Convergent evolution can be a tricky thing, and one of the most celebrated examples of it (at least among creationists) is the case of the extinct marsupial predator the Thylacine (Thylacinus cynocephalus, also known as the Tasmanian Tiger or Tasmanian Wolf). Indeed, when compared to the skull of a wolf (a placental predator), the differences between the skulls of the predators are so slight that anyone but an expert would easily become confused (the primary giveaway being two extra holes in the palate bones of the Thylacine, [see ventral view skull comparison below]). Indeed, mammalian carnivores, whether truly members of the Carnivora or merely carnivorous in habit, have converged on similar body plans over and over again through geologic history, large canines and a battery of shearing teeth being the marks of a mammal that often consumes meat. Some have cited such convergence to support teleological/deterministic ideas of evolution and creationism, though, and apparently Of Pandas of People Mk. III (aka The Design of Life) cites the Thylacine as a “problem” for undirected evolution.
As with many other creationist arguments, the claim that the thylacine presents a problem for evolution is not only overblown but also very old. While the first origins of this “icon of creationism” likely predates my knowledge so far, the earliest reference I’ve yet come across has been in the pamphlet The Predicament of Evolution (1925) by the early “flood geologist” George McCready Price. Price writes;
There is an animal on the other side of the world that is called the thylacine, or the Tasmanian wolf, being confined to the island of Tasmania. It looks so absolutely like a dog or a wolf at a distance that one could hardly tell the two apart. Yet the thylacine is not a true mammal at all, but a marsupial, or pouched animal, carrying its immature young ones around in a sort of pocket, as the opossum does. Thus it is quite impossible to suppose that this animal has been derived from the dog or wolf, or the latter from it. The two types must have been produced quite independently. How did nature come to evolve this absurd parody on the wolf by any system of natural selection or any other form of evolution?
Price doesn’t mention whether we can infer something about God’s sense of humor by studying the Thylacine as an “absurd parody on the wolf,” (which was still alive when Price wrote this precursor to modern-day creationist tracts) but I have seen a similar example mentioned over and over again in creationist literature. Many authors use this marsupial predator as a springboard for even grander claims, however, and as they do so they undercut their own argument. The Design of Life blog post, for instance, features this quote from Michael Denton’s Nature’s Destiny;
The diversification of the marsupials in Australia is very instructive. Almost every type of placental mammal has its counterpart among the marsupials. There is a marsupial lion, cat, wolf, mole, anteater, jerboa, and flying squirrel. There was even a giant wombat equivalent to the placental rhino.
Indeed, according to Denton nature is deterministic, the aspect of contingency that Stephen Jay Gould argued in Wonderful Life and Full House being supposedly shown to be false by marsupial equivalents to placental mammals. The Thylacine is probably the closest to a placental carnivore in overall form (although, as you can see from the video above, it would be somewhat hard to confuse with a wolf if you saw the two side-by-side in the flesh), but what of the marsupial “lion” and the marsupial “rhino”?
I should probably first mention that there was not single “marsupial lion” but instead an entirely family called the Thylacoleonidae, containing the genera Priscileo, Wakaleo, and Thylacoleo. Of these, the species Thylacoleo carnifex is the most well known and celebrated, but to say that it was a “marsupial lion” is a bit misleading. While it likely inhabited a niche similar to that of big cats, Thylacoleo was relatively small, about the size of a leopard (Panthera pardus), and we have no indication that it lived in prides or gregarious social groups. Even within the available fossil evidence, Thylacoleo carnifex has a very distinct and short skull, the premolars being adapted into shearing blades. The carnissal shear of true members of the Carnivora is diagnostic of the group, creating a slicing edge often used to cut open the stomach of prey animals, but the highly derived premolars of Thylacoleo are taken to an extreme that looks like they have more in common with meat cleavers than the teeth of any placental carnivore. In essence, Thylacoleo was a very strange carnivorous marsupial, much more divergent in character from a lion than a thylacine is from a wolf.
