TCS Daily


Evolution's Poker Hand

By Kenneth Silber - May 31, 2005 12:00 AM

As I type these words, my fingers fly across the keyboard -- albeit not as efficiently as I would like. For one thing, I have never really been a 10-finger typist. For another, I have only 10 fingers. Surely, I could get this piece written and emailed to TCS Headquarters faster if I had, say, 12 or 14 fingers. Why don't I?

Although it might sound like I'm on drugs, the question I am raising is one that is of interest in evolutionary biology. Humans and numerous other species have five (or fewer) digits per appendage. Why? And moreover, why have selection pressures not given rise to appendages with more digits? A few extra fingers, after all, would allow more detailed grasping and other operations, and thus a mutation that provided them would seem like a good bet to spread through the gene pool.

The explanation, it turns out, tells us something not only about the evolutionary past but also about humanity's future -- about the potential and limitations of genetic engineering. In one scene in the 1997 sci-fi film Gattaca, a close shot of a pianist's hands reveals that he has six fingers on each (and thus he gives a virtuoso performance of "Impromptu for Twelve Fingers"). Just what would be involved in making such a concert possible?

Some 360 million years ago, at the end of the Devonian era, an evolutionary experiment was underway. Fish, which had long been the only vertebrates on the planet, were giving rise to tetrapods, animals with four limbs. The evolutionary link consisted of fish with two pairs of fins that gave them a survival advantage in shallow water (they could push themselves off if stranded, for instance). Some tetrapods had more than five digits on their appendages. One such animal, Aconthostega (a fossil of which was found in Greenland in 1987), had eight digits on each appendage (and had lungs as well as gills).

By 340 million years ago, there were tetrapods that spent much of their time on land, such as a small, lizard-like animal called Casineria (a fossil of which was found in Scotland in 1992). Casineria had five digits on each limb. Having relatively few digits may have been an advantage in making a land animal better suited for walking rather than aquatic paddling. Casineria also had more flexible digits (the fossil bones have furrows that once held ligaments) and a stronger backbone, key adaptations to land life.

A basic pattern of having five digits thus was set early in the history of land vertebrates. It seems to have taken hold before the evolutionary break between amphibians and amniotes (a broad category that ultimately would include dinosaurs, birds, reptiles and mammals). Many land animals today have fewer than five digits, but there is a notable absence of species that have more than five. This is not entirely surprising; in evolution, losing a feature that is not needed is relatively easy, compared to gaining (or regaining) a potentially useful feature.

Still, in the sweep of evolutionary time, one would expect some species to have found a survival advantage in having extra digits -- for example, in becoming better at tree-climbing or, in the case of more recent human ancestors, tool-making. The fact that this did not happen suggests that there are genetic constraints involved. Clifford J. Tabin of Harvard Medical School has suggested that the genes that control digit formation are structured so as to produce five basic patterns (thumbs, pinkies, etc., in the case of a human hand). Thus, additional digits that arise through genetic abnormalities merely duplicate an existing digit, and thus offer little survival benefit.

An even more daunting constraint appears to arise from pleiotropy, the capacity of genes to influence multiple physical characteristics. A rare genetic disease called Hand-Foot-Genital Syndrome involves malformations of the genito-urinary system as well as limbs; the cause seems to lie in incorrect coding across the same set of genes that handle digit formation. Thus, in the course of evolution, whatever advantage arose from having more digits may have come alongside the disastrous disadvantage of having malformed reproductive organs.

Vocal proponents and critics of genetic engineering both tend to assume that the technology is on the verge of transforming humanity. But the practical difficulties of achieving the hoped-for or feared transformation are too often overlooked. Perhaps, future genetic engineers will be able to create a 12-fingered pianist without unwanted side effects. Yet the complex evolutionary and genetic history behind having five fingers on each hand suggests that such virtuosity, scientific and musical, may be a long time in coming.

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