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.
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If you meant that as a scientific statement, you have, I think, taken the evidence at least one step beyond what it can support. At most, the evidence might be said to be consistent with their being no cosmic plan or direction. But to say that it "shows" the lack of one runs into the same problem we throw in the face of the IDeologists all the time: to either confirm or deny design in the universe, you have to be able to say something about the motive and intent of the designer.
The rules in philosophy, such as they are, may permit this argument more latitute but it seems to me that it still winds up being "there is no cosmic plan or direction of the sort I would impliment if I was in charge."
Incidently, it isn't just crude creationists who have toyed with the directionality claim. Stephen Jay Gould published an argument he had with Simon Conway-Morris over the same subject and Conway-Morris' book Life's Solution: Inevitable humans in a Lonely Universe expanded on it.
Naturally, your mileage is free to vary.
I would have to agree with John, that at most what you can say is that there does not appear to be any evidence for a plan or direction. This is different from saying there is evidence that there is no plan. After all, you cannot in principle rule out the possibility that the 'plan' was to unfold exactly as we see it in the fossil and extant record.
On another note, it would be very useful I think to compare not only wolf with thylacine skulls, but also kangaroo and thylacine skulls. Make a list of all the differences and similarities. Which pair is most similar?
It's interesting that Price thought that marsupials weren't 'true mammals'. In terms of evolutionary history, marsupials could, with some justification, lay greater claim to being the 'true mammals' while we, more recent, placentals are just a curious, interloping offshoot.
Mind you, I suspect the monotremes would have a thing or two to say about that.
Distance? How much distance? Heck, given sufficient distance, one could hardly tell apart a wolf and your little brother. Wesley R Elsberry posted links to nice photos when Cornelius Hunter was last beating this dead mammal.
Gee. A plan to look purposeless. How inspiring.
"Almost every type of placental mammal has its counterpart among the marsupials." Except, of course, God's ultimate creation: humans. It would be interesting to see what a marsupial "hominid" looked like!
As for the Tasmanian tiger, it looked and acted more like a domestic dog than a wolf, if one has to find a canid with which to compare it. What caused their extinction? Or did you cover that and I don't remember now?
Oooh! Maybe Neandertals were the marsupial hominids?
That's fascinating. After seeing those skulls compared, I'm convinced that evolution is teleological.
"Gee. A plan to look purposeless. How inspiring."
Oh, I dunno. Don't you find Nature inspiring? (Or as a wise fellow says in the post above, "[W]hat is apparent from the unity and diversity of life is that nature is far more varied and wonderful than we can possibly imagine.") Not that this is any indication whatever of a "plan" behind it all, much less one so relatively simple as to be humanly discernible.
(For whatever it is or isn't worth, my personal feeling is that there is no Plan, no heavenly equivalent of Gantt charts with the notations, "1.5 billion years - stick little outboard motors on the butts of several bazillion bacteria - phew," and "4.5 billion years - time to kill off the Tasmanian wolf.")
Or as TR Gregory says: "Evolution as fact, theory, and path".
No, that is correct as there are no other mechanisms directing the process. If there were huge gaps in the understanding you could possibly speculate in other theories complementing evolution. But there is not.
Besides, I thought most theists had abandoned gap arguments as they are falsifiable? If the claim is that evolution always results in humans by some previously unknown mechanism, I suspect we will have such data in a few decades. One of the leaders for exoplanet search has gone on record that we should detect life-bearing planets in that time frame.
However many of those shows signs of civilization (radio and light emissions, perhaps detection of industrial effects on atmospheres), I think it would be feasible to predict how often human equivalent intelligence leading to technological civilizations occurs.
Maybe one could argue that human equivalent intelligences wouldn't result in technology, but when we would have to discuss what being "human" is.
But that is precisely what you are forced to do when you accept the simpler theory, you rule out and discard the others.
I'm not sure if you are trying to say that facts and theories are entities with certain and often known amounts of uncertainty associated with them, as that is a rather trivial and uninteresting claim.
Maybe not, I suspect not many observations would be able to detect direct emissions. Even Spitzer who has made the first exoplanet thermal map had a 150 K differential to work with...
Torbjorn:
There are philosophical problems involved in determining causation. The claim that humans can detect all mechanisms that might be in operation through scientific means is a philosophical claim and not a scientific result.
Occham's Razor is a rule of first approximation, not a guarantee of truth. Saying that the evidence is consistent with there being no cosmic plan or direction is accurate. Saying that science can or does rule out plan or direction is philosophy, not science (and, in my opinion, not very good philosophy).
> In terms of evolutionary history, marsupials could,
> with some justification, lay greater claim to being
> the 'true mammals' while we, more recent, placentals
> are just a curious, interloping offshoot.
