Wise Young
11-08-2006, 01:39 PM
Surely this would only imply an evolutionaty link between humans and fish if humans retained remnants of fish-like gills into adult hood. As dolphins retain pelvic-bones into adult-hood I feel the relevance of these bones as evidence about their more recent evolutionary origins is greater, and here I use the word recent on the geologic scale so fifty million years is much more recent than the connection between humans as possible ancestors, of ancestors, of ancestors of organisms that had gills thousands of millions of years ago.
Adrian,
Sorry, I should be more clear in my criticism of the unscientific reasoning that is being used to interpret skeletal changes. I gave the example of gills as something that is simply wrong and has been debunked for decades and yet continues to be taught as science. The linked commentary that I included debunked the gill hypothesis by pointing out that the so-called gill slits are really not gills.
For many years, attempts to identify specific skeletal features were used asarguments for evolution. Many of these attempts led to dead ends or, worse, to wasting of much resources to finding fossil records that support fanciful interpretations. Anthropology was dominated by attempts to attribute evolution of walking and intelligence from observations of the pelvis and skull. The problem with such approaches is that they do not provide testable hypotheses that allow cause-effect conclusions.
In my opinion, many of the skeletal changes theories were flawed by unwarranted assumptions and speculations. For example, for many years, anthropologists attempted to classify humanoids by the size of the brain, assuming that brain size predicts intelligence. However, the Neanderthal brain is quite large. The whale and elephant brains are huge. Likewise, the assumption that the upright posture is somehow superior or allowed the development of hand function is very speculative.
Now, if there were a way to test these hypotheses, e.g. show a direct genetic evolutionary lineage that predicted skeletal changes or a clear-cut demonstration that the appearance of a particular skeletal change resulted in specific changes in behavior and other developmental changes, such hypotheses might be warranted. However, until then, such speculations are nothing more than speculations.
So, returning to the story, a scientist apparently has found additional appendages on a single dolphin and jumped from this finding to the conclusion that dolphins were once land mammals. Why is this not just a developmental abnormality? Yes, it is an attractive theory, that dolphins once walked and somehow went from land to sea, but this finding is not sufficient evidence to warrant the conclusion.
Just because a theory is attractive and "makes sense" doesn't mean that it is true. What if mammals first developed in water and then came onto land? To test this hypothesis, one would need to show that the first mammals appeared on land and then later migrated into the sea. Unfortunately, we don't have such evidence, or at least not convincing evidence of this yet (at least that I know of). The reason is that fossils don't really tell us whether a given fossil was necessarily mammalian. For example, scientists are still debating whether dinosaurs were warm-blooded, bird-like, had hair or feathers.
Wise.
Wise Young
11-12-2006, 10:35 PM
I've been following this thread topic and have to wonder if you have a problem with / are resistant to, the explanation that whales and dolphins evolved (or "devolved":D, as the case would seem to be*) into marine species from land dwelling and ambulatory ones.
*This is in jest as the traditional early through mid 20th century notion of "progress" in evolution has been debunked for at least the last four decades, so the development of legs is in no way superior-to or more advanced than, a development lacking legs. Hence the loss or partial loss of legs is not "devolution".:agog::nono:
Juke,
Sorry. I didn't think that anybody would be sufficiently interested in the details of cetacean evolution to want to know the details. Please understand that I am not necessarily opposed to the idea that dolphins and other marine mammals evolved from land-inhabiting, ambulating mammals. The evolutionary origin of the whales has long been an issue of great contention between evolutionists and creationists. Unfortunately, the dogma of cetacean evolution, as it is taught in science textbooks is not well supported by evidence and is vulnerable to criticism.
Here is the dogma. Dolphins are cetaceans, believed to have derived from a family of early terrestrial animals called Mesonychidae (Source (http://library.advanced.org/17963/evolution.html)). The common ancestor of the cetaceans and the ungulate mammals (artiodactyla) was thought be condylarthrans, which apparently lived on the edges of slow rivers and the seas. All this occurred over 50 million years ago. The condylarthrans were thought to be hippopotamus-like animals that fed on molluscs and slow fish.
There is very strong (and rigorous) genetic evidence that modern cetaceans are genetically closely related to ungulata (hoofed animals), particularly cows and pigs. In fact, analysis of mitochondrial DNA of the pig shows that pigs are a sister group of a whale/cow clade that split at a date that has been molecularly identified as 65 million years ago. The whale/cow clade apparently precedes the appearance of both ungulata and cetaceans, indicating a yet-to-be-identified common ancestor (Source (http://www.springerlink.com/content/qlyhh9pmxwndke67/)). However, the genetic evidence does not rule out the possibility that the common ancestor of the cow and the whale was marine rather than terrestrial.
