Why did Darwin become so chagrined about this passage? His
hypothetical tale may be pure speculation and conjecture, but the scenario is not
entirely absurd. Darwin's discomfort arose, I think, from his failure to follow a
scientific norm of a more sociocultural nature. Scientific conclusions supposedly
rest upon facts and information. Speculation is not entirely taboo, and may
sometimes be necessary faute de mieux. But when scientists propose truly
novel and comprehensive theoriesas Darwin tried to do in advancing natural
selection as the primary mechanism of evolutionthey need particularly good
support, and invented hypothetical cases just don't supply sufficient confidence
for crucial conclusions.
Natural selection (or the human analogue of differential
breeding) clearly worked at a small scalein the production of dog breeds
and strains of wheat, for example. But could such a process account for the
transitions of greater scope that set our concept of evolution in the fullness of
timethe passage of reptilian lineages to birds and mammals; the origin of
humans from an ancestral stock of apes? For these larger changes, Darwin could
provide little direct evidence, for a set of well known and much-lamented reasons
based on the extreme spottiness of the fossil record.
The supposed lack of intermediary forms in the fossil record
remains the fundamental canard of current antievolutionism. Such transitional
forms are sparse, to be sure, and for two sets of good reasonsgeological
(the gappiness of the fossil record) and biological (the episodic nature of
evolutionary change, including patterns of punctuated equilibrium, and transition
within small populations of limited geographic extent). But paleontologists have
discovered several superb examples of intermediary forms and sequences, more
than enough to convince any fair-minded skeptic about the reality of life's
physical genealogy.
The first terrestrial vertebrates retained six to
eight digits on each limb (more like a fish paddle than a hand), a persistent
tailfin, and a lateral-line system for sensing sound vibrations underwater. The
anatomical transition from reptiles to mammals is particularly well documented in
the key anatomical change of jaw articulation to hearing bones. Only one bone,
called the dentary, builds the mammalian jaw, while reptiles retain several small
bones in the rear portion of the jaw. We can trace, through a lovely sequence of
intermediates, the reduction of these small reptilian bones, and their eventual
disappearance or exclusion from the jaw, including the remarkable passage of the
reptilian articulation bones into the mammalian middle ear (where they became our
malleus and incus, or hammer and anvil). We have even found the transitional form
that creationists often proclaim inconceivable in theoryfor how can
jawbones become ear bones if intermediaries must live with an unhinged jaw before
the new joint forms? The transitional species maintains a double jaw joint, with
both the old articulation of reptiles (quadrate to articular bones) and the new
connection of mammals (squamosal to dentary) already in place! Thus, one joint
could be lost, with passage of its bones into the ear, while the other
articulation continued to guarantee a properly hinged jaw.
Still, our creationist incubi, who would never let facts spoil a
favorite argument, refuse to yield, and continue to assert the absence of all
transitional forms by ignoring those that have been found, and continuing to taunt
us with admittedly frequent examples of absence. Darwin's old case for the origin of
whales remains a perennial favorite, for Darwin had to invent a fanciful swimming
bear, and if paleontologists haven't come to the rescue by discovering an
intermediary form with
functional legs and potential motion on land, then Jonah's scourge may gobble up
the evolutionary heathens as well. God's taunt to Job might be sounded again:
Canst thou draw out leviathan with a hook? (The biblical Leviathan is
usually interpreted as a crocodile, but many alternative readings favor whales.)
Every creationist book on my shelf actually cites the absence
of and inherent inconceivability of transitional forms between terrestrial mammals
and whales. Alan Haywood, for example, writes in his Creation and
Evolution (see bibliography):
Duane Gish, creationism's most ardent debater, makes the same argument in his
more colorful style (Evolution: The Challenge of the Fossil Record):
Three major groups of mammals have returned to the ways of
distant ancestors in their seafaring modes of life (while smaller linkages within
several other mammalian orders have become at least semiaquatic, often to a
remarkable degree, as in river and sea otters): the suborder Pinnepedia (seals,
sea lions, and walruses) within the order Carnivora (dogs, cats, and Darwin's
bears among others); and two entire ordersthe Sirenia (dugongs and
manatees) and Cetacea (whales and dolphins). I confess that I have never quite
grasped the creationists' point about inconceivability of transitionfor a
good structural (though admittedly not a phylogenetic) series of intermediate
anatomies may be extracted from these groups. Otters have remarkable aquatic
abilities, but retain fully functional limbs for land. Sea lions are clearly
adapted for water, but can still flop about on land with sufficient dexterity
to negotiate ice floes, breading grounds, and circus rings.
