ing 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 docu- 

 mented both the mode and the excellence 

 of such underwater locomotion, but they 

 too often stop at an expression of engi- 

 neering wonder and do not ask the equally 

 intriguing historian's question. Fishes 

 swim in a truly orthogonal manner — also 

 by propulsion from the rear, but with ver- 

 tical tail flukes that beat from side to side 

 (seals also hold their rear feet vertically 

 and move them from side to side while 

 swimming). The word orthogonal is par- 

 ticularly appropriate here — meaning, liter- 

 ally, "at right angles," but also, in technical 

 scientific parlance, "entirely indepen- 

 dent." 



Both systems work 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 \he other; 

 either way will do, and the one chosen is 

 effecdvely random in any individual 

 case." "Random" is a deep and profound 

 concept of great positive utihty and value, 

 but some vernacular meanings amount to 

 pure cop-out, as in this case. It may not 

 matter in the "great scheme of things" 

 whether optimaUty be achieved vertically 

 or horizontally, but one or the other solu- 

 tion occurs for a reason in any particular 

 case. The reasons may be unique to an in- 

 dividual lineage, and historically bound — 

 that is, not related to any grand concept of 

 pattern or predictability in the overall his- 

 tory 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 some 

 standard examples, but few with any real 

 documentation; most are hypothetical, 

 with no paleontological backup. (For ex- 

 ample, my colleague Dick Lewontin loves 

 to present the following case in our joint 

 introductory course in evolutionary biol- 

 ogy. Some rhinoceros species have two 

 horns; others one horn. Either result is 

 probably just as good for whatever rhinos 

 do with their horns, and the pathway cho- 

 sen may not matter. Two and one are equal 

 solutions, or multiple adaptive peaks. He 

 then points out that a reason must exist for 

 two or one in any case, but that the expla- 

 nation probably resides in happenstances 

 of history, rather than abstract predictions 

 based on universal optimality. So far so 



good. History's quirkiness, by its role in 

 populating the earth with a variety of un- 

 predictable, but sensible and well-work- 

 ing, anatomical designs does constitute the 

 main fascination of this theme in evolution 

 theory. But we can go no further with rhi- 

 nos, for we have no data for understanding 

 the particular pathway chosen in any 

 case.) 



1 love the story of Ambulocetus because 

 it has provided hard data on reasons for a 

 chosen pathway in a classical case of mul- 

 tiple adaptive peaks. Why did both orders 

 of fully marine mammals choose the or- 

 thogonal solution of horizontal tail flukes? 

 Previous discussions have made the plau- 

 sible argument that definite legacies of ter- 

 restrial mammalian ancestry established 

 the anatomical predisposition. In particu- 

 lar, many mammals (but not other terres- 

 trial vertebrates), especially among agile 

 and fast-moving carnivores, run by flexing 

 the spinal column up and down (conjure 

 up a sprinting tiger in your mind, and pic- 

 mre the undulation of the back). Mammals 

 that are not particularly comfortable in 

 water — dogs dog-paddhng for instance — 

 may keep their backs rigid and move only 

 by flailing their legs. But semiaquatic 

 mammals that swim for a hving — 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 flie body undulates. 



Thus, horizontal tail flukes may evolve 

 in fuUy marine mammals because inher- 

 ited spinal flexibihty 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 symbohc support from 



living otters, but no direct evidence from 

 the ancestry of whales or sirenians. Ambu- 

 locetus provides this direct evidence in an 

 elegant manner — ^for all pieces of the puz- 

 zle lie within its recovered 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 flexi- 

 bility for up-and-down movement — and 

 from the large hind legs that undulation of 

 the back must have propelled the power- 

 ful, paddling feet, as in modem otters. 



Thewissen and colleagues draw the 

 proper evolutionary conclusion from these 

 facts, thus supplying beautiful evidence to 

 nail down a classical case of multiple 

 peaks with paleontological data: ''Ambulo- 

 cetus shows that spinal undulation evolved 

 before the tail fluke.... Cetaceans have 

 gone through a stage that combined hind 

 limb paddling and spinal undulation, re- 

 sembling 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 nu- 

 merous theoretical altematives. 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 con- 

 trol. The hand of the past reaches forward 

 right through us and into an uncertain fu- 

 ture that we cannot fuUy specify. History 

 has you and me, brother and sister — the 

 whole world — in her hands. 



Stephen Jay Gould teaches biology, geol- 

 ogy, and the history of science at Har\>ard 

 University. 



"I guess the hardest thing for me growing up was when I realized 

 I wasn 't going to be an eagle. " 



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