In contrast, the fossil evidence that supports the Mesonychidae origin of Cetaceans is weak and limited (Source (http://palaeo-electronica.org/1998_2/ging_uhen/what.htm)). In a book entitled "Emergence of Whales", a passage in chapter 6 illustrates the skepticism and debate regarding fossil-based evidence for the origins of cetaceans:
http://home.tiac.net/~cri/2001/acker06.html
"Unresolved Phylogenetic Issues"
Well, it's the Mesonychians vs. the Artiodactyls again. A much-referenced 1966 paper by Van Valen is apparently the foundation of the Mesonychian ancestry for cetaceans plan.
<snip>
Van Valen noted that the Mesonychids were "weird" Carnivora and moved them to Condylartha. That got fixed in 1975, when everybody was put into Ungulata. In 1997, to sort this mess out, Cetacea was reduced to a "subordinal" level and they, with Mesonychidae, Triisodontinae, and Hapalodectidae, form Order Cete. Class Acreodi excludes Cetacea. This classification scheme was devised to help minimize paraphyletic groups (which apparently bother both cladists and creationists).
Molecular evidence indicated that the Cetacea sister group is within Ungulata. Evidence to that effect has appeared in previous chapters. The authors summarize and give profuse references. It boils down to this:
"The possibility of Cetacea originating within Artiodactyla has been viewed with skepticism by most paleontologists because of the clear morphological evidence for the monophyly of Artiodactyla to the exclusion of Cetacea." (refs) "It should be noted that most of these characters occur on the astragalus. Although they appear to be independent, it is possible that they are related and may be overweighted by morphologists." (Reflective of the comments at the end of Chapter 4.)
Morphological analysis tends to find Cetacea closer to Perissodactyla than Artiodactyla. So obviously questions remain to be answered.
The evolutionary origin of the whale has loomed large in the evolutionist versus creationist debate (Source (http://www.trueorigin.org/whales.asp)). Steven Jay Gould took the subject head-on in an April 1994 article named "Hooking Leviathan by its Past" (Source (http://www.stephenjaygould.org/library/gould_leviathan.html)):
Darwin's Origin of Species contains some wonderful insights and magnificent lines, but this masterpiece also includes a few notable clunkers. Darwin experienced most embarrassment from the following passage, curtailed and largely expunged from later editions of his book:
In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.
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 theories—as Darwin tried to do in advancing natural selection as the primary mechanism of evolution—they need particularly good support, and invented hypothetical cases just don't supply sufficient confidence for crucial conclusion
<snip>
Gould goes on to point out that how creationists keep harping on the lack of transitional forms:
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 reasons—geological (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 theory—for 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.)
Gould then points out that three major groups of mammals have "returned to the ways of distant ancestors"... implying that the cetaceans have evolved to go into the sea, without questioning the assumption:
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 orders—the Sirenia (dugongs and manatees) and Cetacea (whales and dolphins). I confess that I have never quite grasped the creationists' point about inconceivability of transition—for 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, breeding 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 all—and 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 theory—the 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.
Gould goes on to make several "cases" for the evolution of the whale. The first case is as follows:
CASE ONE: Discovery of the oldest whale. Paleontologists have been fairly confident, since Leigh Van Valen's demonstration in 1996, that whales descended from mesonychids, an early group of primarily carnivorous running mammals that spanned a great range of sizes and habits from eating fishes at river edges to crushing bones of carrion. Whales must have evolved during the Eocene epoch, some 50 million years ago, because Late Eocene and Oligocene rocks already contain fully marine cetaceans, well past any point of intermediacy.
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.
Gould then made his second case:
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.
Again, as you can see, not yet convincing. Gould goes on with his third case:
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.
It is not time to light the cigar yet! So, Gould makes his fourth case:
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.
<snip>
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.
In 1997, Gould wrote an epilogue which included the following illustration:
http://www.stephenjaygould.org/images/futuyma.gif
[ 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.
So, is this the end of the story for the evolution of Cetaceans? I am not so sure. Let me ask three questions:
Is there any evidence that Ambulocetus gave rise to Rodhocetus?
Is there any evidence that Mesonychid gave rise to Ambulocetus?
Is there any evidence for linkage of these forms other than their existence within several million years of each other and some remote similarity of their fossils?
The answer to all three of these questions is no. It is a very pretty story and I love Stephen Jay Gould but this is not convincing. I think that if I wrote a paper on spinal cord injury based on such sparse and speculative evidence to argue for the presence of regeneration in the spinal cord, my paper would have been rejected with laughter. It seems to me that, if we are going to do science, we should do good science. The above cases are filled with speculations and unwarranted assumptions, and gaps in evidence. If we don't know, we should say that we don't know.
Wise.