But I admit, of course, that the transition to manatees and
whales represents no trivial extension, for these aquatic mammals propel
themselves to powerful, horizontal tail flukes and have no visible hind limbs
at alland how can a lineage both develop a flat propulsive tail from the
standard mammalian length of rope, and then forfeit the usual equipment of back
feet so completely? (Sirenians have lost every vestige of back legs; whales
often retain tiny, splintlike pelvic and leg bones, but no foot or finger bones,
embedded in musculature of the body wall, but with no visible expression in
external anatomy.)
The loss of back legs, and the development of flukes, fins,
and flippers by whales, therefore stands as a classical case of a supposed
cardinal problem in evolutionary theorythe failure to find intermediary
fossils for major anatomical transitions, or even to imagine how such a bridging
form might look or work. Darwin acknowledged the issue by constructing a much
criticized fable about swimming bears, instead of presenting any direct evidence
at all, when he tried to conceptualize the evolution of whales. Modern
creationists continue to use this example and stress the absence of intermediary
forms in this supposed (they would say impossible) transition to land to sea.
Goethe told us to love those who yearn for the
impossible. But Pliny the Elder, before dying of curiosity by staying too
close to Mount Vesuvius at the worst of all possible moments, urged us to treat
impossibilities as a relative claim: How many things, too, are looked upon
as quite as impossible until they have been actually effected. Armed with
such wisdom of human ages, I am absolutely delighted to report that our usually
recalcitrant fossil record has come through in exemplary fashion. During the past
fifteen years, new discoveries in Africa and Pakistan have greatly added to our
paleontological knowledge of the earliest history of whales. The embarrassment
of past absence has been replaced by a bounty of new evidenceand by the
sweetest series of transitional fossils an evolutionist could ever hope to find.
Truly we have met the enemy and he is now ours. Moreover, to add blessed insult
to the creationists' injury, these discoveries have arrived in a gradual and
sequential fashiona little bit at a time, step by step, from a tentative
hint fifteen years ago to a remarkable smoking gun early in 1994. Intellectual
history has matched life's genealogy by spanning gaps in sequential steps.
Consider the four main events in chronological order.
In 1983, my colleague Phil
Gingerich from the University of Michigan, along with N. A. Wells, D. E. Russell,
and S. M. Ibrahim Shah, reported their discovery of the oldest whale, named
Pakicetus to honor its country of present residence, from Middle Eocene
sediments some 52 million years old in Pakistan. In terms of intermediacy, one could
hardly have hoped for more from the limited material available, for only the skull of
Pakicetus has been found. The teeth strongly resemble those of terrestrial
mesonychids, as anticipated, but the skull, in feature after feature, clearly belongs
to the developing lineage of whales.
Both the anatomy of the skull, particularly in the ear region, and the inferred
habitat of the animal in life, testify to transitional status. The ears of modern
whales contain modified bones and passageways that permit directional hearing in the
dense medium of water. Modern whales have also evolved enlarged sinuses that can be
filled with blood to maintain pressure during diving. The skull of Pakicetus
lacks both these features, and this first whale could neither dive deeply nor hear
directionally with any efficiency in water.
In 1993, J. G. M.
Thewissen and S. T. Hussain affirmed these conclusions and added more details
on the intermediacy of skull architecture in Pakicetus. Modern whales achieve
much of their hearing through their jaws, as sound vibration pass through the jaw
to a fat pad (the technical literature, for once, invents no jargon
and employs the good old English vernacular in naming this structure), and thence
to the middle ear. Terrestrial mammals, by contrast, detect most sound through the
ear hole (called the external auditory meatus, which means the same
thing in more refined language). Since Pakicetus lacked the enlarged jaw
hole that holds the fat pad, this first whale probably continued to hear through
the pathways of its terrestrial ancestors. Gingerich concluded that the
auditory mechanism of Pakicetus appears more similar to that of land
mammals than it is to any group of extant marine mammals.
