Human Evolution And Increase In Body Size Biology Essay

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Human evolution is the origin and evolution of Homo sapiens as a distinct species from other hominids, great apes and placental mammals. Evolution of Human as species led to an increase in body size of humans- Eg early australopithecines were small weighing between 18 and 43 kg. Body weight rose with Homo erectus peaked with early Homo sapiens, has declined slightly over the past 80,000 years.

Reasons for gradual increase in body size:

A move from forest to a more terrestrial habitat relaxed the constraints on the size of arboreal species

In open environment predation risk was greater and selected for increased size.

There was a gradual increase in brain size (doubles its volume in past 0.5 million years) in Human evolution.

Reasons for increase in Brain Size:

Large brain development was a consequence of selection for living in social groups, diet, searching for food and bipedalism. These have been explained in details as follows:

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Living in Social Groups: The environment in which humans lived - open grasslands fostered the development of complex social groupings which required intelligence to cope with such living - co-operation, deception, representational understanding, communication.

Supporting Hypothesis were given by:

Dunbar (1992) pointed out that there is a direct correlation between group size and size of the neocortex across the primate order.

Byrne & Whitten (1998) argued that social living fosters the need for an individual to serve his own interest by interacting with others either co-operatively or manipulatively without disturbing the social cohesion of the group. They referred to this ability as 'Machiavellian intelligence'.

Stanford (1999) stated that a key element of group living is the ability to hunt cooperatively as a group, and then after the hunt there are many other social elements involved such as sharing, bartering etc. Stanford also points out that meat is highly valued (in human hunter-gatherer societies and in chimpanzee groups), partly because the possession of it allows the holder to gain political favours from males, and sexual favours from females - it is a social resource. The control of such a valuable resource confers power and status. In human foraging societies women prefer successful hunters as mating partners.

Diet: Brain size is closely related to diet with leaf-eaters (folivores) having smaller brains with proportionally less neocortex in relation to body size than frugivores (fruit eaters). 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).

Supporting Hypothesis were given by:

Foley & Lee (1991), and Gibons (1998) - Argued that the change in cranial capacity during the course of hominid evolution required dietary and developmental strategies that would sustain the cost of a large brain.

Milton (1993) discussed the behavioural and physiological adaptations concerned with plant-eating, fruit-eating and meat-eating diets. The latter two diets require the development of mental skills such as memory for food locations and increased social co-operation for hunting and food sharing. The Australopithecine's (judging by the size of their dentition) were herbivores and small brained. As the early hominids moved from plant eating to fruit and meat eating their teeth became smaller and the brains increased in size.

Hibbeln (1998) pointed out that Palaeolithic nutrition was probably low in saturated fats and high in polyunsaturated fats (the reverse is true today). Therefore, our ancestors consumed more omega-3 fatty acids (arachidonic acid, docosahexaenoic acid) and these are the major determinants of synaptic membrane fluidity.

Hibbeln proposed that the move from being vegetarian and scavenging for meat to eating fish (rich in these fatty acids) promoted the sudden burst in intellectual/technological/cultural achievement about 35,000 years ago.

Diet and Schizophrenia.

Schizophrenia is found in all known populations in all continents and has a similar prevalence and outcome in each. This indicates that genetic rather than environmental factors are the principal cause, but if this is so we must ask the question what adaptive function does schizophrenia serve? 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. He points out that a large brain per se does not automatically lead to creativity or intelligence as our ancestors did possess large brains, but for many millions of years showed no evidence of using them. Around 35-50,000 years ago there was a sudden burst of intellectual and creative achievement leading to religion, war, art, transport, complex culture and an agricultural revolution. If brain size was not the principal factor, then Horrobin argues that connectivity was the key.

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Neural connectivity is determined largely by the availability of phospholipids, which make up 60% of the brain, and in particular the connections between neurons are made by phospholipid-rich axons and dendrites. These rely on a supply of the essential fatty acids (arachidonic acid, and docosahexaenoic acid) and the essential amino acids, which are vital for cell-function and neuronal signalling 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 a side effect - schizophrenia. This disorder is typically seen in families who were distinguished in many other fields (art, leadership etc), families in which one member is schizophrenic clearly show other behaviours that are referred to as 'schizotypal':

Excess of suspiciousness.

Trace of paranoia.

Difficulty in making social contact.

Hearing voices.

Increased interest in religion and mysticism.

Eccentricity.

Creative abilities.

Interestingly, schizophrenics have been found to have reduced levels of phospholipids, there is increased activity of the phospholipases which removes the fatty acids from membranes. Membrane activity is thus affected and neuronal conductivity suppressed. This will not be a problem provided that the diet is rich in fatty acids (as it was during that part of our evolution when creativity and intelligence boomed). However, modern diets are severely lacking in the essential fatty acids and are replaced with saturated fatty acids, modern farming and agricultural techniques and contributed to this.

