Evolutionary Psychology, Lecture 1:

Hominid Evolution

chimpanzee human

Figure from Allman, 2000, p 164.

Learning Outcomes.

At the end of this session you should be able to:

1) Describe the key advances in mammalian brain structure/function.

2) Describe the evolution of primates and hominids. 3) Discuss the factors involved in hominid brain

expansion.

Key Events in Evolution.

10-16bya - formation of universe. 4.5bya - formation of Earth. 3.5 bya - microorganisms. 600mya - multicellular organisms. 520mya - invertebrates. 460mya - vertebrates. 360mya - terrestrial amphibians. 260mya - reptile-like mammals. 200mya - true mammals. 100mya - marsupials and placentals. 65mya - mass extinction of dinosaurs and most

mammals. 45mya - anthropoid primates. 4mya - first humans?

Mammalian Brain Evolution.

Some crucial steps in brain evolution in mammals were as follows:

Bones of the inner ear (derived from the neural crest) allowing discrimination of higher frequencies.

Haemoglobin increased availability of oxygen to the brain. Specialisation of the telencephalon for olfactory input and

function. Retinal stability via the vestibular system and the

cerebellum. Insulation of axons by myelin. Development of the neocortex in the forebrain. Maintenance of constant body temperature (homeostasis)

which is very energy demanding and so requires increased energy from food and changes in brain, body and behaviour.

The Primate Order.

Closely related to: Scandentia (tree shrews); Dermoptera (flying lemurs); Chiroptera (bats). Characterised by:

Agility. Dexterity - simple tool making. Binocular vision - depth perception. Enhanced facial expressiveness. Colour vision Increased brain size relative to body size, especially in

neocortex. Enhanced parental care.

Divided into: Strepsirhini (lemurs, lorises) and Haplorhini.

Sub-orders of Haplorhini.

Platyrrhines (New World monkeys – capuchins, marmosets).

Cattarrhines (Old World monkeys – baboons, macaques).

Pongids (apes). Hominids (humans). New and Old World monkeys separated from the ape

and human ancestor around 35mya. Human ancestors split from orang-utans 12 mya, from

gorillas 7-9 mya, and chimpanzees 5-8 mya. Modern humans, our extinct ancestors, gorillas,

chimpanzees, and bonobo’s, are all descended from a common ancestor around 10mya (Bradshaw, 1997).

Tree of Relationships Among Anthropoids.

Diagram from Noble & Davidson, 1996, p23.

Possible Characteristics of the Common Ancestor.

Lived in Africa, perhaps on the edge of the tropical rain forests.

Was not bipedal, and probably knuckle-walked (like the chimpanzee).

Had a small body (probably equivalent to a modern baboon) and a relatively small brain (equivalent to a modern chimpanzee).

Did not have a large (if any) meat component to its diet.

Did not use tools to any great extent. Had a closed social network of solitary males, females

grouped in loose associations; mating would not be monogamous with little male parental investment.

These are speculative as no fossil remains have been discovered.

Genetic Relationships Within the Hominoids.

Rogers (1993) summarised the genetic evidence from mitochondrial DNA sequencing and concluded that modern humans are more closely related to chimpanzees than gorillas or orang-utans.

In genetic terms we differ from chimpanzees by around 1.9% of DNA base pairs.

Diagram from Cartwright, 2000, p166.

The Early Hominids.

According to Bradshaw (1997) the hominid lineage is presumed to be as follows but the precise order (if this is the order at all) remains ambiguous.

Ardipithecus ramidus 4.4mya Australopithecus anamensis 4.2mya Australopithecus afarensis 3.8mya Australopithecus africanus 3-2.6mya Homo rudolfensis 2.5mya Homo habilis 2.2mya Homo ergaster 1.9mya Homo erectus 1.9mya Homo heidelbergensis 0.7mya

Homo sapiens 0.2mya

Homo sapiens.

Our ancestors emerged out of Africa around between 0.5-0.7mya and migrated into Europe and Asia between 50,000 and 100,000 years ago. Current racial differences are thus recent in evolutionary terms.

It has been argued that all modern humans descend from a single female ancestor (Eve) who lived around 200,000 years ago with the extreme genetic similarity of all modern humans suggestive of a small ancestral population with an effective population of around 10,000.

However, Ayala (1995) disputed this hypothesis, arguing that human ancestral populations were in fact much larger (around 100,000) and that while we do indeed inherit some mitochondrial DNA from a single individual, the majority of our DNA was inherited from other

contemporaries.

Hominid Brain Evolution.

As Allman (2000) points out, if large brains enable animals to deal better with changing and unpredictable environments then why don't all species have large and complex brains? In fact animals with large brains are rare, probably due to the costs involved:

A large brain is extremely energy demanding, and must compete with other body organs for resources.

A large brain takes a long time to mature, which limits the rate at which an individual can reproduce; large-brained infants are heavily dependent upon their parents.

A large brain is more prone to damage and malfunction. The australopithecines possessed a brain the size to be

expected from a primate of their size but hominid brain size steadily expanded. Modern human brains are around 7 times larger than expected for a primate of our size.

Hominid Brain Expansion.

Diagram from Cartwright, 2000, p175.

Factors Involved in Hominid Brain Expansion.

