What Makes Us Human

And one percent Neanderthal

Back-packing Mommas and Brainy Babies

Did early human mommas create slings to help develop bigger brains?

Pepohoan mother and child (1875)
http://commons.wikimedia.org/wiki/File:Pepohoan_Mother_and_Child.jpg
I routinely describe the research I do as about "how people make things and things make people".

So I am predisposed to like anthropologist Tim Taylor's assertion in The Artificial Ape that the invention of devices to carry babies, leaving early humans with free hands, was the mother of human braininess.

An interview in the British weekly New Scientist gives a clear exposition of the argument:

"Darwin had to put large cranial size down to sexual selection, arguing that women found brainy men sexy. But biomechanical factors make this untenable. I call this the smart biped paradox: once you are an upright ape, all natural selection pressures should be in favour of retaining a small cranium. That's because walking upright means having a narrower pelvis, capping babies' head size, and a shorter digestive tract, making it harder to support big, energy-hungry brains. Clearly our big brains did evolve, but I think Darwin had the wrong mechanism. I believe it was technology."

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The interviewer quickly underlines the implication that human technology precedes the emergence of the quintessentially human trait of large brains. In my characterization of what my research is about, this would be equivalent to saying I study "How things make people and people make things". (Which is just as accurate, but that's another post for another day.)

In the New Scientist interview, there follows an exchange that I think is really a distraction, about whether the existing data showing stone tool production apparently earlier than the first recognized members of the genus Homo should be trusted. This is a classic "spoiler" argument in archaeology: if you don't like the way someone is explaining the existing evidence, point to the possibility that the archaeological record is incomplete (which it always is).

But it isn't stone tools that Taylor is identifying as the trigger for human brain size increase. It's an entirely invisible technology of perishable flexible material:

"Upright female hominins walking the savannah had a real problem: their babies couldn't cling to them the way a chimp baby could cling to its mother. Carrying an infant would have been the highest drain on energy for a hominin female - higher than lactation. So what did they do? I believe they figured out how to carry their newborns using a loop of animal tissue. Evidence of the slings hasn't survived, but in the same way that we infer lungs and organs from the bones of fossils that survive, it is from the stone tools that we can infer the bits that don't last: things made from sinew, wood, leather and grasses."

Now, that last sentence may have lost the non-archaeologists: Taylor is saying that the stone tools are a secondary technology made to create other things, and that the existence of those other things-- although long since rotted-- is still implied by the tools used to make them.

Taylor compares his back-packing mommas to kangaroos (stay with me here, the analogy will make sense): human infants could be born more and more dependent, and continue development outside the womb, especially development of those over-sized brains.

In a review of Taylor's book in the British newspaper The Guardian, science writer Peter Forbes argues that this suggestion and a series of other recent, speculative arguments about early hominin development represent

a reversal of the received idea of evolution through natural selection. In this, a mutation takes place that happens to be useful; it is retained and spreads through the population. In the new theory, proto-human beings, through innovative technologies, created the conditions that led to a rapid spread of new mutations.

This is a fairly grand claim, but not, I think, indefensible. Anthropologists are returning more agency to humans, even very early humans, in shaping our own destiny as a species. And it is our technologies that are identified as the medium for our self-shaping.

But I still have one problem with the proposal, and even more so, with the reporting about it. There is a kind of circular logic built in, and in this case, it is a gendered circular logic. The problem of carrying babies is a problem for females in the early hominin world; so the females developed the technology to solve it. Along the way, they also solved the problem of giving birth to babies with large brains.

This is not an uncommon way to represent archaeological arguments about the emergence of human capacities: as inevitable causal chains leading to what we know finally occurred. We know we have large brained offspring that push the limits of female anatomy during childbirth, so classically we have looked for a factor that favored greater brain size enough to overcome the adaptive disadvantage of having a large number of mothers die in childbirth.

Taylor's argument is different, and reporting about it comes close but does not quite capture that difference. I see his proposal as similar to archaeologist K. D. Vitelli's work on "looking up" in early pottery making in Greece.

"Looking up" implies restoring the arrow of time, not looking backward from the present for the causes that inevitably produced what we see in the recent past.

You have to understand adoption of a practice in its own terms, with its intended consequences; and then separately evaluate the extra effects, or unintended consequences, that it facilitated. In Vitelli's example, the first fired clay vessels in Greece were made in small numbers using closely guarded formulas as part of healing rituals; pots were medicine. Once the technology existed, though, it could be generalized for use in cooking, eating, and storing foods.

Taylor argues that early hominins needed to be able to free their hands in bipedal posture. Slings allowed female hominins to secure dependent young on their backs. That meant that less developed young were no longer at as much of a disadvantage, and these under-developed humans could survive and pass on their genetic propensity to be born earlier and with a less-mature brain. Brainy, less developed young were an unintended consequence of use of slings.

Necessity might have been the mother of invention; but its offspring was an unexpected bundle of joy.

Rosemary Joyce, Ph.D., is a professor of anthropology at UC Berkeley.

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