Change Over Time: Hominin Evolution

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Change Over Time:

Hominin Evolution

1. Humans are primates. Describe the major features that distinguish primates from other mammals.

- Brain larger when compared to other mammals.

- Hands and feet with curling fingers.

- Nails on digits instead of claws (support tips of fingers when gripping).

- An opposable digit which makes possible the power and precision grips.

- Large forward-facing eyes that provide stereotypic vision.

- Reproductive patterns with emphasis on caring for fewer offspring well.

2. Why are opposable digits important to primates? In particular, how is the opposable digit important to humans?

They allow a primate to grip in two ways. The power grip with all digits curled is especially useful to apes climbing trees. The precision grip where the thumb tip meets the fingertips is useful for manipulating small objects. The precision grip is particularly important for humans in making and using tools.

3. What features distinguish hominins from other primates?

- Development of bipedalism – that is, walking upright on two legs. To allow an upright stance, changes to the backbone, hips, knees and feet occurred during evolution of the hominins

- Enlarged brain – the skull had to undergo several changes. Jaw and tooth size decreased along with muscles to support and move big jaws.

- Increased intelligence – allow more complex behaviour and development of language.

- Lack of oestrus – most female primates have a time when they are on heat (obviously sexually receptive). Human females do not show greater interest in mating when fertile. Ovulation is concealed and the male is continually interested in sexual activity. Suggestions have been made that continuous interest promotes more stable relationships and keeps the father around to help look after his offspring. In primates and other mammals the male competes with the other males for females who are on ‘heat’.

- Longer developmental periods – all primates take a relatively long time to mature but this is particularly long in humans. This allows greater time for skill development and learning to problem-solve.
4. Describe the location changes of the foramen magnum.

The foramen magnum is the hole at the bottom of the skull through which the spinal cord connects to the brain. In four-legged mammals the foramen magnum is at the back of the skull. In humans it is in the centre of the base. During the evolution of the primates and with the change to bipedalism the foramen magnum has had to ‘move’ forward.

5. What changes to jaw and tooth structure have occurred during human evolution? How might behaviour have changed during this evolution?

Reduction in jaw and tooth size has occurred while the brain takes up proportionally more room in the skull. The larger and stronger teeth of the apes allows thorough ripping and crushing of food. Human ancestors used their brains to develop tools to do these jobs. Increased brain size must have been more advantageous.

6. Skull shape and size have changed during human evolution. Describe their changes and their significance.

The change in skull shape and size is associated with the huge expansion in brain size. The area of the brain most increased in size is the cerebrum which is associated with reasoning and problem solving. Increased brain size has allowed the development of complex spoken and written language. Increased brain size also allows complex social behaviour. Humans can work and plan together. All organisms affect their environment but humans are able to alter any environment to suit their needs. The changes described are not unique to humans. Unique, however, is the extent of the changes.

7. What has been the limiting factor to a further increase in skull size?

A significant limiting factor to a further increase in skull size is the size of the human female’s pelvis.

8. What selective pressures might have led to an increased brain size?

The major selective pressures acting on human ancestors were probably competition and predation. It is thought that early hominids hunted on the African plains. Other larger animals could catch and kill one lone hominid. A group of well-organised stick-waving hominids would have been more formidable. Good organisation requires more brain power. To hunt animals for food requires planning and co-operation. Again greater brain power would have been an advantage. Hominids with larger brains and problem-solving abilities would have had a selective advantage.

9. Is it true to say that man has evolved from chimps and gorillas? If not, rephrase the statement.

It is not true to say that humans have evolved from gorillas and chimps. Chimps, gorillas and humans have a common ancestor. That common ancestral species would have lived several million years ago.

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