4. Human Evolution

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The Biological Aspects of Language: Life history patterns (1)

It has been calculated that if a female lesser mouse lemur and a female gorilla were born at the same time and had equal reproductive success of males and females, then the mouse lemur would leave 10 million descendants before the gorilla became sexually mature!

Mouse lemur weighs less than 100g, can reproduce within 12 months of birth and can have two litters of each of up to 3 offspring per annum.

Gorilla weighs more than 100kg, does not breed until about 10 years of age and can produce a single offspring every 4 to 5 years.

Amongst mammals lifespan increases with body weight while litter sizes decrease with size.

Larger species have longer gestation periods, later ages at weaning and reproduction, longer lifespans and smaller number of offspring.

Life history patterns (2)

Primates live life slowly. In comparison with other mammals, primates have long gestation periods, long lifespans and mature late. These differences are associated with the state of development of the young at birth.

Slower-developing mammals such as primates produce offspring whose eyes are open at birth (precocial) in contrast to the young of mammals whose offspring are born with the eyes closed (altricial) and develop more rapidly.

Primates also have relatively large brains for their body size. It is possible that brain size, rather than body size, is evolutionarily linked to life history variation.

Life history patterns (3)

The  brain grows slowly and might therefore be a controlling organ that enables primates to have relatively long lives. This is an attractive idea for humans that have huge brains and are the slowest developing and longest living of all primates. If our gestation period was any longer then the neonatal brain could not be delivered through the birth canal.

Whatever the role of the brain, there is a clear correlation between body weight and the timing of life-cycle events and this relationship is similar to that found in other mammals.

This relationship is not always linear as is shown by the gestation period of the 100g mouse lemur (62 days) and that of the 100 kg plus gorilla (256 days) – thus, a 1000 x heavier gorilla does not have a 1000 x longer gestation period.

Life history patterns (4)

Some primates develop more slowly even after an allowance has been made for body size. Species with short gestation periods do not have late ages of weaning while those with long gestation periods to not have short weaning periods. Short gestation periods equate with early weaning and vice versa – thus the whole pace of life is either speeded up or slowed down.

Lemurs tend to be fast developers and in relation to their body/brain sizes have short gestation periods, give birth to small young, have early ages at weaning and maturity and short life spans.

Tarsiers are slow developers with long gestation periods, large neonates, late weaning and late maturity and long lifespans.

Life history patterns (5)

Differences in pace of life are associated with habitat. Tropical rainforest dwellers [e.g. Allen’s bushbaby *(Galago alleni)* in W. Africa] lives longer than the lesser bushbaby (Galago moholi) which lives in wooded savanna regions of southern Africa. *G. Alleni* has single births, breeds relatively slowly and lives longer. *G. Moholi* has twin births & two breeding seasons per annum.

The rain forest is a more stable habitat that favours channelling of resources into more slow developing but more competitive young. The savanna is less stable with periods of plenty (therefore favours rapid breeding) interspersed with famine (populations crash). Ecological correlates tend to be good predictors of life history. Large primates (e.g. gorillas) tend to be terrestrial, diurnal and leaf eaters and those same species will also have long gestation periods, slow post natal development, long lives, low metabolic rates and large brains.

Human Growth & Development (1)

Primates have slow reproductive turnover & drawn-out life histories. The pace of growth from birth to adulthood is retarded. In monkeys, apes & humans it is possible to recognise special features of the growth curve associated with the attainment of sexual maturity (puberty). These characteristics were once thought to be unique to humans but other primates have been found to have them. The velocity of growth reflects child’s state at a particular time better than does actual height which depends largely on how much the child has grown throughout life.

Human Growth & Development (2)

The blood and tissue concentrations of substances that change with age are more likely to accompany the velocity rather than the distance (height) curve. Sometimes acceleration of growth rather than its velocity may reflect physiological events (e.g. elevated endocrine secretions at adolescence clearly seen in an accelerated growth rate). Velocity of growth in height decreases from birth onwards but this decrease is interrupted shortly before the end of the growth period

At age 13-15 for boys there is a marked acceleration in growth – the adolescent or pubertal growth spurt. A slight increase in growth velocity may also occur between 6-8 years – the mid growth spurt.

