So what does make humans unique?

I went to hear Dr. Kim Hill's talk on Thursday on the origins of human uniqueness. Hill began by framing our uniqueness in terms of our energy usage and biological dominance&emdash;for example, the fact that we cycle more nitrogen than all other terrestrial lifeforms combined, and we represent 10 times more biomass than any other large species that ever lived. We also exhibit extreme social complexity and specialization; no other species has anything remotely resembling the New York Stock Exchange or the NCAA basketball tournament, for example. Moreover, even before agriculture, we had colonized every landmass, and hunter-gatherers exhibited unusually complex social behavior compared to that of other animals. However, although we exhibit non-unique traits that arose through non-unique processes, we somehow turned into this distinctive species. The question is, how?

Hill described a combination of critical features that enabled all this to happen: cumulative culture, non-kin cooperation, language, and various cognitive capacities. These features emerged from various preadaptations, including bipedality, a change in our dietary niche, and a shift in the human life history (that is, the timing and duration of the events that make up the human lifecycle) and social structures. He focused on a chain of events involving non-kin cooperation and cumulative culture. To illustrate each link in the chain, he used a great deal of data from his own work and that of others with current-day hunter-gatherers in South America and Africa. The story goes roughly like this:

  • Our feeding niche shifted from collecting foods to extraction and hunting. (Extraction Includes the gathering of roots, nuts, and other things that require some labor or ingenuity to get.)
  • As a consequence, there was more variation in the quantity of food available each day. This in turn led to daily food sharing. In two hunter-gatherer tribes that he mentioned, nuclear families never keep more than 50% of any type of resource that they acquire. This buffers the variability for everyone day by day and on longer timescales, and provides protection against starvation due to illness or injury.
  • The result of this is a fourfold difference in adult mortality between humans and chimpanzees. Adult mortality affects the entire life cycle: delaying maturity, delaying senescence, and extending the productive years of adults.
  • Because maturity is delayed, and children depend on their parents for food, parents have multiple dependent offspring at any one time. As anyone who has bought groceries for a large household can imagine, this is difficult for parents. Several strategies arose to address this problem. One of them is that adults without children, for example, siblings of the parents, may help out (AKA opportunistic cooperative breeding, or helpers in the nest). In fact, the food sharing that arose initially as a way of reducing the variation in the food supply, and thus the risk, became intentional overproduction. That is, people seek out excess, for instance, by gathering more food than they need rather than stopping when they have enough for themselves.
  • An additional consequence is that women live past their reproductive capacity. That is, women go through menopause.
  • These cooperative breeding arrangements favored the evolution of prosocial emotions, or investment in the well-being of other people in the same group. In insect colonies, cooperation occurs because the insects are closely related. Hill presented evidence that shows that this is not the case for hunter-gatherer bands: People cooperate and do things for the benefit of others even if they are not related.
  • Juvenile dependence and the resulting investment of fathers in their children and pair bonding gave rise to a pattern described as the exogamy complex which, in a nutshell, eventually created complex social networks that included people who were not genetically related (e.g., in-laws) that allowed cumulative culture to arise.
  • Cumulative culture is what happens when you not only learn how to build or use a spear or grinding stone or web page, you figure out a better way to do the job. Although other animals learn things from each other, like learning how to use sticks to dig out termites, none of them build on what they learn and improve on what they have learned. Hill suggested that the shift to large social networks may have been as important as evolved cognition in explaining why we begin to produce cumulative culture in the late Middle Pleistocene 200,000 years ago.

Obviously this leaves out important things like language and brain size and other cool things about how we got to be the way we are. However, it's still a fascinating story of our roots. What struck me in particular is the difference between this and Hobbes's description of life in the natural state as nasty, brutish, and short. The hunter-gatherer societies that he described have a form of income insurance that protects against illness or injury, for example, and the entire system of hunter-gatherer life that he described was nothing like “every man for himself.”

At the end of his lecture, Hill noted that maybe we shouldn't be thinking so much about whether we will ever contact intelligent species in the rest of the universe; perhaps we should be thinking instead about highly cooperative social species. A single intelligent human could never have gotten to the moon alone.

One of the most interesting questions that came up in the Q&A afterword was the question of whether it's likely that there could be one more than one such species per planet. The answer, rather sadly, is probably not. It looks as though the price of our success might have been the extermination of all competitors, even distant ones. Hill estimates that we exterminated at least five other hominin species that could have also followed our trajectory, and right now we seem to be in the process of exterminating the chimpanzees.

You can read more about Prof. Hill's work in this New York Times article.