The toe bone’s connected… errr… related to the finger bone

A recently published study has ruffled quite a few feathers in evolutionary circles by implying that Darwin didn’t get human evolution quite right. Darwin believed the unique human ability to knit (or play guitar or do anything else cool with our hands) while walking evolved in two separate steps. First, early humans stood upright. Then, since walking upright left their hands free, our ancestors did clever things like carry stuff, make tools and use tools which resulted in changes to their hands.

For the past 150 years evolutionary folks have tended to accept Darwin’s hypothesis. Not so for Dr. Campbell Rolian of the University of Calgary. Dr. Rolian (whose doppelganger is Ted Allen) and his colleagues say that Darwin was right about bipedalism (walking around on two feet) occurring first, but missed the mark when he implied that changes in the hands of early humans happened after (and more less separately) from learning the two-footed Cha Cha.

When early humans started walking upright they had feet very similar to today’s primates—short “big toes” (first digits) and long “little toes” (lateral digits). But as walking and running caught on, natural selection favored longer big toes and shorter little toes. (Dr. Rolian documented that shorter “little” toes are in fact better for running in some of his earlier research.) So the feet of early humans evolved.

And through a quirk of our genes, our hands changed along with our feet. Our hands and feet share much of the same genetic architecture—that’s science speak for having the same basic set of genes—so early humans had short thumbs to go along with their short big toes, and long fingers to go along with their long little toes. Dr. Rolian’s latest study showed that as our feet changed courtesy of natural selection, our hands mirrored some of the same proportional changes.

Dr. Rolian and his colleagues claim that these changes to thumb and finger length set the stage for tool use. Having a longer thumb and shorter fingers is better suited to handling tools than having a short thumb and long fingers. Modern primates (who still rely on arms and hands for walking) still have the short thumb/long fingers combination which prevents them from touching the tips of their fingers together. To do any kind of detail work with tools (like feed grass into a termite mound), they’re left with the less dexterous option of squeezing a short thumb against the side of their long “lateral digit.”

According to Dr. Rolian, walking upright didn’t just free up our hands, it was responsible for changing our hands so that we could start using tools.

A chat with Dr. Campbell Rolian, PhD

Campbell and I go way back—we’re talking WAY back. So when his paper came out and was splashed across the headlines (a number of news outlets ranging from the BBC to an obscure Vietnamese publication covered the study), I took the opportunity to ask a few behind-the-scenes questions. Here’s what he had to say:

PM: What are you most stoked about with this study?

CR: There’s a tendency, especially among anthropologists, to diagnose everything as an adaptation. You come up with a ‘Just So’ story that describes why a trait exists. Problem is, there’s really no basis for the story and it risks turning the trait into something that it’s not. There have to be some aspects of our development that are merely artifacts—byproducts of the evolution of other traits, or things that just came about by chance in the process of building other parts of our body. For example, there’s nothing adaptive about having white bones. Our bones are white because the calcium phosphate compound that is the main ingredient of our bones happens to reflect all wavelengths of light not because having white bones gives us a particular advantage. So I’m excited that what we’ve found offers an example of a non-adaptive trait. The initial changes in human hands that led to our improved tool use were really just a result of changes to our feet. It was a byproduct of bipedalism. Our study provides a warning out there to human anthropologists to think of alternative hypotheses to some of the ‘Just So’ stories they come up with.

PM: What prompted you to look at the relationship between bipedalism and tool use?

CR: The proportions in our hands and feet are similar, in terms of the relative length of our fingers and toes. Our big toe and thumb are longer, whereas the lateral digits [other fingers and toes] are short and straight compared to early humans. Knowing that our feet and hands share most of the same genetic architecture, it made sense to compare their proportions. It could be that the proportions were similar as a result of coincidence, i.e. the proportions best suited for walking and for tool use just happen to be the same by chance, or there could be something more at work. We wanted to find out if the similarities were in fact the product of co-evolution and not just of coincidence. Fortunately we have the math tools that allow us to distinguish between those two possibilities.

PM: So… why is this study important? (I felt bad asking him why his work mattered, but hard journalism knows no boundaries…)

CR: What we found isn’t going to do anything so dramatic as to prove (to Creationists or Intelligent Design proponents) that evolution is real. But what it does do is shed a little more light on a theory that’s stayed mostly the same for the past 150 years. To my peers it shows that we can take a closer look at the mechanisms of evolution in order to better understand them. And people like to know things about evolutionary history and why the human body looks like it does. Focusing on humans helps get people interested in evolution. Besides, if I had done this study on lizard toes, I don’t think as many people would have been interested.