Why is it that we have evoloved to have the same number of digits on our hands as on our feet? Is there some evolutionary reason or is it simply a pretty big coincidence?

Many features of organisms don't need a selective reason to explain them, and I think this is probably such a case. We have five digits on each limb because we evolved from ancestors with five digits on each limb, and back in the distant past when the first mammals arose, there probably wasn't a powerful reason why five was much better or worse than four or six. The fact that it has remained unchanged suggests that there can't be a strong selective advantage to having more or less.

Actually its quite odd that we have 5 digits on each limb. Although our ancient amphibian ancestors soon settled on 5 digits as the norm (for no obvious reason) a great many animal groups have continually lost digits on both hands and feet. However, there is no instance I know of, of a finger or toe coming back after being lost. So not only do we retain all 5 digits, but none of our direct ancestors ever lost any on hands or feet.

For example Tyrannosaurs famously has only two fingers (though it retains a bony nub of a third). Most other carnivorous dinosaurs had 3 but some of the earliest ones had 4.

Horses of course just have 1 (again with splints of bone from others that remain undeveloped) and snakes have either none, or some boas retain a single spur-like toe.

There shouldn't be any reason for digit reduction to be synchronized between the hands and feet. For instance, as in Dave Hone's example, Tyrannosaurus had two digits on its hands (most other theropods had three) whereas it had four digits on its feet. Birds, which are descendants of theropods, also have three digits on their hands (or wings) while having four on its feet.

Quite right Manabu, reduction has no requirement for a synchronization to occur on the fore- and hindlimbs.  However,  The initial question asked

"Why is it that we have evoloved to have the same number of digits on our hands as on our feet? Is there some evolutionary reason or is it simply a pretty big coincidence?"

This is actually a big question in 'evolution and development', and although I am unaware what the answer is,  The question is often asked "which came first, fore or hind limbs?"

The reason that we have the same number of digits on our feet and toes is that at some point in evolutionary history the genes that control the development of our first set or paired appendages, took over and made another very similar set of paired appendages further up/down our ancestral body.  Remember it is not just the digits that are the same number on our hands and feet, we have two bones in our forearm, the radius and ulna, and two in out lower leg (fibula and tibia).  Working up the limb towards our body, we have one bone in the upper arm (humerous) and a single bone in the thigh (femur).  The arm and leg are surprisingly similar. 
Now obviously it is not the case that there was an ancestor that independently evolved a limb with seven digits, another that evolved a limb with 5, and another with 3 fingers and 4 toes, as we see in fossil tetrapods, living vertebrates, from frogs and salamanders to us, and birds. 
What we know is that the genetic hierarchy, that controls limb development has likely been tweaked at different levels in different groups.  In seven-digit/limb vertebrates, a pattern was set up whereby one set of genes controlled the development of  front and hind limbs.  In 5-digit/limb vertebrates, this genetic control of the number of digits was altered to say make all limbs have 5 digits.  This lineage by luck alone, gave rise to all the living groups of tetrapod vertebrates alive today.  Lastly, the loss of fingers or toes in individual groups like T-rex is down to genetic control of specific limb pairs.  The overall control in groups like this will still be saying 'make limbs that are the same for the front limbs, as the hind', but development of structures like toes can then be tweaked again further down the hierarchy of genes, reducing the amount of cell growth, and so the numbers of fingers that actually result in certain groups.

Last edited by Neil Gostling (14th Oct 2007 14:09:46)

Hi Oli,

what a great question you've asked, obviously there have been a lot of responses! Here are some of my thoughts on the topic:

Few examples of polydactyly occur in the animal kingdom today, the panda’s thumb though, is one classic example. The panda has five digits on its paw plus an opposable 'thumb' but this thumb is not a sixth digit like the others, but actually an unusual outgrowth of a wrist bone. Thus even the panda’s thumb is not truly an example of more than five digits.

So what about the earliest tetrapods, amphibians like Acanthostega (pictured at http://universe-review.ca/I10-72-Acanthostega.jpg) and his buddies? The Late Devonian was a busy time, plant life was diversifying and insect life was gaining ground on land. Tetrapods, were making their first steps on to land and living semi-aquatic lifestyles about 375 million years ago. These animals had many characteristics advantageous to aquatic life, such as streamlined bodies, webbed feet and tail fins. But they also had weight-bearing limbs with which they could lift themselves out of the water. Acanthostega had eight digits on its front and hind limbs and two other early tetrapods, Ichthyostega and Tulerpeton, also had more than five digits.

So how could these animals have had such a wide variety of limb structures when all of their descendants seem to have a variation upon the five digit structure? To be honest, we don't understand how these animals could have been so experimental while their descendants were so conventional. It is thought that pentadactyly evolved in a sinlgle lineage of animals that was ancestral to all present day tetrapods (amphibians, reptiles, birds and mammals) and that this event event happened 340 million years ago in the Lower Carboniferous (about 35 million years after the first tetrapods evolved).

And why 5 anyway? No one knows. And we have no examples to examine. There are few animals who have extra digits. The most common is the novelty polydactyl cat, but this species is the result of selective breeding of animals with a genetic anomaly.

In a general sense, we know that it easier to ‘lose’ a trait then to gain it, hence the large number of animals who have reduced digits. But the striking lack of polydactyl examples in the long history of tetrapods since the Devonian implies there may be an evolutionary constraint at work. For example, pleiotropy is the multiple effects of genes on more than one physical characteristic. Hand-Foot-Genital syndrome illustrates such a condition. This rare disorder malforms limbs and the urinary system because the same defective genes pattern both systems. So perhaps the constraint on tetrapod limb structure is part of a greater pattern.