A few weeks ago I found myself reading up about the evolution of language in preparation for a talk I gave to the Applied Linguistics Association of Australia’s conference in Perth. It turns out the evolutionary forces that shaped humanity’s capacity for language remain far from well resolved.

Just about everybody agrees that language is awfully useful, but how much of that usefulness reveals the purpose for which language originally evolved? Do we owe the gift of the gab to the fact language can be used to talk our way out of trouble, or to eavesdrop, or to cooperate with others, or to trade, or to court and seduce mates?

Some of the most influential thinkers in this area, including Noam Chomsky, have argued that our capacity for language might be a byproduct of adaptations with little to do with communication and lots to do with computation. Perhaps, they argue, language is a recent embellishment to our evolved capacity to calculate quantities, to navigate, and discern the complex web of who-owes-what-to-whom on which human social life depends.

While the researchers involved each have their favourite theories, nobody seriously claims that language has a single adaptive purpose. Few traits, and particularly not complex traits like language, have a single adaptive function. The excitement and the controversies surrounding these traits arise out of questions about the relative importance of different evolutionary forces, and how they act.

Speak hands for me

Yet so many popular stories about evolution strive to pinpoint a single evolutionary force responsible for a particular trait. I am expecting that today will provide a new tide of stories that are just so, when news of a new biomechanical study of human hands and fists hits the presses.

This intriguing study was inspired by an argument about the adaptive forces that shaped, of all things, the spermaceti organ in the sperm whale’s head. These immense organs produce the waxy substance that made sperm whales such prized quarry for whalers. Spermaceti organs are thought to help whales regulate their buoyancy, or to act as a lens for the whale’s echolocation system.

When bio-mechanics expert David Carrier published a paper arguing that the spermaceti organ functioned as a battering ram in fights, his friend and colleague Frank Fish disagreed. Professor Fish, to illustrate his point, waved his fist and exclaimed ʻI can hit you in the face with this, but it did not evolve for that!ʼ

Which got Carrier thinking about fists, and earned Fish an acknowledgement in Carrier’s most recent paper. That paper, published today in the Journal of Experimental Biology, under the title “Protective buttressing of the human fist and the evolution of hominin hands”.

Human hands can rival language for expressiveness. They can perform neurosurgery or 12-bar blues, lift a pencil or twice their owner’s body weight. Five million years ago our ancestors probably had hands like chimpanzees, with short thumbs, long fingers and elongate palms. Unlike language, good fossils document the evolution of hominid hands. Our palms and fingers have shortened but our thumbs have become longer over the last five million years.

And yet we know little about the selective forces that produced these changes. Certainly a move away from tree-dwelling freed our hands to evolve in new ways. Feet and hands develop via shared pathways, and some models suggest that changes to our hands are a mere byproduct of the optimisation of feet for upright walking and running. Most discussion about the evolution of hands concerns the benefits of dexterity and delicateness, especially for making and using tools.

But Fish’s brandished fist suggested to Carrier and his colleague Michael Morgan that fighting might also have shaped the human hand. In a series of experiments that involved subjects hitting punching bags, Morgan and Carrier tested the effectiveness of the human hand as a weapon. In short they found that a clenched fist buttressed by the thumb doubles the force that a punch can pack.

They conclude that the shorter, squarer hands but longer thumbs of modern humans combine to make the fist a surprisingly effective weapon. And so, they argue, that the human hand was probably forged as much in combat as it was in more refined activities.

Fighting

It might be difficult, from a 21st-century vantage point, to fully appreciate the evolutionary benefits of fighting well. But men in particular have long fought one another for resources, status, and respect, all of which improve a male’s prospects on the mating market.

The presence of genes that predispose carriers to the unfortunate condition known as left-handedness provides a clue to the importance of fighting in our evolutionary past. Southpaws enjoy a strategic advantage over their right-handed rivals because most lefties are used to fighting right-handers, but those right-handers are unused to left-handed opponents. The same advantage, I might add, now applies to almost all sports involving a one on one contest: tennis, boxing, baseball, and cricket. Genes that raise the chance of left-handedness are maintained by the advantage left-handers enjoy in fighting right-handers.

Returning to today’s paper about fists, Morton and Carrier are very careful not to claim that sexual selection on fighting ability is entirely responsible for the present shape of the human hand. Only that it could plausibly be one of the adaptive forces involved. Critics are already lining up to take a crack at their idea, citing other arguments that fists are seldom the best available weapon.

This is a paper in which the sound-bite certainly isn’t the message. Like most science, it doesn’t bestow a punchy, perfectly-formed new fact. Instead it forms a small part of a long discussion about how evolution works and how science operates.

Like language, and spermaceti organs, and just about every other trait that has ever evolved, our hands have been fashioned by a variety of forces. Which is why they are such versatile appendages.