Why are primates so smart?
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[edit] Introduction
Primates are perhaps the most intelligent of the mammalian orders, and include humans, the most intelligent animal species. Primates have a number of anatomical and physiological features that account for their high intelligence, and there are many suggested selection pressures that have led to these adaptations. (Note, Whales & Dolphins, Elephants, Pigs, Crows & Parrots are also intelligent.
[edit] Climate change
During the last 5 million years the Earth has experienced many glaciations and inter-glacial periods. This epoch is called the Pleistocene. There were many periods when climate changed substantially during the lifetime of a single animal. Pre-adapted behaviour “hard wired” into the brain cannot possibly evolve sufficiently fast to adapt to such changes. Therefore in primates and some other species Natural selection favoured ability to reason and learn.
[edit] Adaptations
There are many features of primates that allow them to have a higher intelligence than other mammals.
[edit] Large Brains
Most obviously primates tend to have a larger brain size than mammals of a similar size. This alone cannot explain our intelligence, however, as elephant brains are five times the size of human brains. Perhaps more importantly, the cortex (the thin outer layer and surface of the brain) is more folded than those of most other animals, though again this can not be the only adaptation, as dolphins have even more convoluted brain surfaces. There is also a correlation between intelligence and the size of the prefrontal cortex, a region associated with emotion, personality and decision making. During mammalian evolution the prefrontal cortex has grown three per cent in cats, 17 per cent in chimps and 29 per cent in humans (Greenfield 1997).
[edit] Vision versus smell
Most primates are microsmatic, having reduced olfactory bulbs in the brain, in favour of larger visual regions, a change which may account for some of the difference in intelligence. Reduction of the sense of smell in favour of vision is associated with intelligence, with the lemurs resembling other mammals both in brain size and visual and olfactory systems, while the apes and related groups have the least developed olfactory systems.
[edit] Diet
Most primates have diets which include some insects or meat, supplying the protein needed for a larger brain, though this does not apply to all primates. Catching prey that doesn’t want to be caught needs more intelligence than eating plant material.
[edit] Small litters
Primates tend to have smaller litters and a longer gestation period than other mammals, which allows for the larger brain and head size, but at some cost in reproductive fitness. Many primates, most notably humans, are better able to learn and mimic than other animals, allowing intelligence to be passed on by learning as well as genetically.
[edit] Evolution
There are various reasons suggested for primates evolving higher intelligence. Each theory has merits and problems, and in reality there have probably been many different selection pressures acting on primate brains during their evolution.
[edit] Complex environment
There is a general principle for brain size in animals that states that in simple, unchanging environments the brain will be more simple and hard wired, but as the environment gets more complex the brain is expected to be larger and more dynamic, able to learn and model the world (Dennett 1995). So, the evolution of higher intelligence was probably a response to a more complex environment, with a larger range of selection pressures. It has been suggested that this change was a move from a terrestrial life to the more complex and dangerous arboreal life. Many arboreal primates are leapers, and when leaping between trees large eyes, depth perception, good hand-eye coordination, a larger visual cortex and an ability to plan moves in advance are all useful adaptations. However, other arboreal animals do not have the same adaptations, so it is unlikely that this theory alone can explain them. The early primates were probably nocturnal insectivores, and the visual adaptations would also be beneficial to this lifestyle. Many primates also eat fruits, and unlike most other mammals have colour vision which allows them to easily differentiate between ripe and unripe fruits. In macaques the visual systems make up 50% of the neocortex, supporting the hypothesis that vision was crucial in the initial growth of the brain.
[edit] Complex social groups
Many primate species have relatively complex social groups, often based on reciprocal altruism, kin relationships and manipulation, in which individuals remember their friends as well as family and enemies or cheaters. This system alone requires higher intelligence and larger memory, as well as systems for reinforcing societies, such as grooming or some form of communication. The initial brain size increase, possibly due to vision, hunting and arboreal life, may have made more complex social structures possible, which then led to further increases in intelligence.
[edit] Costs of increased intelligence
It should be noted that increased intelligence is not necessarily an obviously "good idea" in evolutionary terms, as there are examples of increased intelligence reducing evolutionary fitness, for example, by reducing the size of litter and increasing gestation period meaning that fewer individuals can be born, and by making the young spend longer in an underdeveloped state where they are vulnerable to predation. Large brains are also costly, for example, shrews' brains make up 3.33 per cent of body mass (compared to 2.33 for humans and less than 1 per cent for most mammals) and shrews must therefore eat their body weight in insects each day to power this brain. The selection pressures that led to increased intelligence must have outweighed these.
[edit] Humans
Humans have an especially high intelligence, with a skull capacity of over three times that of other apes. There are some anatomical and dietary adaptations that allow for the increased brain size. In apes the newborn brain size is about half that of the adult, while human babies are born especially premature with a brain size just a third that of the adult. This gives humans a longer period of brain growth and, more importantly, learning. Humans, even as hunter-gatherers, tend to eat much more meat than other apes, providing the protein and the calories required for our brains (Milton 2003), which consume 20% of our energy.
[edit] Culture may promote intelligence
There are many hypothesis for the selection pressures acting on human intelligence. These include the food-sharing hypothesis, that suggests that culture and language began as groups came together to share food and perform rituals. As well as food-sharing other notable human characteristics include bipedal locomotion, spoken language, living in home bases and organised hunting of large prey, all of which may have created selection pressures for higher intelligence (Leakey 1994). Miller (2000) and others have also suggested that intelligence in humans has been sexually selected. The high intelligence in humans is not just the result of larger brain size, however, but culture and learning that the larger brain allows to take place to an extent not possible in other animals. Culture and learning are an adaptation that allows adaptation to an environment by individuals, rather than over many generations by natural selection. Blackmore (1998) has tied this to the theory of memetics, and suggests that once humans began communicating with language, meme evolution caused positive feedback on the genetic evolution of higher intelligence.
[edit] Conclusions
Primates are more intelligent than other mammals because of a number of adaptations, including diet, visual system and social arrangement. The primate brain, through learning, is intrinsically more adaptable than those of other mammals. The selection pressures that led to these adaptations are complex, and many different pressures, acting at different times, have caused the evolution of intelligence in primates.
[edit] External links:
[edit] References & Sources
- Blackmore, Susan, 1998. The Meme Machine. Oxford University Press, ISBN 019286212X
- Dennett, Daniel, 1995. Darwin's Dangerous Idea. New York: Simon & Schuster, ISBN 014016734X
- Leakey, Richard, 1994. The Origin of Humankind. London: Weidonfeld & Nicolson, ISBN 0465053130
- Miller, Geoffrey, 2000. The Mating Mind. London: Heinemann, ISBN 0099288249
- Milton, K., 2003. “The critical role played by animal source foods in human (Homo) evolution.��? in J Nutr. 2003 Nov;133(11 Suppl 2):3886S-3892S.
- Osborne, Richard, personal communication.
[edit] Further reading
- Jared Diamond, 1992. The Third Chimpanzee. New York: HarperCollins, ISBN 0060183071
- Susan Greenfield, 1997. The Human Brain. London: Weidenfeld & Nicolson, ISBN 0753801558
[edit] Acknowledgments
- First edition by Joe Dunckley in November 2004 [1], with minor additions suggested by Richard Osborne.
