Last week I wrote about the physical evolution of humans. Today I write about the evolution of human characteristics. I have drawn on the account given by Ernst Mayr in This is Biology (Harvard Univ. Press, 1997).
The traditional explanation for the evolution of human characteristics was that as Africa's climate became drier in the Miocene period (five to 23 million years ago), many groups of our human ancestors became isolated in open landscapes where walking upright was an advantage. This freed arms and hands for tool use. This, in turn, created an advantage for invention and use of new tools, which created a selection pressure for brain enlargement for this purpose.
Although consistent upright two-legged walking (bipedalism) is uniquely human, it is now believed that this alone does not account for tool use and tool use alone does not explain the dramatic growth of the human brain. The widespread use of tools by chimps suggests that tool use was common before bipedalism evolved. Also, there was little advance in tool technology or brain size two million years after the first human tools appear in the fossil record.
The early australopithecines, from which humans evolved, were built for a semi-arboreal life with feet adapted for climbing and arms longer then later fossil hominids and modern humans. Therefore the young had to be born well-developed in order to cling to their mother as she climbed.
The shift to full terrestrial living 2.5 million years ago freed the mother's arms for carrying babies. This allowed a lengthening of the helpless newborn state and facilitated continued growth of the brain in early infancy, which is a very human characteristic.
The australopithecines were mostly vegetarian. It was believed that the shift to terrestrial living when Homo erectus evolved and hunting began, caused a shift to an animal diet. However, we now know that meat was only an occasional part of the diet of Homo erectus. Large animal hunting was a late evolutionary development. Early humans derived much food from wild cereals and grass seeds.
The eventual development of hunting probably played a large part in humanisation. There was a requirement for elaborate base camps, planning of hunting strategies, invention of more effective weapons, and so on. There was also a need for improved communication - speech.
Homo erectus had a larger brain than the australopithecines, but really large brains only developed within the last 150,000 years.Obviously, strong selection pressures pushed this explosive development. In addition to infant-carrying and hunting these included speech development, which allowed the acquisition and transmission of culture across generations.
The Homo line was pre-adapted anatomically for the development of speech. Neanderthals lacked some of these anatomical features and are believed to have been inferior in forming sounds. This may explain why they did not make better use of their relatively large brains.
Human speech developed about 250,000 years ago. However, about 100,000 years ago the increase in human brain size stopped and has not increased since. There is no convincing explanation for this fact.
It may be that the increasing social integration of humans into larger groups afforded increased protection to individuals with relatively smaller brains and inhibited further evolution of the genome.
One of the most important developments in human cultural evolution was the social integration of the hominid group. With the terrestrial mode of living, group size increased, offering better protection against predators, and greater efficiency in gathering food. Many physiological and behavioural changes that facilitated group survival were favoured - continuous sexual receptivity of females, concealment of oestrus (animal heat), development of menopause, expanded life expectancy and other human characteristics.
Competition between neighbouring groups must have been fierce at times, with stronger groups often eliminating weaker groups. This may explain the extinction of the Neanderthals.
Within social groups the benefits of co-operation are opposed by the potential for conflict, particularly male competition for females. In humans, some of this conflict was toned down by a cultural trend towards monogamy. Marriage also became a strategy to forge connections between families that might otherwise be competitors. Rules to avoid incest were developed to lessen conflict within families and to enhance genetic variation.
The foundation of group structure throughout the hominid line has always been the family. Family cohesion exists both in the core (parents/children) and extended (uncles, aunts, cousins) family. The extended family is important not just for general help but also for cementing culture and transmitting it to the next generation. The weakening of the family is one of the biggest problems facing modern Western civilisation and we would be well advised to protect this vital institution.
(William Reville is Associate Professor of Biochemistry and Director of Microscopy at UCC.)