Cultural and Biological Evolution

Bipedalism 

Evidence from the fossil record suggests that some bipedal tendencies were present as far back as Orrorin tugenesis (6MYA). There is some possibility that O. tugenesis was not in fact in the hominin line, suggesting either that bipedalism evolved in more than one taxa or that bipedalism had started to develop before the split between the last common ancestor of apes and humans. The well known female Australopithecus afarensis skeleton, “Lucy” (Fig 4) is dated three million years ago and is believed to have been habitually bipedal, adapted for bipedal walking but not able to run efficiently. A. afarensis existed from 4‐3MYA so Lucy was a relatively late example of this species. Evidence from earlier fossils of A. afarensis show that some arboreal behaviours were also part of the niche of A. afarensis, suggesting that this was the period of gradual evolution from quadrupedal to bipedal locomotion.

Selection for bipedalism in this period must have offered survival advantages to the species, which are likely to have resulted from changes in the environment which made this form of locomotion preferential.

Evidence suggests that selection for bipedalism was driven by environmental change seen in eastern Africa around 4MYA in which near continuous forest changed to wooded savannah, creating more open habitats in which food resources became more dispersed (Fig 3). In such habitats, bipedal movement is energetically more efficient than quadripedal movement, particularly at walking speeds. Calculations of the net costs of transport for different gaits (Fig 5) show that human walking is more efficient than most other forms of locomotion in animals (Sockol et al 2007). The paleoanthropologist Robert Foley has calculated that, provided the early hominid spent the majority of its time in terrestrial (as opposed to arboreal) foraging, then bipedalism would be adaptive in such environments. Thus the evolution of bipedalism can be seen to have been progressively adaptive as the hominin line evolved, driven by habitat change. 

Other arguments that have been put forward for the origin of bipedalism include: 

• Improved thermoregulatory efficiency, leading to the ability to forage through the heat of the day 
• Improved ability to see and therefore avoid predators 
• Improved ability to carry food, tools and infants during their period of dependence 

These adaptations are likely to be secondary to the drive for selection based on energetic efficiency, and could have arisen from the initial adaptation of standing upright and adopting a bipedal gait. While bipedalism was selected for and therefore had adaptive advantage, it also had adaptive costs that resulted from the structural changes that are associated with the upright posture and bipedal gait, as shown in the table below.  

Tools, Brain Expansion and Social Development

Bipedalism provided the potential for a number of significant behavioural adaptations and innovations that contributed to the evolution of modern Homo sapiens and the relative success of the species. Tool culture, brain expansion and social development are amongst these adaptations. The earliest evidence of the use of tools to modify or manipulate objects and elements of the environment is seen in Homo habilis. This is the same time that we observe an increase in brain size in both absolute terms and relative to body size. Australopithecines had absolute and relative brain sizes not greatly different from those of the modern apes. In comparison, the brain size of the Homo lineage showed an exponential increase from H.habilis onwards (Fig 6).

Brain size is not inherently linked to evolutionary success. The Neanderthal brain had a larger volume than the modern human brain, yet the species was not as successful. Indeed, brains are energetically expensive and there are many large animals with highly successful evolutionary histories yet relatively small brains (witness the dinosaur prior to the asteroid‐induced extinction). Thus the question of why hominins evolved large brains and Homo sapiens is endowed with its unique set of capacities is not self‐evident. There are several inter‐related theories on the evolutionary origin of brain expansion. Effectively they either place emphasis on Homo species finding adaptive advantage in social interactions within their group, or in planning their affairs for hunting and tool making. The weight of evidence now favours the former.

The Expensive Brain 

Humans have a relatively large brain compared to body mass. This increase in brain size evolved rapidly from the Homo erectus onwards. Evolution of the large brain came at a high metabolic cost. The brain requires a higher level of energy to sustain activity than any other tissue in the body. Humans expend around 20% of their energy intake on the brain compared to 8‐10% in most other primates and 3‐5% in non‐primate mammals. Humans have evolved to eat an energy and nutrient rich diet which supports the high level of metabolic need of the brain. Fossil evidence suggests that major changes in brain size seen in Homo erectus coincides with the evolution of hunting and gathering communities. These communities consumed a higher proportion of animal foods than ever seen before among primates, had home bases to which they transported foods and were in the habit of sharing food within a community group. These improvements in diet quality are thought to be important in the development of the large brain in relation to body size (Leonard et al 2007).

Humans today interact socially with multiple individuals within multiple communities. Just think about the number of groups you interact with on a daily basis; family, school, work, friends from sports clubs, church groups, cultural groups, Facebook, Twitter and Instagram. Technology means we now have multiple interactions with people who we do not see face to face.

Anthropologist Robin Dunbar from the University of Oxford has suggested that living in larger groups had an evolutionary advantage for our ancestors. The larger group size offered security of food supply and increased protection, Dunbar has also shown that living in large groups requires intelligence.  Fig 7 shows that group size in primates is correlated positively to brain size. In particular the neocortex, a part of the brain associated with higher order functions such as sensory perception, motor control, spatial reasoning, conscious thought and in humans, language is larger in comparison to the rest of the brain in species that live in larger groups. Based on correlations with other primates such as chimpanzees who live in groups of 50, Dunbar predicted that humans have evolved to live in groups of about 150 individuals.

The Reproductive Cost of Brain Expansion 

The development of the large brain had other consequences for hominins. Most significant are the problems associated with giving birth to a baby with a large head through the narrow and rigid birth canal that arose as a result of the pelvic changes associated with bipedalism. If the human infant was born at the same stage of maturity as other primates, then pregnancy would last about 21 months. This would require a pelvic canal so wide that it would be impractical for efficient bipedal locomotion. The compromise has been that humans give birth to an infant at a stage when the head can fit through the birth canal, but this means that the human infant is entirely dependent on its mother for many months after birth. This need to give birth to a totally dependent infant has determined human social structure - if the mother is to support the infant she in turn needs to be confident of support from the father. Thus while early hominids showed great sexual dimorphism, with the males much larger than the females, implying a harem type mating system with fighting between males for mating rights, Homo sapiens has a much lesser degree of sexual dimorphism, suggesting that in general females were able to have continued support from one male.