Take the question, “how did the rhinoceros get its skin”. On the one hand, we have a scientific study in 2009; that compares folding across many rhinoceroses and examines the blood vessel network. They suggest that the skin evolved its folds to help keep the rhino cool. Rudyard Kipling, however, gives us another hypothesis.
In Just-So stories, the rhinoceros got cake crumbs inside its skin. It was so itchy that the rhino scratched and scratched and rubbed and rubbed until his skin went dry and stretched, and began to fold over.
Kipling’s fun children’s stories have given their name to a particular kind of fallacy in evolutionary biology: the Just-So story. This rather damning sobriquet is often leveled at the more preposterous claims of evolutionary psychology (and often deservedly so). We tend to use it more generally to refer to stories that seem too far-fetched and fanciful to be true, yet are ultimately untestable.
In essence, however, Just-So stories are an extreme form of a more general fallacy within evolutionary biology called adaptionism. To put it succinctly: function does not prove purpose. Simply, it is fallacious to assume that any structure is perfectly adapted for the function it now serves. Firstly, a selective pressure is rarely great enough to act on such individual body parts, and secondly – and perhaps, more importantly - it is often impossible to implement the changes required. It's certainly true that adaptation is perhaps the major driving force of evolution, but it is not necessarily true that organisms evolve a tailor made body plan in response to this.
So, lets turn our attention to palaeontology, and ask a question that has probably attracted more speculative hypotheses than any other. How did T. rex get its tiny arms? For an animal the size of a bus, the arms were no longer than a human’s. We also know that this body plan was relatively stable, and T. rex probably inherited it from a long line of ancestors with short arms. Perversely, these arms were not long enough to reach the mouth, and yet, evidence shows that they were heavily muscularized and were in regular use, clearly discounting any suggestion that they might be useless and vestigial.
This paradox has been a major bugbear for palaeontologists ever since Henry Osborn discovered the first specimen in 1905. He had “grave doubts” about the true size of the arms and substituted longer arms, despite related species being found with significantly smaller arms. Final confirmation of their embarrassingly disproportionate size came with the discovery of the first complete specimen in 1989.
A quick literature search throws up hundreds of suggestions for functions. One of the earliest suggestions I can find is from Osborn himself. He suggests the arms were used for grasping during copulation. Another leading hypothesis was from Newman (1970), who suggested the arms were adaptedto brace the body in getting up. Meanwhile, Carpenter and Smith (2001) have suggested that the arms were used to function as meathooks while T. rex ate.
Intentionally or unintentionally, it’s very easy here to stray into adaptionism. As I said above, it is certainly true that one of a fundamental driving forces of evolution is adaptation. It is not the be all and end all, however. The point is; as tempting as it may seem, any speculation about the function of the arms is not strictly necessary in a discussion about why they are so small. To do so would be to imply that T. rex’s arms were tiny because they were selected to be tiny. It would assume that, had T. rex needed long arms, it could have had them.
This may be true, but it is not the only explanation. If we use any discussion about their function in a discussion about their evolution, we run the risk of creating a Just So story for ourselves. I am sure that the authors had no such intention either; it would be wrong to trash legitimate scientific speculation about their function. But the fact remains, we still need a good hypothesis to explain the origins of T. rex’s tiny forearms.
So, where does this leave us? We’re left in search of a hypothesis to explain the diminutive size of T. rex’s arms that may or may not have anything to do with its function – something which is difficult enough to deduce as it is. To put it mildly, it’s an intractable – and pretty annoying place to be.
There is one other way of approaching the problem, however. Instead of thinking about it in terms of why they we so small, some have thought about the problem in terms of, why couldn’t they be larger? In other words, perhaps long dangerous forearms would have been very useful for T. rex; it’s just that it couldn’t have them because of constraints in evolution.
This concept of constraints in evolution is well understood but often overlooked. It’s easy to forget that evolution is not a free-for-all. Just because a structure would be particularly useful for an organism does not mean that it can be evolved. Instead, its evolution is strongly dictated by two main factors: firstly, constraints from its ancestor’s body plan, which form the blueprints as raw material for evolution, and secondly, constraints from development. Other than that, we really know very, very little about what inhibits the course of evolution.
