Why does a cow have four legs?

Aug 01 2012 Published by under Uncategorized

I think that I have talked enough in these posts about variational and transformational evolution, and so today I will write something about how phylogenies affect the way biologists work as scientists.  Why is the concept of evolutionary history so important in biology?

In one sense, science is about problem solving. We set out to find the answer to a particular question, and in order to do so we have to solve the puzzle posed by that question. So, a relevant point is this: Is problem solving different in the biological sciences compared to the physical sciences (eg. physics and chemistry)? My answer is "yes and no".

To me, there is a way in which they are all the same, and this involves performing some sort of experiment to test any ideas we have that might solve the puzzle. So, as I noted in an earlier post, we all attempt to explain natural phenomena in terms of other natural phenomena.

However, in another way biology is different from physics. First, in biology we have to deal with the fact that organisms respond to their environment, which other physical objects do not do. If I drop a stone then it falls downward, and it will do so no matter where I am on earth. However, if I release a migrating bird it will adjust its flight depending on where it is released. This complicates the study of biology. Second, organisms pass information between generations, via their genes. There is no equivalent concept of inherited information in physics.

The consequence of these differences is that in biology unique historical events can have effects that last for millions of years. Something that happened to one of my ancestors thousands or millions of years ago can still affect me now, because the information about that event has been passed down to me in the genes that I have inherited from that ancestor. There may be no other evidence of that past event except in my genes. No physicist has to deal with this concept — in physics, those past events that have an effect now do so by leaving observable traces in the environment. The laws of physics that operated back then are still operating now, and we can therefore study them now.

This ultimately means that cause and effect can be separated in biology by a great deal of time. The explanation for the natural phenomena that I see now may be a long time in the past. For example, the cause may no longer be important in the modern world but it was in the past. How do I know about its importance back then? The cause may no longer be apparent in the modern world, so maybe I don't even know about its existence. Under these circumstances, even imagining the cause may be difficult. Perhaps most importantly, the cause may be an historical “accident” — a one-off event that has not happened since. We believe that modern biology is actually the result of a whole series of historically unique accidents!

How do I study natural phenomena under these circumstances? This is where a phylogeny comes into play. We use phylogenies as the framework for studying biodiversity, because they take into account the historical component of biological studies. In biology we use them to describe the natural phenomena, to explain them, and to predict them.

Consider the question posed in the title: Why does a cow have four legs? This is a question about explanation, not description or prediction. So, merely describing the legs of a cow is insufficient to answer it, and predicting how many legs the next cow will have is also irrelevant (although it may be interesting in its own right!).

A physicist might attempt to answer this question in terms of balance and stability, particularly while the cow is moving. The idea is that four legs are stable while allowing the animal to graze, walk or run. This tries to explain the presence of four legs in terms of what we know about the number of legs on other other objects in the modern world, and how stable they are while moving.

For example, we know that a 3-legged object can keep its balance while stationary, but three legs is very awkward for moving. (It is usually suggested that a 3-legged object would have to rotate itself to walk, alternately standing on each of its legs.)

Alternatively, we know that two legs is okay for walking and running, since we do that outselves; and if you are Swedish then you are well aware that at least one cow walks on two legs! We also know that insects have six legs and spiders have eight legs, so these are okay for balanced movement as well.

This is Mamma Mu (on the left) and Kråkan (on the right).

However, a biologist would not approach the question in this way. A biologist knows that a cow has four legs because it inherited that characteristic from its parents, both of whom also had four legs. Furthermore, those parents inherited the characteristic from their parents, and so on. Therefore, to a biologist the explanation for four legs may have little to do with cows in the modern world. The cows have a set of genes inherited from their ancestors, and it is those genes that cause them to have four legs (rather than some other number). There may be no particular relevance to having four legs (as opposed to two or six) in the modern world — modern organisms have characteristics because they inherited them, and not necessarily because they need them.

This starts a search backwards in time, through a series of ancestors in the phylogeny, searching for the one who first acquired four legs. The answer to the question about cows' legs then becomes a question about why that ancestor had four legs when its ancestors did not.

Thus, all cows have four legs, and so we conclude that the common ancestor of their species, Bos taurus, also had four legs. Indeed, all cow-like organisms have four legs, and so the common ancestor of the genus Bos had four legs. Furthermore,  all bovine organisms (cattle, bison, buffalo, yaks, etc) have four legs, and so the common ancestor of the subfamily Bovinae had four legs. Continuing, we work our way backwards through the common ancestors of (respectively) the Ruminantia, Cetartiodactyla, Laurasiatheria and Mammalia, all four of which we conclude had four legs.

Eventually we come to the common ancestor of the superclass Tetrapoda, which also had four legs. (After all, that is what the name says — tetra = four, pod = limb). Not all descendants of this ancestor still retain four legs in the modern world, of course. Primates have modified the front pair of legs into arms, birds and bats have modified them into wings, whales (and dolphins and porpoises) and seals (and sea lions and walruses) have modified their front legs into flippers and greatly reduced their hind legs, manatees and dugongs have front flippers but no hind legs at all, and snakes have lost all four legs almost entirely. Still, the common ancestor of all tetrapods had four legs.

The part of the phylogeny involving the origin of tetrapods is shown in the next diagram. There are four groups of organisms involved.

The part of the phylogeny involving the origin of tetrapods.

