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Evolutionary Psychology, Lecture 1:
Hominid Evolution
chimpanzee
Figure from Allman, 2000, p 164.
human
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.
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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.
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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 0.50.7mya 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.
Neocortex ratio
 Byrne & Whitten (1988)
argued that social living
requires 'Machiavellian
intelligence'.
Neocortex ratio
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).
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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 proteinbased 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.
Photos from Allman, 2000, p 166
 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 leafeating howler monkey.
howler monkey
spider monkey
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 nonessential 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.