Warning: Lot of Largish Graphics Below
For years Creationists have attacked evolution. It continues to this day, sometimes with breathtaking ignorance and outright dishonesty as in Creationism for Dummies. And one of their many lines of assault has been the incessantly repeated claim that "There are no transitional fossils!" For years in the 1970s and 1980s Young Earth Creationists used whales as an angle with which to pursue their agenda. 'Where are the transitional whales' they would ask in mock surprise, and then go on to conclude that because there were none, yet, we never would have any, and therefore, somehow, Young Earth Creationism "wins". They don't say that any longer ...
A word on what a transitional fossil specimen really is: A transitional is not necessarily directly ancestral to the later organism. It might be, but we really can't know for certain if a given transitional is the direct forebear of a later species or not. Consider the paleontologist who finds the partial skeleton of a Dachshund, a red fox, and a wolf. Is one ancestral to the other? Which way does the lineage flow; from big to little or vice-versa? What if you have only a few leg bones and some pelvis for the red fox, a partial skull of the Dachshund, and the lower jaw and a scrap of spine for the wolf? And what if you'd never seen a living canid of any kind! That's the dilemma paleontologists are in when they try to assign ancestry. It's pretty amazing, a testament to the dedication and expertise of paleobiologists, that they've been as successful as they have.
A transitional fossil only means that the intermediate exemplar possesses morphological characteristics (Physical features) which are also present in both modified and/or unmodified versions in a later, likely descendent, species. In a very broad sense, every fish is a transitional tetrapod, because every fish has a spinal column and fins which are present in later modified (Fins to limbs) and relatively unmodified (Spinal column) versions in all in land vertebrates such as amphibians or mammals. This means that a transitional population can over lap descendent populations in time: wolves are transitional to Dachshunds and directly ancestral to them, but wolf populations didn't magically disappear when Dachshunds came to be!
Likewise, in another broad sense, every reptile is a transitional bird, because birds have modified arms they use as wings and relatively unmodified spines both of which are present in reptiles. That doesn't mean every fish is directly ancestral to every species of reptiles or rodent or that every reptile is ancestral to every species of bird; all fish and all reptiles are obviously not directly ancestral to those respective clades!
Whales are closely related to Mesonychids but not directly derived from them. Mesonychids are a diverse group of proto-hoofed predators which first arose in the late Paleocene Epoch, beginning about 65 million years ago, in the ashes of a world still reeling from the K-T Impact which may have escorted the dinosaurs off the evolutionary stage. They diversified into a number of species. Some of them, especially early on, were the size of a collie, others rank as the largest mammalian land predator to ever tip the scales, such as Andrewsarchus above.
Because of this semi-hoofed ancestry, DNA comparisons, and recent fossil evidence, Whales are now technically classified in the order of Artiodactyls, meaning 'even toed'. This order includes hippos, giraffes, and pigs.
Illustration courtesy UTI commentator, contributor, and renowned wildlife artist, O'George
The first notable transition in our tale of the whale is a kind of cute looking critter called Pakicetus, from over fifty-million years ago early in the Eocene Epoch. This fellow was a lot like a big, bad, wolf in both size and habits, and probably occupied a similar eco-niche as an ambush predator does today.
The next stop in our whale's tale is what looks for all the world like the mammalian version of a crocodile: Ambulocetus meaning "walking whale", at about 49 million years ago. This is a bad boy, a mean looking customer, long, powerful, low slung, partially aquatic, and the jaws are outfitted to clamp down, hold on to, and crush prey. It probably made it's living a lot like crocs do; snapping up fish from the water and unwary mammals from the beckoning shores of lakes and bays.
Speaking of intermediates, how does Rodhocetus above look for a transitional whale candidate in both skeleton and artist's rendition? This fellow was roaming the south Asian seas at 45-50 million years ago.
