Both humans and African apes have hands with a thumb that is sufficiently separate from the other fingers to allow them to be opposable for precision grip. Possession of an opposable thumb means that objects can be carried more easily and manipulated. There is considerable evidence to suggest that, by being able to throw and powerfully grip an object, early humans were better at protecting themselves from animals and other humans (Young 2003). The development of the opposable thumb, however, primarily helped humans to make tools, which was an essential advantage in human cultural evolution. But the mere presence of the opposable thumb does not explain why humans make sophisticated tools: if it did, then chimpanzees would make complex tools too (and they do not).
opposable thumb A thumb that is sufficiently separate from the other fingers of the hand to allow for precision grip
Figure 1.1 Hand of an African ape and of a human. (Denise Morgan for the University of Utah / Wikimedia Commons)
ACTIVITY
Take off your shoes and socks and sit on the floor. Try to open a banana skin with only your feet. What happens? What does this activity tell us about the advantages of having opposable thumbs?
Make a list of things that early humans might have been able to do as a result of having opposable thumbs. How might these activities have helped humans to survive?
Sexual dimorphism
Sexual dimorphism refers to both the internal and the external differences between males and females found in a variety of animals and plants. The earliest fossil evidence to show sexual dimorphism in early primates demonstrates that canine teeth and body shapes were different in males and females (Krishtalka et al. 1990). Hominins have not shown dimorphism in canine size, but there was a significant level of body size dimorphism in early hominins such as australopithecines. However, sexual dimorphism was significantly reduced in the larger-brained Homo erectus and their descendants (including Homo sapiens). This suggests an important development in social organization, with a possible change from polygamy (frequently associated with larger males) to monogamy (often characterized by low sexual dimorphism). Modern humans are sexually dimorphic to some degree. It is estimated that males are 5 to 10 per cent larger on average and have greater upper body muscular development. This is small compared to over 100 per cent body size dimorphism in gorillas and at least 15–20 per cent in chimpanzees and bonobos.
sexual dimorphism This refers to both the internal and the external differences between males and females found in a variety of animals and plants
STOP & THINK
Suggest some reasons for the gradual reduction in human sexual dimorphism.
Diet and internal organs
Like chimpanzees and bonobos, humans are omnivorous; this means that humans and chimpanzees kill other animals for food in addition to eating a wide variety of plants. Essentially, the human body is similar to that of the great apes; humans have the same arrangement of internal organs and bones, share several important blood types and suffer from many of the same diseases. However, there is a significant difference in the amount of meat that humans eat compared to chimpanzees. While chimpanzees extract at most 5 to 10 per cent of their calories from animals (including both small monkeys and termites extracted with tools), Homo erectus obtained most energy from meat (either from scavenging or from hunting). This heavy reliance on meat lasted until very recently, when agriculture appeared. There are only a handful of human populations (fewer than fifty across the globe) still able to survive without agriculture: they are known as hunter-gatherers.
omnivorous Ability to eat and survive on both plant and animal matter
hunter-gatherers Members of a nomadic people who live chiefly by hunting and fishing and harvesting wild food
STOP & THINK
What are the advantages of an omnivorous diet?
Competitiveness, hierarchy and aggression
One controversial characteristic that we may share with other primates is the potential for aggression. Field studies have shown differences among nonhuman primate species in the incidence and circumstances of actual intraspecific (within-species) violence. In anthropology, there has been much debate about the human capacity for violence and aggression. Sociobiology, for example, is an area of scientific research and thinking that claims that some social behaviour is a product of evolution (although not all sociobiologists agree on the extent of this). Some argue that certain behaviours (such as aggression and competitiveness) may have been advantageous to human survival. E. O. Wilson (1978), one of the founders of modern sociobiology, described some behaviours thought to be human universals as genetically based, such as male–female bonds, male dominance over females and aggression. These claims have been disputed by many human scientists, who have shown that there is much more variation in patterns of social behaviour across human societies.
sociobiology An area of biology that aims to explain social behaviour in terms of evolution
More recently, biological anthropologists such as Richard Wrangham and Dale Peterson (1997) have argued that violence played an important role in the evolution of humans and chimpanzees. However, such views have been heavily criticized for their lack of supporting evidence. An alternative view gaining strength in evolutionary anthropology is that our capacity for cooperation is what made humans behaviourally unique. While other species of primates mostly show alliances among closely related kin, studies of hunter-gatherers have shown that we frequently exhibit cooperative behaviours (such as food-sharing) towards those who are unrelated. Some of us are even able to display cooperation and altruism towards unknown individuals (think about giving blood or about charity work). The reasons for the coexistence of extreme cooperation and aggression in human societies is one of the most debated issues in evolutionary anthropology.
Social relations
Humans and some nonhuman primates share certain similarities in their behaviour towards each other. However, social behaviour varies from species to species. Two examples of nonhuman primate behaviour are explored here: of bonobos and of chimpanzees, our two closest evolutionary relatives.
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Bonobos
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