Prenatal growth (1)

Although velocity of growth in length is greater at birth than at any later period, in foetal life the velocity is greater still. Peak velocity occurs at about 18th week of postmenstrual age (foetal age usually calculated from first day of mother’s last menstruation which is an average of 2 weeks prior to actual fertilization). Growth in foetal weight follows the same pattern except that peak velocity occurs 34th postmenstrual week. From 36-40 weeks rate of growth slows down – space available in uterus may be becoming fully occupied. Growth of twins slows down earlier when their combined weight is approximately the weight of a 36-week singleton. Birth weight & size reflect maternal environment more than the child’s genotype – the slowing down mechanism allows a genetically large child developing in the uterus of a small mother to be delivered successfully.

Prenatal growth (2)

Growth rate increases again after birth especially in genetically large children and peaks at about 2 months. Velocity of growth in length in the first two months of foetal life is low because regionalisation of the originally homogenous whole into regions such as head and arms and histogenesis (differentiation of cells into specialised tissues such as nerve and muscle) occurs. Each region becomes moulded, by differential growth of cells or by cell migration, into a definite shape (morphogenesis).

These processes are complete by the 8th postmenstrual week & by then the embryo has a recognisably human appearance. High rate of growth of the foetus compared to that of a child is mainly during to cell multiplication. The proportion of cells undergoing mitosis becomes less as the foetus gets older.

Few, if any, new muscle (skeletal & cardiac) or nerve cells appear after the 6th month of foetal life. Foetal nerve and muscle cells have very little cytoplasm around the nucleus. Foetal muscle has more intercellular substance and a much higher proportion of water than mature muscle.

Prenatal growth (3)

Later foetal and postnatal growth of muscle consists of building up cytoplasm of muscle cells and forming proteins such that the cells become bigger, the intercellular substance almost disappears and amount of water decreases. This process continues up to about age 3 and slowly thereafter. At adolescence it briefly speeds up again especially in boys under the influence of androgenic hormones.

In the nervous system cytoplasm is added to neurones, nucleoprotein bodies appear and axons and dendrites grow. For most tissues, postnatal growth is a period of development and enlargement of existing cells rather than the formation of new cells.

Postnatal growth (1)

Most tissues follow the same general growth curve as height. The brain & skull, lymphoid tissue, reproductive organs & subcutaneous fat have different curves. Subcutaneous fat has a complicated growth curve being laid down in week 34 of foetal life and peaking at 6-15 months post partum and decreasing thereafter. At age 6-8 subcutaneous fate begins to increase in both sexes. At adolescence, limb fat decreases in boys and is not regained until about age 20 – the loss of trunk fat is smaller. During adolescence in girls, the increase in limb fat slows down but no loss occurs and trunk fat shows a steady rise.

Adolescent growth

Adolescent growth spurt in girls occurs at about 10.5-13.5 years and at 12.5-15.5 years in boys. Peak growth velocity in height averages about 10cm for boys and slightly less for girls. Girls are more advanced in maturity at all ages from birth onwards. Sex difference in height in adulthood is caused by the longer period of male growth.

Postnatal growth (2)

Of the average 13cm difference: 2cm result from differences in prepubertal growth, 7cm from the later occurrence of the growth spurt in males and 4cm from its greater intensity. All parts of the muscles and skeleton are involved in the growth spurt but not to the same extent. Most of the spurt comes from growth of the trunk rather than the legs. Many sex differences in adult body size and shape result from differential growth patterns at puberty. Male shoulder width shows a greater growth spurt than female shoulder width whereas the reverse is true for the hip width growth spurt. The greater male leg length is a consequence of the longer prepubertal period of male growth when leg length is growing faster than the trunk throughout this time.

Postnatal growth (3)

This period of growth is also associated with rapid growth and development of the reproductive system. The tempo of growth at adolescence varies greatly between individuals with some 14 year old males being able to pass as 12-year old boys and others appearing to almost fully grown. A similar situation will exist for 12 year old girls.