Then we get into Dalanistes at circa 45 MYA. Followed by Protcetidae some of which were the first fully aquatic whale ancestors, and then on into full blown early whales such as archaic Odontocetids (Early toothed whales), like Dorudons and Basilisaurids by about 40 million years ago-the latter incidentally still trailing tiny, but fully formed, rear legs
Basilosaurid skeleton. Note the vestigial rear leg bones. It possible Baslisaurus may have used these small limbs to lock together during mating
How do we know that these fossils, in some case just partial skulls, are really whales ancestors or early whales? Well, it happens that the Whale Ear, from the Pakicetus to modern whales, is highly distinctive and easily recognized if you know what to look for. The ear slowly turned into the receiver for cetacean sonar, preserving and modifying those distinguishable features during that transition. Since the ear cavity and inner structure is part of the skull and the crania in general is one of the most likely portions of the skeleton to be preserved as a fossil, the morphology of that ear cavity is a fantastically useful diagnostic trait for determining if you're dealing with a whale and for discerning the particular stage of evolution of that specimen. Evolutionary biologist PZ Myers provides an outline:
Start with the top left diagram. This is the ear of a typical modern land mammal. The horn shaped structure angling down towards 8 o'clock is the external auditory meatus (EAM), your ear hole, which leads to the dark gray oval, the tympanic membrane (TyMe), better known as the ear drum. Sound in air travels down the EAM to the TyMe, which vibrates. The vibrations are amplified the the chain of inner ear bones, the malleus (Mal), incus (Inc), and stapes (Sta), or hammer, anvil, and stirrup (I know--the terminology gets a little dense). Finally, the amplified vibrations are transmitted to the cochlea, where they are transduced into localized deflections of hair cells that trigger pitch-specific nerve impulses.
That's the path that works well in the air, but it doesn't work so well in water. Try immersing your head in the bathtub or swimming pool, though, and sounds are immediately dampened; the EAM fills with water that puts pressure on the eardrum, reducing the amplitude. Instead, the vibrations are transmitted through the bones and tissues of the head, vibrating the tympanic bone (TyBo) and by that path the inner ear bones.
The next three diagrams show the progression of changes in the whale lineage. The top right picture (b) is a pakicetid from about 50 million years ago. It's not much different from the generic land mammal, with an EAM, eardrum, etc., but note the one special feature: the tympanic bone isn't connected to the periotic bone (Per), and it's actually thickened into a structure called the involucrum. Basically, the bony structure of the ear is less tightly attached to the skull, and is more free to vibrate in response to sound transmitted through the tissue of the head.
The next step is seen in a group of whales called the remingtonocetid/protocetids, from 43-46 million years ago (c). The ear capsule is even less strongly attached to the skull, and the involucrum is more robust and even more remote from the skull--the whole thing is better at moving freely. The ear drum is reduced and conical in shape, and the malleus is fused to the bone, so although the pieces are all there, it's not going to be particularly effective at capturing sound waves in air. Another feature is a deep groove in the mandible that indicates that these animals had a fat pad (FaPa) in the jaw that would better transmit vibrations from the jaw bone to the ear capsule.
Last (d) is the ear structure of a modern whale. All of the trends of the previous organisms are accentuated: the ear capsule is specialized to receive sounds transmitted through the fat pad, and has completely given up on sounds transmitted through air--the external auditory meatus is closed off and gone, and while the eardrum is present, it's not connected to the external world.
In addition to the unique morphology of the ear canal and bones, whales also have a distinctive spine which was modified over time to provide motive force underwater. Most folks understand that whales move by swinging their large tail flukes up and down, rather than side to side as most fish. The modifications in the spine, especially around the pelvic girdle, is a dead giveaway if you have the requisite vertebrae.
