July 23, 2010
So what really makes us human?
Here’s the beginning of the Chapter 2 draft of my book, Finding the Li: Towards a Democracy of Consciousness. This section covers C. Owen Lovejoy’s theory of an “early hominid adaptive suite” arising from the discovery of Ardi (Ardipithecus ramidus). All constructive comments from readers of my blog are greatly appreciated.
Chapter 2: So What Really Makes Us Human?
Elephants have trunks; giraffes have necks; anteaters have tongues. What do we humans have that makes us unique? If you’ve read the first chapter of this book, you’re probably ready to answer: our pfc. Right, but other mammals also have pfcs, and advanced primates such as chimpanzees and bonobos* have pfcs almost the same size as ours. So what specifically was the change that led humans to populate the world with seven billion of us, building cities, surfing the internet, and sending rockets into space, while chimps and bonobos still live in the jungle and face the threat of extinction?
There are in fact a whole mess of things that are biologically different about humans when compared with other primates. There’s very little difference in size between males and females (known as “reduced sexual dimorphism.”) We walk upright on two legs (“bipedality”). We’re capable of fine manipulation with our hands and can throw projectiles accurately and powerfully. We have much smaller upper canine teeth. We have larger brains, factoring in the size of our bodies (known as “encephalization quotient.”) Our infants are helpless for years, relying entirely on adult support. Females don’t show any signs when they’re ovulating (known as “concealed ovulation”) and experience menopause. And we’re virtually hairless, except for our heads and genitals. Now, this seems like a highly varied set of differences. Which of these were the major drivers on the road to worldwide domination?
A basic timeframe may help to crystallize this question. It’s now generally agreed, as a result of genetic sequencing, that we humans shared our last common ancestor with chimpanzees and bonobos a little more than six million years ago. By way of perspective, that’s about the same time frame that horses and zebras, lions and tigers, and rats and mice also shared their common ancestors. If you just look at a physical comparison between humans and chimpanzees, that doesn’t seem too unreasonable. But it you look at the massive differences in what we’ve accomplished, there’s clearly something else going on. Michael Tomasello, a leading figure in the field of evolutionary anthropology, makes the point that “if we are searching for the origins of uniquely human cognition, therefore, our search must be for some small difference that made a big difference, some adaptation, or small set of adaptations, that changed the process of primate cognitive evolution in fundamental ways.” But where does the search begin?
A plausible story of our ancestors
In 2009, a major worldwide event occurred in the usually quiet and dusty corridors of evolutionary archaeology. In what was billed as the “breakthrough of the year,” Science magazine published a slew of articles on a newly discovered human ancestor which lived 4.4 million years ago in Ethiopia, known as Ardipithecus ramidus, or more affectionately, Ardi. What made the find so noteworthy was that enough remains had been found to reconstruct Ardi’s whole skeleton, which a large team had painstakingly done over fifteen years of intense excavation and analysis. Ardi’s reconstructed skeleton surprised conventional wisdom because, even though she was fairly close in time to our last common ancestor, she was already walking upright, as opposed to the knuckle-walking or swinging from tree branches that modern chimpanzees do.
In that issue of Science, team leader C. Owen Lovejoy used his findings to suggest a plausible story for a set of new behaviors that launched our ancestors on the road to homo sapiens, which he called “an early hominid adaptive suite.” In an analysis that rivals the best detective stories, he focused on three key characteristics differentiating Ardi and other later hominids from chimpanzees: bipedality; the loss of the big upper canine teeth that other primates have; and female concealed ovulation. What on earth could these three apparently unrelated developments have in common?
Lovejoy’s story begins with the thinning out of dense tropical vegetation due to changing climate. Ardi is thought to have lived in woodlands with small patches of forest. In order to get enough to eat, Ardi’s species were becoming omnivores, increasingly leaving the cover of the woodlands to venture out onto the plains, most likely scavenging for carrion that other predators had left behind. But the plains were a dangerous place for hominids that were used to living in the forest. Most predators could easily outrun them and cut them down. So, it made more sense for groups of males to cooperate closely with each other, going out together on multi-day foraging missions in the savanna. On successful missions, the males could bring meat back to their families, which would become an increasingly important source of fuel. Bringing back provisions was a lot easier if you could walk upright, using your arms for carrying, and this may have accelerated the evolution of bipedality. Gradually, the females began to choose males who were good providers for them and their infants, instead of the traditional choice of the most aggressive male. As cooperation between males became more important than aggressive competition, those large upper canines, used mostly for fighting rivals, became less important. In fact, females may have started to select mates with smaller upper canines, preferring a mate that would focus more on bringing back the bacon.
But there was still one problem with this otherwise happy scenario. No male in his right mind is going to go foraging with his buddies for days at a time if he sees his partner is in heat when he’s leaving. He’d be spending his whole time worrying about who’s getting into bed with her while he’s gone. This is where the development of concealed ovulation becomes so important. When ovulation is no longer advertised with a clarion call, the risk of cuckoldry decreases and becomes more manageable.
These are the set of behaviors that Lovejoy calls an early hominid adaptive suite, which he defines as a set of interrelated characteristics which, together, form a pattern that optimizes a species for enhanced evolutionary success. While some of Lovejoy’s arguments remain controversial, perhaps the most important part of the story, which has major ramifications for our own species, is the shift away from individual competitiveness to increased cooperation within a group. This cooperation was the launching pad for homo sapiens, as we will soon see.
 Bonobos, previously known as “pygmy chimpanzees” are one of the two species comprising the genus Pan, along with the “common chimpanzee.”
 For a useful table listing characteristically human attributes, see Flinn, M. V., Geary, D. C., and Ward, C. V. (2005). “Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence.” Evolution and Human Behavior, 26(1), 10-46.
 Tomasello, M. (1999). “The Human Adaptation for Culture.” Annual Review of Anthropology(28), 509-29.
 Gibbons, A. (2009). “Breakthrough of the Year: Ardipithecus Ramidus.” Science, 326, 598-599.
 White, T. D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C. O., Suwa, G., and WoldeGabriel, G. (2009). “Ardipithecus ramidus and the Paleobiology of Early Hominids.” Science, 326, 64-86.
 Lovejoy, C. O. (2009). “Reexamining Human Origins in Light of Ardipithecus ramidus.” Science, 326(2 October 2009), 74e1-74e8.