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Social Interaction and Complexity

This page supports specialist and non-specialist teachers by providing background information about the concepts that underpin the LENScience resources on social interaction and complexity.

What is Human Evolution?

Brain & Skull

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.


Evolutionary Fitness

The concept of fitness is central to understanding evolution. Evolutionary fitness is a measure of the match between an individual and its environment to best enable successful reproduction. It is important to note that fitness is measured as the ability to reproduce and live long enough to support offspring to the point where they reach reproductive age. Selection is based on adaptations that contribute to the success of reproduction. Health and longevity beyond reproductive years are not generally drivers in evolutionary selection processes.


Humans and Their Relationship to Primates


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.   


Adaptive Radiation

Adaptive radiation is a term used to describe an evolutionary phenomenon where there is the rapid appearance of multiple related species from a common ancestor. This is generally thought to occur when a species enters a new ecosystem where there are multiple ecological niches available. Natural variation within the ancestral species means that, over time, individuals that are best adapted for a particular niche will be selected for, and thrive in that niche, eventually forming new species. A classical example of adaptive radiation is the finches of the Galapagos where at least 13 species rapidly evolved from a common ancestral species which reached the Galapagos Islands. Each variant is adapted for a particular niche so that they are usually not competing with each other for resources and they can thus diverge and eventually form distinct new species. 


The primates and the hominins display adaptive radiation. We cannot be sure of the factors that drove the pattern of adaptive radiation seen in hominins, but they are likely to reflect a change in forest environment, perhaps reflecting climatic change.   


Currently the earliest fossil for which there is reasonable evidence to suggest that it was an ancestral hominin is a specimen of Sahelanthropus tchadensis which is dated at least 6 MYA and was found in the region of Lake Chad. Ardipithecus ramidus is dated to 4.4 MYA and is the first fully bipedal hominin, which was also partially arboreal and lived in a woodland environment. The Australopithecines are several species of early hominid found in both eastern and southern Africa and which date to between 4 and 2 MYA. Their appearance and radiation has been related to a change in global temperatures, which fell during that time and led to a more open habitat in eastern Africa. The earliest members of the genus Homo are found approximately 2.5MYA. Homo rudolfensis and Homo habilis had smaller jaws and teeth, a larger brain and are known to have used tools. Exact relationships between the hominins continue to be debated, however, it is clear that the hominin clade has included many species, some of which will have coexisted (Figure 3).