Now that we've met a few of the actors in our Whale's Tale, and armed with new found fossil specimens, ear analysis, and DNA comparisons, we can construct a rough, provisional evolutionary progression, showing a plausible series of whale transitional, starting with a fully terrestrial animal and ending with modern cetaceans. Thanks again to O'George's artistic skills here it is:
It's not meant to be a perfect line of ascending cetaceans and the sporadic nature of the fossil record ensures it never will be. We don't know for a fact who or what is directly ancestral to later specimens or any intermediate, or which ones are ancestral to which whales, or even if any of early lineage's shown survived. Modern whales may be the descendants of a small offshoot of one of the earlier protocetids and never preserved, which we will therefore never know about in detail. But sequentially as a gross series, that's all pretty damn reasonable. Each intermediate is definitely a whale or an indirect/direct precursor to a whale. Each is found in the correct chronological order in the geological record and exhibits the clear signs of a transitional form progressing from a four legged terrestrial mammal to a full blown, aquatic one.
Creationists claim we have No transitional fossils? Bullshit: Utterly fallacious! Here's hundreds of vertebrate intermediates alone, whales are just the tip of the iceberg. For the most part Creationist claims that we have no intermediate candidates for whales have fallen silent. Now when they address the topic of whale evolution, they focus on disinformation regarding the rich series of transitionals whales we do have.
Of course the everyday Creationist victim you're likely to encounter at work, church, or school, will sometimes pipe up with "Why aren't dogs evolving into whales now, right now, before my very eyes?" Well obviously the question is loaded: Dogs did not evolve into whales in the first place and any future evolution is not going to produce a creature exactly like whales. Nor would the scope of human life permit such an observation. But we certainly have all kinds of examples of clearly terrestrial creatures which appear to be caught in mid evolution in various states of marine adaptivity, changing forms from land dwellers to water dwellers. Seals, walruses, sea otters, beavers, and Penguins, just to name a few, come to mind. Given the opportunity and time, any one of those groups could go on to become fully aquatic and get as big as a diesel rig, just like the descendants of ambulocetus did.
Top left: Blue Whale Top Right: Sperm Whale Bottom Left: A small pod of Narwhales (Note single tusk in males. It's actually an elongated tooth) Bottom Right: Bowhead Whale with Beluga Whale escort
A few whales and other creatures by size
Enlarge (Hi-res image warning)
Most nations are curbing whaling as an industry. Even the Blue-whale is making something of a comeback. But pollution and over fishing are just as much of a threat to the delicate balance of the marine ecosystem whales depend on as a Japanese Trawler with an explosive tipped harpoon gun.
Whales and their four-limbed ancestors were here long before humans or hominids or even the great apes and monkeys. They rose from the tableau of a shattered world, the smoldering remains of the long lost planet of the dinosaurs. In a little ten million years the grew from the size of antelope to the most massive animals known, and now dominate the ocean in both intelligence and size. They have survived ice ages, continental drift, super-volcanic eruptions, and greenhouse gases. Now they face their biggest threat yet: The fleets of whalers both past and present, and pollution led by the United States headed by a cabal of neo-cons ansd their swindled right-wing Christian followrs who enable them in the belief that God put whales and everything else on this planet to be plundered until the apporaching glorious rapture. Regardless of where they came from and how they evolved, looking forward, our world without whales would be a tragedy; a world poorer, a loss of riches, their extinction at our hands unacceptable.
But we must bear in mind these are wild creatures, born free in an alien three-dimensional realm of deep ocean; they are neither the Demons which tomented Captain Ahab or the tame, gentle giants of Seaworld. They can and have attacked humans, at times for no apparent reason, albeit rarely. And they have saved drowning people, again we have no idea why. They do not exist for crass amusement, but they surely provide us with wonder if observed with respect.
They are mysterious, mostly just curious, and generally harmless. But after their recent experiences with the walking ape, I'd guess what they would really like, if they could speak for themselves, is simply to be left alone.
The ocean is vast, surely big enough for both whales, and ourselves, to co-exist. Maybe that's what this guy is trying to tell us ... If only we could decipher his song.