Well, at least they are more derived than eutherians in the dental department, because they replace fewer teeth. Replacable teeth are the primitive condition.
> As for the Tasmanian tiger, it looked and acted more
> like a domestic dog than a wolf, if one has to find
> a canid with which to compare it.
A 'doggish' look appears very early in the evolution of many carnivorous mammalian clades: mesonychians, triisodonts, basal cetaceans, to name but a few. The same holds true for the dentition of both canids and thylacines: It is essentially a basal mammalian set of teeth with a minimum of adaptions (carnassials) for flesh-shearing.
> What caused their extinction?
Hunting by humans in Tasmania. Competition by Dingoes is said to be the cause of extinction on the Australian Mainland. Some say a stronger bite gave the Canids the edge, but usually marsupials have a stronger bite than their placental counterparts, the large braincase of placentals being in conflict with the jaw muscles. Another explanation is that Dingoes are social animals, while Thylacines are solitary hunters. If observations of dingo/fox and dingo/feral cat interactions in Australia are representative, pack hunters dominate during times of draught, which are of course quite common in Australia.
Is "placental" the correct opposite of "marsupial?" After all, Carl Zimmer corrected his latest whale post when he learned that Kangaroos do indeed have placentas:
http://scienceblogs.com/loom/2007/12/27/return_to_the_dawn_of_whales_c…
As has already been pointed out, Ockham's Razor is a handy rule of thumb, but it doesn't decide the issue. To equivocate 'no evidence that it is directed' to 'is not directed' is a sort of god of the gaps style argument, isn't it? Indeed there may well be no design there, and I personally find that position compelling. But compelling is not conclusive.
To me, saying it was not directed is making the same mistake as those who say it was. Both involve presumptions of what a universe would or wouldn't look like if supernaturally directed. This is simply not a scientific question to me.
After all, you cannot in principle rule out the possibility that the 'plan' was to unfold exactly as we see it in the fossil and extant record.
If there was a "plan," then it wasn't planned very well. In which case, saying it says nothing. It's vacuous. How would you feel if you were watching your favorite sport (if any), and I said, "But the players, the rules, the situation, and blind chance are not sufficient to create the game, which was predetermined from the outset and unfolded according to "plan"? Would you find that inspiring?
If anything, convergent evolution seems to indicate that life repeatedly encounters the same problems, and evolution solves them in similar ways but doesn't "learn" to prevent them, therefore, no plan.
Melanie - that gave me a good laugh!
While I agree with the essence of Gould's statement (we are not the goal of evolution), I've always had trouble with the "no drive to complexity" part. A drunk stumbling around randomly at the South Pole has "no drive to Northerliness", but he will nevertheless tend to visit higher and higher latitudes as time goes on. When you start out simple, there's nowhere to go but more complex. Even though the evolution of any individual lineage is completely undirected and goal-free, the fact that complexity space is bounded at the simple end but open at the complex end ensures that over time, life will expand into ever more complex niches, just by pure random walk, like perfume diffusing out of a bottle.
Then compare the marsupial lion and the lion.
And then Tyrannosaurus.
Sheesh.
This is precisely what Gould said, except he made it clearer that most of life, on average, stays where it started -- the extreme moves, so the mean and the median move, too, but the mode doesn't.
Biological nomenclature is not about what words etymologically mean...
It's been a few years since I read the books, but my sense of Gould's argument was that you shouldn't expect the extreme to keep moving toward greater complexity. It did in our case, but in his view that was a fluke. In a replay of evolution, he expects the overall curve to stay narrow, with no tendency to diffuse outward along the complexity axis ("no drive to complexity").
At least that was my (possibly incorrect) reading of it.
Kristine:
But how can you say that without making assumptions about what "the plan" entails? As I said before, you wind up merely asserting that there is no "good" cosmic plan of the sort you would impliment if you were in charge.
Perhaps not inspiring but, having lived during the college basketball point-shaving scandals, I'd hardly find it impossible. ;-)
I'm amazed at how convergent the thylacine is to the wolf. Has the marsupial wolf lost its seashell teeth?
Thanks for the comments everyone! I thought this post was going to be a one-off that I wrote at about 11 PM last night, but I guess I was wrong!
There are some interesting points brought up here that I think merit expanding, specifically whether evolution has a purpose/direction or not. As is apparent from the post, I don't think evolution is directed, has a "purpose," or has an end-point, and I don't think this idea is somehow off-limits or out of bounds. As I said I'm working on something now that I hope to have up tonight so I won't tip my hand too much as yet, but I see no reason why saying that evolution is undirected should be controversial given what we've come to understand how it works so far.
