Delayed Development and Human Evolution

For hundreds of thousands of years, Homo neanderthalensis was the dominant hominid species of Europe and the Middle East. Then, somewhere in the range of 80,000 to 50,000 years ago, modern humans (Homo sapiens) expanded out of Africa and came in contact with the Neanderthals. Although there is some evidence of limited interbreeding between Homo sapiens and Homo neanderthalensis, for the most part it does not seem that theirs was an overly cordial relationship: as humans spread into Europe and the Middle East, Neanderthal populations receded into small isolated pockets before becoming completely extinct. Whether we outcompeted our closest hominid relatives through direct or indirect means is not clear, but what is certain is that we maintained some kind of advantage over the Neanderthals. What features gave us the slight edge that eventually led to our dominance?

A recent National Public Radio All Things Considered segment entitled “Growing Slowly, Humans Outsmarted Neanderthals” tackles this problem by featuring the work of several prominent anthropologists who specialize on Neanderthals. Tanya Smith and Jean-Jacques Hublin, both of whom make short statements in the piece, head up large scientific teams that have performed investigations using Neanderthal remains to reconstruct the developmental history of our near relatives. In a recent article in the Proceedings of the National Academy of Sciences, Smith and colleagues (2010) describe the differences in maturation rate that can be read from the daily growth patterns left behind in teeth. Comparing the teeth of Neanderthals with those of ancient and current-day Homo sapiens, the study shows that Neanderthals grew at a slightly faster pace than the humans they competed with. Interestingly, the growth rates of ancient and current-day humans were indistinguishable. In the words of Smith, the human strategy was and is “live slow and die old”.

The other major research paper featured in the NPR segment was published by Gunz and colleagues (2010) in Current Biology. Using CT scans of modern human and Neanderthal skulls, the authors consider how developmental pathways of these sister species might differ based on a combination of simulations designed to extrapolate forward from neonatal skulls and backwards from adults skulls. What they find is that postnatal development of human and Neanderthal brains differ. Although both species end up with a relatively similar-sized brain, it seems clear that the brain of each species is undergoing a different kind of development, which would lead us to believe that the cognitive capacities of each brain would differ.

I think that there are a couple of interesting issues that emerge from this work. The first is that although this kind of work is valuable because it begins to define how humans and Neanderthals differ, we are still a long way from explaining how these differences gave us the competitive advantage. Given that the Neanderthals are not here to defend themselves, it is all together too easy to assume that whatever we find in humans is better. This gets us into the dangerous zone of telling just-so stories. I think it is reasonable to hypothesize that slower development gave humans an advantage, but what advantage that was is not certain based on available evidence. Was it an advantage in language? The ability to work cooperatively in social groups? To make more sophisticated use of the tools we made? Unfortunately, the teeth and skulls cannot tell us the answer. Perhaps the best hope for discovering the true difference lies in the fossil DNA that can be extracted from Neanderthal remains, and the Current Biology article alludes to recent evidence that human genes responsible for certain forms of neurological development are absent in Neanderthals.

The second issue, which is not really covered in this segment, is that delayed development continues to be a hallmark of human evolution. Although I do not know if anyone has looked at the actual physical development of modern humans, it is clear that we do not reach social and cultural adulthood as fast as we used to. Our economic and educational development push further and further into physical adulthood, a change that has occurred in a very short period of time. I do not know if this change is exerting any selection on our gene pool, but clearly our culture demands that we put off reproduction and continue neurological development (in the form of extended learning) for many years more than even our grandparents.

Although I am glad to see NPR providing this kind of coverage, I do not love this particular piece. It plays up several human conceits that I frankly do think not need to be further encouraged. It starts out on the wrong foot by claiming that humans are the smartest species on the planet, backing up this claim by asserting “after all, what other species is going to argue with you?”. Clearly if you define intelligence by what we do best we will come out on top, but as any evolutionary biologist knows all forms of adaptational merit are environment-specific and nature can change what is on the test pretty quickly.

The segment also plays on old and long-rejected stereotypes of Neanderthals as “dull, brutish and poor conversationalists”. I guess in the defense of the writers down at NPR I should point out that the piece says that we think of the Neanderthals in this way, but who cares what we think if we have it wrong? No one really knows what kinds of intelligence Neanderthals had, and we do not know if, how, or how well they communicated. Although the articles highlighted do show differences between Neanderthals and humans, interpreting these differences is difficult and they are at best pretty slight. Maybe Neanderthals were just a little bit less adept at one thing but otherwise pretty competent: after all, slight advantages can still cause one species to outcompete another, and our ancestors took a long time to outrun or overpower the Neanderthals. I would much rather have the public informed about what we do not know than reminded of the old misconceptions that most people carry around.

And if these factual and conceptual distortions do not bother you, there is always this fabulous line, which NPR features in marquee quotes:

Like the ‘slow food’ movement, ‘slow growth’ gave complex brains more time to ‘cook’ and then learn all those things a fancy brain could learn.

I understand the use of metaphors to convey key concepts, and this one is a real clunker. I seriously doubt that the “development as cooking” metaphor is all that effective at conveying the significance of the slight differences in developmental timing that distinguish humans from Neanderthals.