The main point about this phylogeny, for our question, is that modern lungfishes and modern tetrapods all have four limbs, while modern coelacanths  and other fish do not. The number of limbs is:

Tetrapod = 4 legs + 1 tail
Lungfish = 4 lobe-fins + unpaired tailfin
Coelacanth = 7 lobe-fins + paired tailfin
Ray-finned fish (most fish) = 7 ray-fins + paired tailfin.

Each fin on a fish is designed to perform a specific function, as shown in the next picture. (NB. MostSome fish also have a small adipose fin behind the dorsal fin, for stability.) Note that pectoral and pelvic fins come in pairs, one on each side of the body. The picture provides the physical explanation for why fish have so many fins,  in terms of balance and stability while moving in water.

So, modern lungfish all have fleshy, paired pectoral and pelvic fins and a single unpaired caudal fin. The other fins of most fishes are absent from lungfish. Modern tetrapods have muscular, paired pectoral and pelvic limbs and a single tail. So, lungfish and tetrapods have the same number of fins/limbs in the same places. We conclude that this arrangement has been inherited from their common ancestor, which is therefore the one we are looking for to answer our question. So, what did the common ancestor of these four-limbed lungfish and tetrapods look like?

We are sometimes incorrectly told that: "Scientists have long known that ancient lungfish species are the ancestors of the tetrapods." This idea is largely discredited today. Lungfish are the closest extant relatives of tetrapods, not their ancestors. That is, they are not primitive — they are well adapted to their natural environment. However, lungfish are one of many relict species of fish that share many ancestral characters.

So, did the common ancestor look something like this?

Tiktaalik.

Or more like this?

Acanthostega.

Probably neither, because we think that both of these fossil species were descendants of the common ancestor in question. But it was presumably more similar to these than to either modern lungfish or modern tetrapods.

So, the initial question about cows' legs becomes: why did the common ancestor of the tetrapods and lungfish reduce their number of fins from seven to four (and no less)? The answer I was given, by the fish biologist W.J.R. (Pim) Lanzing, when I was a student, was that this is the minimum number of fins that can maintain stability and movement in an aquatic environment.

Most of you know roughly how fish move, and how agile they are in water, so I don't need to describe it. However, locomotion of coelacanths is unique to their kind. They have high maneuverability and can orient their bodies in almost any direction in the water. They have apparently been seen doing headstands and swimming belly up. Lungfish, on the other hand, are essentially sedentary. They are reputed to be sluggish and inactive, but still capable of rapid escape movements using their tail. They can can use their paired fins to "walk" underwater, with alternating movements — first one fin moves forward, then the other. They can also use the fins simultaneously to move forward with a lunging action. So, apparently one is not agile in water when one only has four fins, but one can still move around efficiently.

The first tetrapods are thought to have evolved in coastal and brackish marine environments, and in shallow and swampy freshwater habitats. They used their modified, limb-like fins to get around in the water, as do modern lungfish. That is, the origin of legs wasn’t a transformation that happened on land. Limbs were an aquatic innovation that just so happened to be advantageous when tetrapods began to venture out of the water.

So, why does a cow have four legs? Our phylogeny reveals that this is because one of its ancestors (c. 360 million years ago) was aquatic, and four fins is the minimum in liquid. Most descendants have never changed this number, but they did use it to leave the water and live on land. The first part of this answer is a biological one (about ancestry), although the second part is a physical one (about stability and movement).

This is not the sort of answer that can be determined by an experiment. So, scientific problem-solving in this situation is quite different, making biology distinct from physics.

It is sometimes said that physics lies at the heart of science because all explanations ultimately involve physics. That is, the physics explanation lies beyond the biological one — the biological explanation is a proximal one while the physics explanation is the ultimate explanation. This may be so to a physicist, but I do not see why it should be so to anyone else. The biological explanation (in terms of ancestry) is equal to the physical one (in terms of aquatic balance), rather than inferior to it. Moreover, there are explanations beyond the ones that the physicists consider, although that takes us outside science.

In finishing, you might like to try this physics question, instead: Why are tables generally made with four legs and scientific instruments with three?

4 responses so far

  • Jim Thomerson says:

    A small correction, Almost all the fishes with adipose fins are catfishes, characoids, or salmonioids. Maybe some 3000 species (off the top of my unreliable memory), some 10 to 15% of some 25,000 to 30,000 fish species.

    Ernst Mayer characterized biological explanation as having three aspects: (1) mechanical or physiological, how does the system work?(2) adaptive, what good, if any, is it? and (3)What is its evolutionary history?

    The three leg vs four leg question is answered by plane geometry, not physics. Three points define a plane.

    • Jim, For the adipose fin, I should of course have said "some" instead of "most" — this is my botanical rather than zoological training! Also, three legs is only geometry if the object does not move — my point is that as soon as it tries to move then physical forces come into play. As for Ernst Mayr, he does indeed add "adaptation" as a third type of explanation to the two that I talk about in my post. However, that is a far more complex topic, which causes much heated discussion in evolutionary biology. /David

  • Jim Thomerson says:

    Actually, the question is more general. Why do animals have even numbers of legs rather than odd numbers? I've seen videos of dogs with a hind leg amputated still getting around very well.

    • Jim, Actually, I think that most biologists find the answer to that question in the fact that all organisms with an even number of legs are bilaterally symmetrical, and their legs thus come in pairs (one either side). The common ancestor thus had paired legs, and an odd number of legs would have to have a big selective advantage for it to take over, should it ever arise. A three-legged dog, for example, does not have a big advantage over a four-legged dog. /David