As for place of discovery, Gingerich and colleagues found Pakicetus
in river sediments bordering an ancient sea an ideal habitat for the
first stages of such an evolutionary transition (and a good explanation for lack
of diving specialization if Pakicetus inhabited the mouths of rivers and
adjacent shallow seas). My colleagues judged Pakicetus as an
amphibious stage in the gradual evolutionary transition of primitive whales from
land to sea
Pakicetus was well equipped to feed on fishes in the
surface waters of shallow seas, but it lacked auditory adaptations necessary for
a fully marine existence.
Verdict: In terms of intermediacy, one could hardly hope for more from
the limited material of skull bones alone. But the limit remains severe, and the
results therefore inconclusive. We know nothing of the limbs, tail, or body form
of Pakicetus, and therefore cannot judge transitional status in these key
features of anyone's ordinary conception of a whale.
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
requisite 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
optimumand 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.
I think of the old spiritual: Sometimes I get discouraged, and think
my work's in vein. But then the Holy Spirit revives my soul again. I'm
actually a fairly cheerful soul, but we all need replenishment now and then. If
there is a balm in Gilead (the song's title) for scientists, that elixir,
that infusion of the holy spirit, takes the form of new discoveries. on the very
week of my essay's publication, Phil Gingerich and colleagues (see bibliography)
published their description of yet another intermediate fossil whale, a fifth tale
for this gorgeous sequence of evolutionary and paleontological affirmation. (I did
feel a bit funny about the superannuation of my essay on the day of its birth, but
all exciting science must be obsolescent from inception and I knew I could
write this epilogue for my next book!)
Gingerich and colleagues discovered and named a new fossil Eocene whale from
Pakistan, Rodhocetus kasrani (Rodho for the local name of the
region, kasrani for the group of Baluchi people living in the area).
Rodhocetus, estimated at some ten feet in length, lived about 46.5 million
years ago. This new whale is thus about 3 million years younger than the
smoking gun Ambulocetus (Case Four and the key story in the main
essay), and about the same age as Indocetus (stage three in the main essay).
No forelimb bones have been found, and the spinal column lacks tail vertebrae, but
much of the skull has been recovered with, perhaps more important, a nearly complete
vertebral column from the neck all the way back to the beginning of the tail. Most
of the pelvis has also been found and, crucial to evidence about intermediacy,
a complete femur (but no other elements of the hind limb).
We may summarize the importance of Rodhocetus, and its gratifying extension
of our story about hard evidence for intermediacy in the evolution of
whales from terrestrial ancestors, by summarizing evidence in the three great categories
of paleontological data: form (anatomy), habitat (environment), and function.
FORM. I was most struck by two features of Rodhocetus's
anatomy. First, the excellent preservation of the vertebral column provides good
evidence of intermediacy in a mixture of features retained from a terrestrial past
with others newly acquired for an aquatic present. The high neural spines (upward
projections) of the anterior thoracic vertebrae (just behind the neck) support muscles
that help to hold up the head in terrestrial animals (not a functional necessity in
the buoyancy of marine environments; whales evolved from a terrestrial group, the
mesonychids, with particularly large heads). Direct articulation of the pelvis with
the sacrum (the adjacent region of the vertebral column) also characterizes both
Rodhocetus and terrestrial mammals (where gravity requires this extra
strength), but does not occur in modern whales. Gingerich and colleagues conclude:
These are primitive characteristics of mammals that support their weight on
land, and both suggest that Rodhocetus or an immediate predecessor was still
partly terrestrial.
But other features of the spinal column indicate adaptation for swimming: short
cervical (neck) vertebrae, implying rigidity for the front end of the body (good for
cutting through the water as the rear parts of the animal provide propulsion); and,
especially, the seamless flexibility of posterior vertebrae (sacral vertebrae are
fused together in most large terrestrial mammals, but unfused in both modern whales
and Rodhocetus), an important configuration for providing forward thrust in
swimming. Gingerich and colleagues conclude: These are derived characteristics
of later archaeocetes [ancient whales] and modern whales associated with aquatic
locomotion.