That depends on what the meaning of the word "is" is. That statement certainly doesn't seem descriptive of Australia today.
Jared Diamond suggested one reason there aren't many big modern marsupial carnivores like Thylacinus on the Australian islands might be fierce competitive pressure from coldblooded dragons.
Of course it could be that Godidit, but later decided to have them all killed off just to piss off Noah.
Brian, I'm looking forward to your next post on this. By the way, here is the Gould/Conway-Morris debate I mentioned:
www.stephenjaygould.org/library/naturalhistory_cambrian.html
And here is an excerpt from Conway-Morris' The Crucible of Creation.
www.stephenjaygould.org/library/conwaymorris_crucible.html
I've seen the footage of Benjamin taken so long ago, and one thing I remember is his gape. He could open his mouth wide. Wider than a canid as a matter of fact.
At this point we're going into the realm of cryptozoology. Some years back a photograph was taken In mainland Australia of an animal that some identify as a thylacine, while others identify it as a dog. I've seen the picture, and Benjamin looked more like a dog than the subject does. The muzzle is longer, more delicate than Benjamin's was, and the subject's hind legs are not as robust as Benjamin's, or a canids. One final difference, the hindquarters were striped.
So, if it was a mainland version of the thylacine it's evidently even less doglike than the Tasmanian version. It could probably be put into its own genus. Maybe even its own family.
The point is, if mainland Australia has its own doglike marsupial that animal is an even less perfect imitation of the dog than old Benjamin was. Which means it can serve as an even better example of how convergent evolution doesn't necessarily produce results that are virtually identical to a species that arose somewhere else under similar---but not identical---conditions.
Form follows function is the operative rule here. Ambush hunters come to resemble cats. Much as the nimravids did. Chase hunters come to resemble wolves. At one time apparently the thylacine and thylacine- like marsupials were chase hunters originally, and even when they became primarily scavengers they retained the chase hunter body shape.
In short, evolution is not entirely random. There are limitations on what works and what doesn't. So life will come to resemble what arises in other places in similar circumstances, but only so far as chance allows it to. If the changes that allow for the appearance of a dog-like animal don't occur, then a dog-like animal simply won't appear in that situation. In other words, convergent evolution is not guaranteed.
Now, if you could help me clarify what I've just said, I'd be very grateful to you.
@ John Pieret:
Perhaps so, but not empirically. QM theory shows, with data of impressive certainty (~ 15 sigma, IIRC), that there are no hidden local variables when you have a full description of the observables.
It is you who claim a gap. Empirically, there isn't much of one in most sciences, and in evolution especially not any of importance.
This is simply not a philosophical question, contrary to the blanket claim, and I can't see how it can be claimed to be one. Meanwhile I have supported my claim.
I agree that it could be bad philosophy, as philosophy and science are widely different. The first has a lazy approach and doesn't discard ideas until they are proven internally inconsistent, the second greedily search for knowledge and discards invalidated ideas as soon as it can.
@ John Pieret and Dave S:
Nope, not comparison by parsimony, but testing of theories. Occam's (or Ockham's) razor comes into play when we have theories that have equivalent predictive power, but it doesn't describe that we discard the theories that fails the test, whatever the reason.
The reason could be that one theory is inferior to another by parsimony, which then shows up in the number of parameters used, and there are measures of parsimony that can be used to do the test. This is commonly done AFAIK in cladistics, and was in any case used when the current concordance cosmology was tested.
If the protestation is that we can't model any directional agent, I think it is misdirected. The model would push for human equivalent intelligence by your definition, that would be fulfilled by a vast class of models supplementing existing evolution, and we could exclude them by fiat by way of parsimony.
Torbjorn:
But empiricism itself is a philosophy, not a scientific result, as Hume showed 200+ years ago. Empiricism cannot be used to justify reliance on empiricism except circularly. I'm sorry, but all your supposed impressive certainty is a case of studying your own philosophical navel.
I don't need to refute circular reasoning. I need only to show that it is circular to demonstrate that it is not reliable. Note I said "reliable" and further note that we are discussing an issue at the rarified level of a "cosmic plan or direction," well beyond the ordinary day-to-day workings of science.
The sole question here is whether we can say scientifically (as opposed to the philosophical conclusions you may want to draw from science) that there is no such cosmic plan or direction with the kind of certainty you are claiming. The circular nature of your argument says "no."
Say what you will about his circular reasoning, John, but if he's reasoning in a circle, he's at least being consistent!
FWIW, catching up on old threads:
@ John:
I wasn't discussing a philosophy over empirical methods, I was referring to the empirical results themselves. (Which makes it hilarious when you discuss staring at philosophical body parts.)
And as the rest of your comment relied on this mistake, you haven't been able to respond.