This leads us to a hypothesis from Martin Lockley, published in a book about Tyrannosaurus, which I’m betting you won’t have heard before. This was an idea that I heard about in my undergraduate, and at the time I thought was absolutely fantastic. Unfortunately, as Huxley said, there is nothing more tragic in science than the slaying of a beautiful hypothesis by an ugly fact, and in the case of this gorgeous hypothesis, it has been veritably slashed to a pulp by hordes and hordes of grotesque and glaring facts.
I’ll go through the counter evidence afterwards, but for the time being, I’ll explore this beautiful but ultimately wrong hypothesis.
Lockley’s hypothesis is to do with constraints in evolution. As I mentioned above, we have two major constraints in evolution. The first constraint, ancestral constraints, is probably the most familiar one. It might seem obvious, but we can only work with what we’ve got. This means that evolution recruits pre-existing structures, a principle that often leads to surprising results. Take our inner ear.
The inner ear works by transferring the vibrations on the eardrum via three small bones: the incus, the malleus and the stapes. The system, I’m sure you’ll agree, works beautifully. But, it would be bizarre to design a hearing system like this from first principles. Yes, it works well, but an engineer would never design such a convoluted system. The structure only makes sense if we remember that these bones originally evolved as part of the lower jaw.
Indeed, in crocodiles, a distant relative of ours, they still do. Through evolution, one bone in the lower jaw, the dentary, got larger and more important. Though they had always been used to transmit sound, as evolution progressed, these bones were eventually recruited for the sole purpose of hearing. So, even though they work incredibly well, it was not their original function.
The second constraint is related to the first, but it’s considerably more important than most people realize. Many of the changes required in evolution involve changing the size of body parts. We all change the size of our body parts as we grow from an embryo up to adulthood, so one way of changing the sizes of body parts through evolution is simply to slow down or speed up the development of the whole body through evolution. Think of the axolotl, which is developmentally a baby salamander that never grows up.
In fact, it’s often suggested that we adult humans are almost like chimps that never grew up, spending a long time in childhood and never developing the brow ridges of adult chimpanzees.
So it follows that to tweak the size of an individual body part through evolution, we speed up or slow down its developmental timing. Crucially, however, we still need the raw material of evolution to act on; in this case, a selection of forms of the body parts grown during development for evolution to take its pick from; a principle known as allometry. If there is no allometry, no variation in form, then there is little that evolution can use.
Take cats and dogs. The shape of wolves’ skulls, the ancestors of dogs, change dramatically through growth, offering ample differing skull shapes to select from. Hence we have an enormous range of dog skull forms; from the Chihuahua to the Great Dane. The skulls of cats, however, have very little allometry through their growth; they get larger and that is all. In this way, we might think of cats as being constrained by their evolution, because, if required, evolution would not have the range of skull forms to pick from.
Similarly, if all dogs were decimated save for the Chihuahua, evolution would not be able to regenerate the range of dog breeds we previously had because the Chihuahua has little allometry to work with.
Allometry in dogs. McNamara 2012.
From McNamara (2012)
Here we come to the crux of Lockley’s argument as to why T.rex had tiny arms. His idea in essence is this: the development of T. rex gave no scope for the growth of long arms. He argues that they were a trade off; a necessary sacrifice for T. rex to grow a large head and large body.
The reason why this is so is ultimately rooted in genetics.The genes that govern how we grow are simply not clean enough to be able to tweak the development of individual body parts. Instead, changes in body part size have to be changed by broad-brush changes to the genetics.
Lockley hypothesises that the most important goal in T. rex’s evolution was to grow a large body, and have exceptionally long hind limbs. But there is no way of doing this without making a trade-off for smaller forelimbs and a smaller neck.
T. rex was big. The way that T.rex generated this height, among many other dinosaurs, was to tweak its developmental timings. T. rex did this by growing very, very quickly. In fact its been estimated from looking at the structure of T. rex that in the early stages of its life it would havegained about 2 kg a day, piling on considerably more than many of its close relatives like Gorgosaurus and Albertosaurus, which would have barely gained half a kilo per day. T. rex also kept growing and growing as it got older.
While Gorgosaurus and Albertosaurus reached maturity at around 14, T. rex didn’t reach maturity until around 18. So Tyrannosaurus grew faster, and for longerthan most dinosaurs.