Early maturers, especially girls, are at an increased psychological risk while slow developers can have a particularly difficult adolescence as their peer group may be 15cm shorter, much weaker and sexually less developed (they catch up!). Hormones, heredity and the environment each play a role in growth and development.

Evolution of the human growth curve

During the latter part of foetal life & for a short time after birth Gn-RH is released from cells of the arcuate nucleus of the CNS. About 12 months after birth in humans & six months in rhesus macaques, the secretion of Gn-RH stops & the  nucleus becomes silent or almost so.

This period of silence has become lengthened progressively during primate evolution.

We do not understand the mechanism of this suppression but evolutionary reasons for the progressive lengthening of this period are easy to find and include. learning to co-operate in a group or family life while the individual remains relatively docile and before he/she comes into sexual competition with adult males or females.

The brain & language (1)

Compared with other primates humans stand out in 3 main respects:

(1) their skeleton is adapted for striding locomotion

(2) they have modified teeth and jaws

(3) they have very large brains

Compared with the average for simian primates, the human brain is 3x bigger than expected which represents a dramatic increase in just a few million years. Such a rate of increase in brain size is unique. It seems that bigger brains are better but why?

The brain & language (2)

Large brains are expensive  – an adult human brain consumes 20% of the body’s energy and the imbalance is even greater in early life. The brain of a human newborn represents 10% of body weight and consumes about 60% of the baby’s energy. How do humans afford large brains?. The remodelled jaw apparatus, bipedalism and large brain may all be intimately connected. Thus (as with all mammals) locomotion is crucial in food collection, teeth & jaws are crucial in food processing while the energy obtained from food meets the high running costs of a brain (which plays a key role in obtaining food and survival)

The brain & language (3)

The evolution of the human brain can be explained at 2 levels:

(1) the increase in size from australopithecines to early modern *Homo* probably depended on a shift in feeding habits that led to increased energy turnover

(2) specific selection pressures must have led to changes in the internal wiring of the increasing mass of brain tissue that allowed improvements in problem-solving, the origin of sophisticated tool-making and, most important, the emergence of language and culture

Primate brains and senses (1)

Many similarities between primate and human mental abilities despite great differences in shape and size. All primate brains have a distinct temporal lobe which is separated from the parietal and frontal regions by the Sylvian fissure. The temporal lobe includes the auditory areas (on the dorsal surface), some association visual areas (on the ventral surface) and some limbic structures (the amygdala & hippocampus on the medial surface) which are associated with emotion, attention and memory. The shift to a more upright posture in early primates led to a change in the foramen magnum

The exit of the brainstem and the position of the face (including optic and olfactory nerves) have converged to form almost a right angle in primates. This pulls the cerebellum and brainstem beneath the cortex and draws the posterior end of the cortex beneath the frontal & parietal lobes to form the temporal lobe. There is considerable variability in the surface of primate brains which arises from differences in the extent of folding of the cortex.

Primate brains and senses (2)

The average primate has a larger brain for its body size (at all stages of its lifespan) than most non-primate mammals. Often assumed that the relatively larger brain of primates correlates with an increase in intelligence. The superior visual and manual skills of monkeys could make them well adapted to life in the trees. Nearly complete convergence of the eyes and corresponding changes in the brain’s visual structures are common to all primates. Their depth perception is improved as the visual overlap of the visual fields means that most part of the visual fields are viewed from two perspectives at once

Inputs to the brain from each eye are more evenly divided than in other mammals. This has produced striking changes in the superior colliculus an ancient visual area of the midbrain involved in orientation to visual stimuli. The parallel evolution of a high degree of visual convergence in owls, cats and some bats suggests that extensive visual overlap is particularly valuable to animals that navigate by sight by night.