Denton’s proposed “marsupial rhino” is also a bust. I do not own Denton’s book (nor is it on my list of books I intend to read anytime soon), but I can only imagine that the giant marsupial herbivore he is referring to is Diprotodon, an animal that resembled a giant wombat and belongs to the Suborder Vombatiformes. While Denton may have been impressed with a Diprotodon skeleton and its very superficial resemblance to that of a rhino, Diprotodon in no way represents a marsupial version of a rhinoceros outside of being a large herbivorous quadruped (and if we are to say that Diprotodon effectively is a maruspial rhino, we then need to ask which rhino genus and species Diprotodon would represent). As with the other animals mentioned on the list, the famous argument of “maruspial equivalents” is based on very vague associations at best. Even in cases where a placental and marsupial are relatively close, such associations merely show that there are similar evolutionary solutions to similar problems that are bound to show up over and over again. If we’re talking about “flying squirrels,” gliding by use of skin stretched between limbs or digits has been arrived at by lizards (Draco), frogs (Rhacophorus nigropalmatus), snakes (Chrysopelea), colugos (Family Cynocephalidae) and if we’re going to be generous, even fish (Family Exocoetidae). Indeed, gliding shows up again and again among vertebrates but not all gliding structures are homologous; a flying fish has modifications of its fins while colugos have enlarged patches of skin between their limbs on their flanks. Even among animals that have skin flaps that allow for gliding, the skin glaps can be between digits or attached to limbs or have differing arrangements, so there clearly is not only one “design” or body plan that all vertebrates adhere to. Even within convergence there is contingency.
What the DoL post and Denton’s book are concerned with does not primarily involved marsupials or placentals, however. The question of whether humans were “meant” to evolve (or exist at all) is posed, specifically counter to Gould’s notion that if we were somehow able to “replay the [evolutionary] tape,” life on earth would be quite different. As Gould has pointed out himself, though, is that he is going further back to a much deeper branch than the split between the ancestors of thylacines and wolves. Gould’s book Wonderful Life is primarily concerned with the Cambrian, a time when representatives of many different phyla appeared in a geologically short period of time, and as it stands now the Chenjiang fauna contains the earliest known chordates dating between 520 and 525 million years ago. Indeed, perhaps our own line would have been snuffed out if the history of life on earth was allowed a “do-over” and creatures like Opabinia would have left more descendants rather than disappeared, but we can’t know this for sure as such a retrial is impossible to carry out. Still, our own species is a very unusual and rare sort of animal indeed, the product of contingency and undirected evolution rather than some sort of orthogenic force that drives creatures towards a form that combines bipedalism with large brain size and all the cognitive abilities that go with it. As Gould himself wrote in the conclusion to Full House;
If one small and odd lineage of fishes had not evolved fins capable of bearing weight on land (though evolved for different reasons in lakes and seas), terrestrial vertebrates would never have arisen. If a large extraterrestrial object – the ultimate random bolt from the blue – had not triggered the extinction of dinosaurs 65 million years ago, mammals would still be small creatures, confined to the nooks and crannies of a dinosaur’s world, and incapable of evolving the larger size that brains big enough for self-consciousness require. If a small and tenuous population of protohumans had not survived a hundred slings and arrows of outrageous fortune (and potential extinction) on the savannas of Africa, then Homo sapiens would never have emerged to spread throughout the globe. We are glorious accidents of an unpredictable process with no drive to complexity, not the expected results of evolutionary principles that yearn to produce a creature capable of understanding the mode of its own necessary construction.
Still, even in this we cannot be certain, and therein lies the glory and the frustration of evolution. Perhaps terrestrial vertebrates still would have arisen even if it did not occur during the same time, but the descendants of the more derived fish may have left descendants that followed evolutionary parthways vastly different from those present in the fossil record, for example. Still, what is known from evolution shows that there is no cosmic plan or direction, but the linked contingent consequences of a world teeming with life. We speak of the Cambrian “Explosion” (often ignoring the Cambrian extinctions) but in truth life has been “exploding” on this planet even leading up to the Cambrian, evolutionary innovation and turnover occurring at a breathtaking rate. How different life would be if predators took advantage of the first “fishapods” to make it onto land or if big cats became specialized hominid predators in Africa during our more recent evolution is impossible to know, but what is apparent from the unity and diversity of life is that nature is far more varied and wonderful than we can possibly imagine. Insisting that we are the crown of Creation or the “end of evolution” represents a kind of hubris represented only by those that have dimmed their eyes to the grand history of our own development through time immemorial.