Social Interaction and Complexity

The brain of modern Homo sapiens is a sophisticated organ with the capacity for complex and abstract thought, communication and metacognition (thinking about thinking). The evolution of this capability is linked to biological and cultural factors such as development of the brain as an organ and development of social complexity and cultural capabilities. 

The evolutionary success of humans and their direct ancestors can be linked to selection that led to the development of intelligence. This is not the result of selection relating to one characteristic. Rather it is the selection for a combination of characteristics which collectively contribute to the development of intelligence, and the accompanying evolutionary success. Bipedalism, tool making, increased brain size, the emergence of language and the development of culture are key contributors to the success of the human species.   

Human evolution is the process by which, over time, humans have changed to optimise the fit between the individual members of a population and their environment. Evolution of a species (macroevolution) and evolutionary change within a species (microevolution) operate to produce an organism that is matched or adapted to its environment; that match is not dependent on a particularly long or comfortable life for an individual, but rather on the successful passage of that individual’s genes to the next generation. So, in contrast to a modern human perspective where success is measured by a long and healthy life, evolutionary success is measured by successful reproduction.   

All organisms on the planet today are here because their ancestors successfully reproduced. Lineages that did not do so are now extinct. This is the core concept of fitness, which is fundamental to evolutionary biology. Evolution is the process whereby a population changes over time to optimise fitness of its individual members within a particular environment – so Homo sapiens evolved by adaptations that maximised its fitness in the environments of eastern Africa, the region in which our species first emerged. Biological fitness for a human was – and still is – achieved by a strategy of supporting a small number of offspring to grow successfully to adulthood, reproduce and live long enough to support their own children reproducing successfully. Evolutionary pressures on our lineage operated to ensure this. Health and longevity beyond the reproductive period of the life course, or the period necessary to support offspring into adulthood, are not drivers in the process of evolutionary selection.

There are approximately 200 existing species of primate. They are typically arboreal (tree dwelling) species generally living in tropical and subtropical environments. Some common physical features of the primates include (Figure 1): 

• Opposable thumb/big toe on the hands and feet 
• Locomotion which tends to be hind‐limb dominated 
• Enhanced vision and a reduced sense of smell compared to many other mammals 
• A specific pattern of dentition (teeth) 
• A flattened face

Primate brains are large relative to body size. As a result there are associated changes in life history traits, generally reflected in relatively long life spans, late onset of reproduction, having few offspring and singleton pregnancies. 

Humans or Homo sapiens are upright apes belonging within the primate group. Humans have a particular form of locomotion, communication, cognitive capacity, capacity for intentionality and foresight and the capacity to develop and use technologies. We are a species that lives in social groups and have uniquely developed cultural capacities, defined as knowledge, behaviour and tradition within a particular community or population. 

The Hominoid Clade 

Hominoids include the superfamily of apes and humans. This group is made up of the currently living great apes (the orang‐utan, the gorilla, the bonobo, the chimpanzee and the human), their extinct ancestors and other extinct species which had evolved from the last common ancestor of these five species. Molecular evidence shows that the orang‐utan lineage was the first to split from the primitive hominoid lineage some 12‐15 million years ago and that the gorilla lineage split perhaps 7 million years ago (Figure 2). We shared a last common ancestor with the chimpanzee and the bonobo as recently as about 5‐6 million years ago. The hominin group consists of humans and their direct ancestors, which split from the rest of the hominoid clade 5‐6 million years ago. These dates are based on estimates from the molecular clock which measures the rate of change at a molecular level (DNA, RNA or proteins). The molecular clock assumes that the rate of mutation of these molecules is relatively constant over time and amongst different species. This means that the molecular difference between two species is proportional to the time since they split from a common ancestor.