In a post last month, I pointed out how aerospace engineers can learn a lot from looking at the fossils of ancient flying reptiles. Today's issue of Nature contains a variation on that theme: ancient swimming reptiles can teach geneticists a lot as well.

Almost all humans have five fingers. Genetic disorders can produce extra fingers and toes, but only rarely. Five fingers is generally the upper limit not just for humans, but for all vertebrates on land. You can find plenty of tetrapods whose ancestors lost one or more of those five fingers. Horses have just one; snakes none. But tetrapods with more than five digits are incredibly rare. In most cases, these aren't true digits--wrists bones or other parts of limbs have evolved into finger-like appendages (the panda's "thumb" made so famous by Stephen Jay Gould, for example). But if you're looking for seven or eight real digits--made of three or four rod-shaped bones extending from the wrist or ankle, you're out of luck.

As Gould pointed out in his essay "Eight Little Piggies" (from his book of the same name), nineteenth century biologists treated this pattern as a geometrical law. Five digits were part of the tetrapod "archetype"--the divine blueprint on which all variations were built. But that turned out not to be the case. In the 1980s, the paleontologists Jenny Clack and Michael Coates discovered that the earliest tetrapods that lived some 360 million years ago--vertebrates with legs and toes--had six, seven, or even eight toes.

At the time, biologists were just starting to figure out how genes build toes--and limbs in general. They also were studying how genes build fish fins, and they found evidence that some simple tinkering with just a handful of genes could turn a cluster of ray-shaped bones in a fin into a wrist complete with fingers. Clack and Coates showed that the way in which the extra fingers were arranged on their eight-fingered fossils was consistent with such a flip. (I summarized the state of this research as of 1998 in my book At the Water's Edge .)

Clack and other paleontologists have discovered more tetrapods from this same early stage, and they have many different arrangements of digits. A lot of evolutionary experimenting was going on, probably in part because mutations could produce radical changes in the limbs of early tetrapods. There weren't yet a lot of the regulatory genes in place for producing the standard five-fingered plan. Only about 20 million years after fins became hands and feet, tetrapods were finally sticking to the plan.

So why did our ancestors eventually settle on five fingers? One possibility is that for walking five fingers are better than six or twelve or any other higher number. Bear in mind that the very earliest tetrapods were more like fish with fingers. They had gills and could not have stood upright on their own limbs, suggesting that they lived mostly underwater. There they clambered over submerged logs and debris, much as frogfish do today. It may be no coincidence that the trend towards a consistent set of five fingers roughly matches the trend towards living ashore. Multiple toes probably helped give tetrapods better balance than just a couple, but there may have been a counterforce that put a cap at five--perhaps five digits are the most that can fit around a wrist or an ankle and still allow an animal to walk on dry land.

It's likely that part of the answer to this question has to do with how genes build the digits. Digits are already beginning to form when the limb is a tiny bud on the side of an embryo. There may be a tradeoff between the size of digits and the number of digits that can form in such a limited space. In order to make digits big enough to support a tetrapod on land, perhaps five is the most that constraints will allow. (Jenny Clack investigates these ideas in her excellent 2002 book Gaining Ground.)

Many of the most powerful genes in the construction of hands and feet also play just as important a role in building the entire skeleton, as well as brains and other organs. Any change in the way they work in a hand can have complicated effects on the way other parts of the body develop. Indeed, when people who have extra fingers or toes often suffer other disorders such as in the eyes or the skeleton. There may even be a connection between polydactyly and cancer. Tetrapods that could regulate the development of their fingers may have been favored by natural selection not only because they could do a better job of walking, but because they didn't get sick.

Maybe. Any hypothesis about the evolution of our hands and feet now has to contend with an intriguing fossil from China reported in Nature. The fossil was formed by an early marine reptile that lived 242 million years ago--over 100 million years after tetrapods had settled on five digits. The researchers report that the reptile (which has yet to be named) had six toes on their hindlimbs and seven on their forelimbs. It harkens back to the earliest tetrapods not only in having extra digits, but in where the digits form. In both the reptile and the earliest tetrapods, the new digits are tacked on to the wrist beyond the thumb.

The scientific report is unfortunately very short. The authors don't even name the creature or investigate what its closest relatives were. That's important to get a handle on how its strange digits evolved. But the paper certainly leaves me hungry for more. The authors point out, for example, that this reptile belonged to a lineage that had returned to the water and suggest that it had converged back on the frogfish-like hands and feet of early tetrapods that had not yet moved on land. Most marine reptiles I know about are more like sharks or dolphins, cruising open water and using their hands as steering paddles. So this particular reptile might represent an intermediate stage from land to sea.

What's even more intriguing is what these fossils say about this reptile's genes. Its extra digits are not malformed like the ones that people get from genetic disorders. That suggests that seven or eight digits was normal for the species, and that this single reptile wasn't a freak of nature. How did this reptile overcome the web of constraining genes that have prevented so many other species from acquiring extra fingers? How did they turn back the clock 100 million years? And why didn't any other known tetrapod that went back to the water (whales, seals, turtles, etc.) turn back the clock this way? The answers to these questions are not just a matter of paleontological curiosity. They may help geneticists understand how our own bodies are built, and how weaknesses are built into its design.