Becoming Human

Just over seven million years ago our lineage and that of the chimpanzees diverged from a common path. Given the lack of fossil evidence, we don’t know much about our closest living relative’s evolutionary history. For humans though, there is a remarkable amount of fossil evidence that records three major adaptive regimes since our last common ancestor with chimpanzees. From ~4-6 million years ago the earliest adaptive suite is characterized by the genus Ardipithecus. The two major human-like innovations seen in this genus are the small, feminized canine teeth in the males and standing upright to walk on two legs. While these two features may seem unrelated, they evince a major shift in reproductive strategy – how males and females interacted around the care of their young. Selection had favored males that no longer fought over access to females but who were, instead, pair-bonded to a mate. Males and females were committed so much to caregiving that they walked on two legs, a locomotion style that greatly increases one’s ability to carry food and/or children for long distances. When you see a father today carrying his little child, remember that behavior is one of the earliest adaptive strategies of our lineage. The second adaptive regime existed from ~4 – 1.5 million years ago, represented by the genus Australopithecus. Bipedalism had been so advantageous that the skeletal features that enabled Ardipithecus to still be good at climbing in trees (i.e., a divergent big toe) were lost as selection gradually reconfigured the skeleton to the much more efficient bipedalism we see in Australopithecus. This shift in locomotion is seen alongside an increase in the size of the teeth. We also find fossils of Australopithecus in a wider range of environments. Where Ardipithecus is found only in woodland environments, Australopithecus’ larger teeth and efficient walking provided them with more varied survival strategies; in a sense, they were more catholic. This expansion of food types and environments dramatically increased for the third adaptive regime, that of the genus Homo. Today, with the dramatic increase in reliance on technology, our genus has the widest and most varied geographic range of any living mammal, the closest would be the gray wolf. Perhaps then, this shift towards a more carnivore-like strategy was one of the key innovations to our genus’ evolutionary success. The detailed evidence lies within the archaeological and the genetic evidence, the next two talks in this session.