Second, and even more striking for this essay's case of graded intermediacy,
sequentially discovered during the past twenty years, Rodhocetus is about
3 million years younger than the smoking gun Ambulocetus (a
marine whale with limbs large enough for movement on land as well), and a good deal
older than later whales that had already crossed the bridge to fully marine life
(Basilosaurus, my Case Two, with well-formed but tiny hind limbs that could
not have functioned on land, and probably didn't do much in water either). In the
most exciting discovery in this new Case Five, the femur of Rodhocetus is
about two thirds as long as the same bone in the older Ambulocetus
still functional on land (probably), but already further reduced after 3 million
additional years of evolution.
[ Adapted from Douglas Futuyma,
Evolutionary
Biology, New York: Sinauer, 1998, p. 196. ]
HABITAT. Rodhocetus is the oldest whale from
fully and fairly deep marine waters. The oldest of all whales, Pakicetus of
Case One, lived around the mouths of rivers; Ambulocetus and Indocetus
of Cases Three and Four inhabited very shallow marine waters. Interestingly, the more
fully marine habitat of Rodhocetus correlates with greater reduction of the
hind limb, for Indocetus is a contemporary of Rodhocetus, yet grew a
larger femur comparable in length with the earlier Ambulocetus. (All three
creatures had about the same body size). Thus, admittedly on limited evidence, limbs
decreased in size over time and became smaller faster in whales from more fully
marine environments. (Perhaps Rodhocetus had already ceased making excursions
on land, while the earlier Ambulocetus, with a larger femur, almost surly
inhabited both land and water.) In any case, the contemporaneity of Rodhocetus
(shorter femur and deeper water) and Indocetus (longer femur with life in
shallower water) illustrates the diversity that already existed in cetacean evolution.
Evolution, as I always say, no doubt to the point of reader's boredom, is a copiously
branching bush, not a ladder.
FUNCTION. Rodhocetus lacks tail vertebrae, so we
can't tell for sure whether or not this whale had yet evolved a tail fluke. But
evidence of the beautifully preserved spinal column particularly the unfused
sacral vertebrae, making, in the words of Gingerich et al., the
lumbocaudal [back to tail] column seamlessly flexible indicates strong
dorsoventral (back to belly) flexion at the rear end of the body the
prerequisites for swimming in the style of modern whales (with propulsion provided
by a horizontal tail fluke, driven up and down by bending the vertebral column).
I was particularly pleased by this result, since I closed my essay with a
mini-disquisition on multiple adaptive peaks and the importance of historical
legacies, as illustrated by vertical tail fins in fishes vs. horizontal flukes in
whales both solutions working equally well, but with whales limited to this
less familiar alternative because they evolved from terrestrial ancestors with backs
that flexed dorsoventrally in running. Gingerich and colleagues conclude: This
indicates that the characteristic cetacean mode of swimming by dorsoventral
oscillation of a heavy muscled tail evolved within the first three million years or
so of the appearance of the archaeocetes.
A tangential comment in closing. The sociology of science includes much that I
do not like, but let us praise what we do well. Science at its best is happily and
vigorously international
(see essay 20)
and I can only take great pleasure in the following list of authors for research
done in an American lab based on fieldwork in Asia, supported by the Geological
Survey of Pakistan: Philip D. Gingerich, S. Mahmood Raza, Muhammad Arif, Mohammad
Anwar, and Xiaoyuan Zhou. Bravo to you all. I also couldn't help noting the paper's
first sentence: The early evolution of whales is illustrated by partial skulls
and skeletons of five archaeocetes of Ypresian (Early Eocene) . . . age. The
geological time scale is just as international, for our fossil record is a global
scheme for correlating the ages of rocks. So a layer of sediments in Pakistan may be
identical as representing a time named in a place that later became the bloodiest
European battle site of World War I the dreaded Ypres (or Wipers
as British solders named and pronounced their hecatomb).
But so much for lugubrious and sentimental thoughts. Let's just end in the main
essay's format for our new case of Rodhocetus:
CASE FIVE. Open and shut.
Verdict: sustained in spades, wine and roses.