The second factor at play is to do with an ancient set of genes called the Hox genes, which dictate in what sequence bodies grow. These dictate that the tyrannosaurs, like most other animals, would have grown its head first, followed by the feet. The very last parts to develop were the arms and the neck region. So, to develop its abnormally large head and legs, T. rex had to spend abnormally long in this earliest phase of development. Not only that, but the form of the arms is also dictated by the sequence of development.
It’s easy to think that limbs grow by sprouting from the body, growing sequentially, but in fact the opposite is true; the outer parts – the feet and shins, and the hands and forearms – grow first, and the inner parts – the thigh and the upper arm – develop afterward. As we can see in T. rex’s arms, the humerus is severely reduced.
Lockley’s central hypothesis is, then, that T. rex and many of its relatives had short arms because abandoned the chance of having long arms so that they could grow very large and have a large head. They made the best of this bum deal by heavily strengthening and muscularizing them, and as we can see from their extensive fossil record, it must have been worth the trade off.
Beautiful, isn’t it? Well, it’s time to start introducing some facts and letting the bloodbath begin.
If big size and big heads in tyrannosaurs limits arm size, we wouldn’t expect to see tyrannosaur-sized allosaurs and carcharodontosaurs with big heads and long arms... but we do. Secondly, sauropods are a big problem, as they have have long necks and tails with big forelimbs... which shouldn’t be possible. On the flip side of the argument, there are the abelisaurs which have even smaller arms. Lockley’s hypothesis predicts that they should have relatively larger heads... but they don’t.
As neat a hypothesis as it is, then, it simply doesn’t stack up to the evidence, and it is rejected by most paleontologists worldwide. Despite this, it may well be possible that this approach might be closer to the truth than a more adaptionist approach. It’s just unlikely to be the case that it was a trade off for large head size.
It remains true that there is no tit-for-tat, body part allowance scheme during evolution. The point is, it is simply not possible – or more, it’s difficult, and therefore implausible in evolution – to fine tune changes in particular body parts. Like spandrels in large domed churches, which have an important aesthetic function but are there because of structural constraints from the large dome, many body parts can be highly functional, but take their form because of constraints imposed by other factors. Any organism is not a mosaic of independently evolving body parts, as is often implied by cladistics; they are all parts of a complex whole.
There we have it. We’re still in search of a decent hypothesis to explain T. rex’s tiny arms. All we have are hypotheses that rely too heavily on adaptionism or hypotheses about constraints that don’t stack up to the evidence. But the answer is probably not simple, and is unlikely to be related purely to function. We should be constantly on our guard about confusing function with purpose.
The fact is, evolution is a messy, albeit highly directional process, which must wriggle through the constraints imposed on it. Organisms are generally only good enough for what they do. These Just So stories are often too good to be true. Simply, nothing created by evolution can ever be perfect.
(1) Endo, Hideki, Hiroshi Kobayashi, Daisuke Koyabu, Akiko Hayashida,Takamichi Jogahara, Hajime Taru, Motoharu Oishi, Takuya Itou, Hiroshi Koie, andTakeo Sakai. "The morphological basis of the armor-like folded skin of thegreater Indian rhinoceros as a thermoregulator." Mammal Study 34,no. 4 (2009): 195-200.
(2) Osborn, Henry Fairfield, Barnum Brown, and Richard Swann Lull."Tyrannosaurus and other Cretaceous carnivorous dinosaurs. Bulletin of theAMNH; v. 21, article 14." (1905).
(3) Osborn, Henry Fairfield, and Barnum Brown. "Tyrannosaurus, UpperCretaceous carnivorous dinosaur:(second communication). Bulletin of the AMNH;v. 22, article 16." (1906).
(4) Long, John A., and Kenneth J. McNamara. "Heterochrony in dinosaurevolution." Evolutionary change and heterochrony (1995): 151-168.
(5) Lockley, Martin, Reiji Kukihara, and Laura Mitchell. "WhyTyrannosaurus rex had puny arms: An integral morphodynamic solution to a simplepuzzle in theropod paleobiology." Tyrannosaurus Rex, the Tyrant King(2008): 131.
Many, many thanks to Dave Hone for suggesting some rebuttals and for feedback on a draft of this post.