The human brain is very energy demanding, at only 2% of body weight it accounts for 20% of energy expenditure, this compares with 9% in macaques or chimpanzees, and 3% for elephants. Having a large brain must have produced an advantage it would not have been selected for. There are several explanations:

1. Living in Social Groups. Open grasslands foster the development of complex

social groups which require intelligence (co-operation, deception, representational understanding, communication).

Environment Social Complexity Intelligence.

Neocortex Size and Group Factors.

Dunbar (1992) argues there is a direct correlation between group size and neocortex size across the primate order.

Byrne & Whitten (1988) argued that social living requires 'Machiavellian intelligence'.

Neocortex ratio

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Hunting/Gathering.

Acting cooperatively in a group. Shared information and communication. Making tools. Tracking and navigation. Sharing. Bartering. Food preparation and ritual. Meat is highly valued both in hunter-gatherer societies

and chimpanzees. It is a valuable resource conferring power and status.

In human foraging societies women prefer successful hunters as mating partners.

– (Stanford 1999).

2. Diet.

Milton (1993) discussed the behavioural and physiological adaptations concerned with different diets. Finding fruit and meat require memory for food locations, and increased social co-operation.

The Australopithecines were herbivores and small brained. As the early hominids moved from vegetarian to more protein-based diets (meat and fish) their teeth and jaws became smaller and brain size increased.

Leaf-eaters (folivores) have smaller brains with proportionally less neocortex in relation to body size than frugivores (fruit eaters) and carnivores.

Brains of Fruit-Eaters and Meat-Eaters. Howler monkeys and spider monkeys are

approximately the same size but the fruit-eating spider monkey has a much larger brain and well-developed brain than the leaf-eating howler monkey.

howler monkey spider monkey

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Brains and Guts!

Gibbons (1998) pointed out that while the human brain has expanded greatly, our basal metabolic rate is similar to our ape relatives.

Brain expansion was accompanied by a reduction in the gastrointestinal tract due to a shift from relatively indigestible vegetarian matter to a diet rich in protein and fat which requires less digestion and provides more energy.

The human gut is around 60% smaller than would be expected for an animal of our size

The guts of chimpanzees and gorillas are optimised for fruit eating and leaf eating respectively, the human gut is optimised for high energy diets (principally derived

from eating meat).

The Palaeolithic Diet.

Eaton & Konner (1985) pointed out that Palaeolithic nutrition probably consisted mainly of meat from hunting/scavenging, fruit, fish, roots, beans, nuts and berries.

Importantly, the meat they ate contained 5 times more polyunsaturated fat per gram than that found in modern domestic livestock and significantly more eicosapentaenoic acid - an essential omega-3 fatty acid.

These essential fatty acids are major determinants of synaptic membrane fluidity.

The Intellectual Explosion.

Our ancestors had large brains but did not show creativity or intelligence. Between 50,000-35,000 years ago the following appeared:

Advanced culture. Sophisticated tools. Long-distance transport. Social networks. Large dwellings. Tailored clothing. Rituals. Art. Linguistic communication. Warfare.

Dietary Changes and Creativity.

Horrobin (1998) proposed that during the course of human evolution specific biochemical alterations led to changes in metabolism which enabled the human brain to expand in size and function more efficiently.

Neural connectivity is determined by the availability of phospholipids, which make up 60% of the brain.

Axons and dendrites rely on a supply of the essential fatty acids (arachidonic acid, and docosahexaenoic acid) and the essential amino acids, and can only be obtained from a diet rich in animal protein (meat, fish, eggs).

Horrobin points out that the sudden rise in creativity paralleled dietary changes as hominids moved from eating vegetation and fruit, to eating meat and fish.

While these changes led to improvements in brain function, they may also have had side effects:

Schizotypal Features.

The improvements in neural connectivity may have led to an increase in schizotypal features:

Excess of suspiciousness. Trace of paranoia. Difficulty in making social contact. Hearing voices. Increased interest in religion and mysticism. Eccentricity. Creative abilities. Charismatic personality. Schizophrenics display lower phospholipid levels and

increased activity of phospholipases (which remove fatty acids from membranes). This is not a problem if the diet is rich in fatty acids (as it was during palaeolithic times).

Modern Diet and Schizophrenia.

Modern diets lack the essential fatty acids and are replaced with saturated fatty acids.

Horrobin proposed that biochemical alterations caused by dietary changes increased brain size, improved neural connectivity and led to creative intelligence, but also produced paranoia, mild sociopathy, and religious experiences.

These were kept in check by sufficient dietary levels of the essential fatty acids.

In modern societies, the very behaviours that made us human are expressed in more extreme behaviours of schizophrenia and manic depression.

Schizophrenia has increased over the past few hundred years, and is more common in cultures with a high rate of dietary non-essential fatty acids, and is reduced in patients who increase their intake of essential fatty acids.

4. Bipedalism

Advantages: Changes in blood flow - Increased cooling and further

brain expansion (Falk, 1990). Greater mobility - foraging distance increases. Minimises surface area and so can stay in the sun for

longer. Mother has to carry the infant in front of her -

increasing social interaction. The hands are freed. Regions of the brain for foot control are freed. Better view of approaching predators. Makes birth more difficult -increases social cooperation. Permits the modulation of the oropharyngeal cavity -

allowing a wide range of speech sounds to be produced.