Horrobin proposes that biochemical alterations caused by dietary changes increased brain size, improved neural connectivity and led to creative intelligence, but also produced a series of behaviour patterns such as paranoia, visual artistic skills, mild sociopathy, religious experiences which were kept in check by sufficient dietary levels of the essential fatty acids. As these levels have dropped in modern society, the very behaviours that made us human are expressed in more extreme behaviours of schizophrenia and manic depression. Schizophrenia has certainly increased over the past few hundred years, it has a higher incidence 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.

3. Searching for Food: Early-human ancestors began to hunt and forage for food and this had a series of benefits:

It took them away from the home range that required superior memory and tracking abilities to enable efficient food exploitation and to be able to find one's way home again.

It also fostered communal social activities, as groups of hunters are always more successful than single ones.

This form of food gathering also requires the development of tools to hunt, and carve up the food, and to extract insects. Our closest relative the chimpanzee has also been known to use simple tools.

Hunting and foraging in a group entails the development of some form of communication - even if very rudimentary such as simply pointing to the location of the prey.

A small band of hunters may catch far more than they could eat at the time, the food could then be distributed amongst kin and non-kin, which needed social exchange mechanisms and co-operation.

4. Bipedalism: There are several consequences of bipedalism:

Physiological alterations were needed to adapt to the differing effects of gravity on blood flow. The blood flow system in and around the brain gradually altered and had a cooling effect which then led to further expansion in brain size (Falk, 1990).

A bipedal animal of a given weight can forage over a much greater area than one using four limbs. As much of early human evolution took place in the Savannah this would have enabled successful hunting and gathering.

An upright posture minimised the surface area of the body exposed to the sun so that our ancestors could have stayed in the sun longer and thus gathered more food and explored their environment. This also meant that humans could become hairless which took advantage of cooling by increased sweating.

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Instead of having the infant cling to the back of the mother, bipedalism meant that the mother would have to carry the infant in front of her - this is presumed to have greatly increased social interaction (observational learning and the development of language).

Bipedalism freed the hands for doing other things - holding and manipulating tools.

The regions of the brain that were concerned with foot control (grasping branches) became free for other functions.

It provides a survival advantage as it enables a better view of approaching predators.

Bipedal posture and locomotion led to changes in the structure of the pelvis which made the human birth process more difficult - the babies head emerges in an extended backward-facing position and so human births require assistance thereby increasing social cooperation.

In the upright bipedal posture the head is balanced over the centre of the spine, and the tongue partially lowered down the throat. This permits the modulation of the oropharyngeal cavity thus allowing a wide range of speech sounds to be produced.

Problems Posed by Increased Brain Size

Human devotes 22% of its basal metabolic rate to maintain a healthy brain. Larger brain require better source of nourishment. This posed two problems for early hominids:

How to obtain enough nourishment to support energetically expensive neural tissue

How to give birth to human babies with large heads.

Resolution of above problems (why our babies were so helpless and consequence of this on human social structure)

First problem was solved by switching over to meat eating diet about 2 million years ago. Second problem was solved by what is in effect a premature birth of all human babies. One way to squeeze a large-brained infant through a pelvic canal is to allow the brain to continue to grow after birth. In non-human primates, the rate of brain growth slows relative to body growth after birth. Non-human primate mothers have a relatively easy time and birth is usually over in a few minutes. Human mothers suffer hours of childbirth pains, and the brain of the infant still continues to grow at prebirth rates for about another 13 months. Measured in terms of brain weight development, a full term for a human pregnancy would, if we were like other primates, be about 21 months, by which time the head of the infant would be too large to pass through the pelvic canal. As in so many other ways, natural selection has forced a compromise between the benefits of bipedalism and risks to mother and child during and after childbirth. Human infants are born effectively 12 months premature.

The premature birth of human infants required a different social system for its support than the unimale groups of our distant australopithecine ancestors. As brain size grew, so infants became more dependent on parental care. Women would lead to the emergence of a monogamous mating pattern since a single male could not provision may females. The body size sexual dimorphism of hominids was such that males were sometimes 50% larger than females. This dimorphism was probably selected b y intrasexual selection as males fought with males to control sizeable harems. By the time of Homo sapiens this figure had reduced to 10-20 % signalling a move from polygamy to monogamy. Women probably ensured male care and provisioning for their offspring by their evolution of concealed ovulation. The continual sexual receptivity of the female and the low probability of conception per act of intercourse ensured that male remained attentive

Conclusion:

The evolution of human species led to an increase in their body size and brain size which posed some new requirements of care and attention for a dependent infant and an increased nutritional requirement. The adaptation of humans in an effort to adjust to these requirements, led to a development of men's social structure.

Summary