Primate brains and senses (3)

The ancestors of all primates were almost certainly nocturnal, arboreal, insect-eating creatures as are many present-day prosimians

The retention of visual convergence in diurnal primates, even though it has disadvantages for predator defence, may reflect the difficulties in reversing this evolutionary change. Primates also have 3-colour vision unlike many other mammals – colour-blindedness of mammals is probably derived as it is common in birds, reptiles and fish.  It may reflect our ancestry as nocturnal predators. The secondary evolution of colour in monkeys and apes required specialisations of the retina and of the cerebral cortex. The rediscovery of colour vision in primate evolution probably occurred at the same time as activity moved towards the day and perhaps to accompany a shift to feeding on fruits that advertise ripeness by changing colour. Many monkey species utilise bright colours for social communication. Primate visual cortices are the most complex visual processing system ever evolved. Nearly 50% of the cerebral cortex in macaques is directly involved in visual processing.

Primate brains and senses (4)

The nasal cavity is reduced in primates because of convergence of the eyes. Although the olfactory organs are reduced in many primates the parts of the brain to which they connect have not become similarly diminished. The structures receiving smell information form part of the limbic system form part of the limbic system and are central in regulating emotional arousal. The parts of the cortex and spinal tracts associated with touch and movement are particularly well developed in primates which correlates with primate specialisations such as hands for grasping and manipulation & the face for communicating by gestures. Although the muscles of the mouth and throat are well represented in the primate motor cortex the production of sounds is not under skilled motor control. Primate calls are similar to automatic and stereotypic human sounds such as laughter, sobbing and shrieks than to human speech. The auditory system of primates is no more or less specialised than that of many other land-living mammals. In some species processing of different classes of sounds is specialised to opposite sides of the brain which parallels lateralisation of language processes in the human brain.

The human brain (1)

In absolute terms the human brain is smaller than that of an elephant or a whale but is capable of intricate mental processes that no other brain can approach. The average human brain weighs 1330g with those of men heavier than those of women only because of differences in body size. Human brain is 2% of body weight and receive up to 20% of metabolic energy at rest (16-20% of cardiac output). Brain may contain 10,000 million neurones each of which makes 1,000 to 8,000 synapses with other neurones. The human brain must be unique in anatomy and function because it controls such human adaptations as symbolic communication, speech, tool production and “culture”. Three unique features of the human brains are visibly obvious: its large size with respect to the body; the asymmetry between left and right hemispheres; and the reduction of its olfactory apparatus.

The human brain (2)

We know far more about the functional uniqueness of the human brain than its structural uniqueness. Theories to explain these extraordinary abilities include:

(1) expansion of the brain to increase intelligence and memory thereby enabling humans to learn complicated skills such as tool-making and language.

(2) addition of new brain structures to provide new functions such as specialised language abilities.

(3) re-organisation of the connections of existing brain structures to allow them to serve novel functions such as analysis of grammar.

(4) changes in the relative sizes of different brain areas expanding certain structures to augment particular abilities.

Evolution of brain size (1)

Australopithecine brain had a mean weight of 450g (assuming than 1cc of brain weighs 1g) which may or may not be a significant departure from the apes. Encephalisation probably first exceeded the ape range at least 2 million years ago with appearance of *H. Habilis*

By 1.5 million years ago the brains of *H. Erectus* weighed about 1000g. Brain size continued to increase without a corresponding increase in body size until the appearance of *H. Sapiens* about 400,000 years ago. During human evolution, the brain has increased in size with respect to the body, especially during the past 2 million years when the stature of early humans hardly changed.

Evolution of brain size (2)

Growth curve for human brain and body compared with that for other primate species.  The length of the human foetal phase (in which brain & body grow at the same rate) is extended but the postnatal phase (in which body growth continues at high rates but brain growth slows) is not longer in proportion. If the body grew at the primate rate in a manner appropriate to brain growth, adult humans would weigh 454kg (1000 lb) and stand nearly 3.1m (10 feet) tall.

The capacity for language is the most important specialisation of the human brain. Iconic reference depends on the physical correspondence between the sign and object (e.g. that between the vertical orientation of the  bee dance and the flight path to the flower with respect to the sun). Indexical reference depends on the temporal and spatial linkage of the sign and object (e.g.: 1. production of an alarm call, the arrival of a predator and the elicitation of appropriate responses, 2. learned associations between stimuli and responses are indexical).

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Sources:

I have mislaid the sources of information for this chapter. If any of the above is your work please let me know and I will gladly give you credit.