real documentation; most are hypothetical, with no paleontological backup. (For example, my colleague Dick Lewontin loves to present the following case in our joint introductory course in evolutionary biology: some rhinoceros species have two horns, others one horn. The two alternatives may work equally well for whatever rhinos do with their horns, and the pathway chosen may not matter. Two and one may be compatible solutions, or multiple adaptive peaks. Lewontin then points out that a reason must exist for two or one in any case, but that the explanation probably resides in happenstances of history, rather than in abstract predictions based on universal optimality. So far, so good. History’s quirkiness, by populating the earth with a variety of unpredictable but sensible and well-working anatomical designs, does constitute the main fascination of evolution as a subject. But we can go no further with rhinos, for we have no data for understanding the particular pathway chosen in any individual case.)
I love the story of Ambulocetus because this transitional whale has provided hard data on reasons for a chosen pathway in one of our best examples of multiple adaptive peaks. Why did both orders of fully marine mammals choose the solution of horizontal tail flukes? Previous discussions have made the plausible argument that particular legacies of terrestrial mammalian ancestry established an anatomical predisposition. In particular, many mammals (but not other terrestrial vertebrates), especially among agile and fast-moving carnivores, run by flexing the spinal column up and down (conjure up a running tiger in your mind, and picture the undulating back). Mammals that are not particularly comfortable in water — dogs dog-paddling, for example — may keep their backs rigid and move only by flailing their legs. But semiaquatic mammals that swim for a living — notably the river otter (Lutra) and the sea otter (Enhydra) — move in water by powerful vertical bending of the spinal column in the rear part of the body. This vertical bending propels the body forward both by itself (and by driving the tail up and down), and by sweeping the hind limbs back and forth in paddling as the body undulates.
Thus, horizontal tail flukes may evolve in fully marine mammals because inherited spinal flexibility for movement up and down (rather than side to side) directed this pathway from a terrestrial past. This scenario has only been a good story up to now, with limited symbolic support from living otters, but no direct evidence at all from the ancestry of whales or sirenians. Ambulocetus provides this direct evidence in a most elegant manner — for all pieces of the puzzle lie within the
recovered fossil skeleton.
We may infer from a tail vertebra that Ambulocetus retained a long and thin mammalian tail, and had not yet evolved the horizontal fluke. We know from the spinal column that this transitional whale retained its mammalian signature of flexibility for up and down movement — and from the large hind legs that undulation of the back must have supplied propulsion to powerful paddling feet, as in modern otters.
Thewissen and colleagues draw the proper evolutionary conclusion from these facts, thus supplying beautiful evidence to nail down a classic case of multiple adaptive peaks with paleontological data: “Ambulocetus shows that spinal undulation evolved before the tail fluke ... Cetaccans have gone through a stage that combined hindlirnb paddling and spinal undulation, resembling the aquatic locomotion of fast swimming otters.” The horizontal tail fluke, in other words, evolved because whales carried their terrestrial system of spinal motion to the water.
History channels a pathway among numerous theoretical alternatives. In his last play, Shakespeare noted that “what’s past is prologue; what to come, in yours and my discharge.” But present moments build no such wall of separation between a past that molds us and a future under our control. The hand of the past reaches forward right through us and into an uncertain future that we cannot fully specify.
I wrote this essay in a flush of excitement during the week that Thewissen and colleagues published their discovery of the definitive intermediate whale Ambulocetus, in January 1994. With my lead time of three months from composition to the first publication of these essays in Natural History magazine, “Hooking Leviathan by Its Past” appeared in April 1994 — complete with central theme of a chronologically developing story in four stages.
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