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the various species of human (those species which belonged to the genus Homo) shared many characteristics with each other

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had extraordinarily large brains

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mammals weighting 60kg usually have a brain 200cc (cubic centimeters)

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homo sapiens today who weigh 60kg have a brain weighing 1200-1400cc

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2.5 million years ago, the earliest men and women had smaller brains than we do today but compared to other animals it was still proportionally large and increased as humans evolved

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when we think of reasons why humans ended up dominating the earth and the fact that their brain size is relatively larger than other animals, we tend to think that in terms of brain size, bigger is better, but if this were the case, evolutionary pressures would have also produced cats with big brains, dogs with big brains, and horses with big brains, but this did not happen. The fact is that a big brain, apart from its advantages, is also a big problem: you have to carry it around, and you have to protect it so it is usually encased within a massive, heavy skull. What is even more difficult is to fuel the brain with energy. In Homo sapiens, the brain accounts for 3% of body weight but consumes 25% of the body's energy at rest. In comparison, chimpanzee's brains require only 8% of their body's energy at rest.

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Ancient humans paid for their larger and larger brains in two main ways: (1) they had to spend much more time than other animals looking for food, whereas a baboon with his smaller brain could sit around in the sun much of day, humans spent their day time largely looking for food to fulfill their brain's demanding energy budget, and (2) as their brains increased in size, humans became less muscular, their muscles in general became smaller and weaker as energy was used less to power their biceps and leg muscles and more to power neurons. It is far from obvious that giving up muscle to have a bigger brain is a good strategy to have for surviving in the jungle or savanna. A large ape or chimpanzee could not win an argument with a human but could rip his body apart quite easily. So apart from their flint knifes and pointed sticks, humans had very little to show for their big brains. But according to the social brain hypothesis, when hominids started living in large groups, selection favored greater intelligence. The ability to communicate symbolically helped humans be able to live in larger groups and protect themselves more efficiently."

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walked upright on two legs

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most other animals walk on all fours

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advantages of standing up

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see predators and food from further away

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hands are free while you walk e.g to throw stones or sticks

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as hands became free, humans developed finely tuned muscles to perform delicate tasks with their hands to build tools

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2.5 million years ago, first sign that humans were making tools

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disadvantages of walking upright

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the skeleton of our primate ancestors evolved to support a create to walk on all fours and had a relatively small head and light brain

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walking upright with a larger head and heavier brain created stress on spine and skeleton

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muscles and skeleton evolved to support it better but it was never perfect

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people today suffer from backaches and stiff necks which results from moving to an upright position

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women paid extra for walking upright on two legs

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when you walk upright, your can move more efficiently the closer your hips are to one another, which put evolutionary pressure on human bodies which resulted in the birth canal in general becoming smaller, while at the same time babies heads had evolutionary pressure to become larger

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the result was the human mothers and children began to die more often during child birth

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death of the mother and child during childbirth became far more common among humans than among chimpanzees, baboons, zebras or elephants

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women who tended to give birth later in their pregnancy cycle had a greater chance of dying during childbirth as did their children, and those women who tended to give birth earlier while their babies heads were relatively smaller and softer had a greater chance of surviving childbirth, as did their children, so over the eons humans began to give birth to babies earlier and earlier

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this is why, compared to other animals, humans are born prematurely, when many of their vital systems, especially in their brain, are still not developed

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compare this to a baby horse who can begin trotting several hours after birth

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kittens will begin looking for food for themselves when they are only a few weeks old

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human babies, on the other hand, remain completely helpless for months and even years

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because human children are born prematurely, unlike other animals, a single human mother could not give adequate care to her children during the long period of time the children are not yet developed enough to take care of themselves, which led to humans living in larger and larger groups and become more and more social

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this led to the fact that humans can be socialized and educated to a far greater extent than any other animal

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whereas other animals are born with their brain functionality almost completely formed, humans have a long period of time after birth to develop their cognitive functions and hence their cognitive functions depend much more on their environment

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we tend to assume that having a large brain, being able to produce and use tools, having complex social skills would make humans dominant, but the fact is that humans enjoyed these advantages for more than two million years yet remained weak and marginal without much impact on their environment. A million years ago, there were only about a million humans in the world and they were not on the top of the food chain, they were preyed upon and hunted by lions, bear, and alligators. They were not very good hunters, hardly able to hunt and kill big animals like giraffes and elephants, even in groups. Most humans subsisted by eating vegetables such as nuts, fruits and mushrooms, by hunting small animals like rabbits, frogs and turtles or eating the leftovers from the kills of other animals such as the leftovers of a giraffe killed by a lion.

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one of the most common uses of early stone tools was not to hunt animals, but to crack open the bones of dead animals in order to get at the marrow.

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every animal has a special niche

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woodpeckers can peck inside wood and eat termites and other insects that live inside the niche

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using stones to break open bones in order to eat the marrow was the original specialty of humans in the world

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lions kill giraffe and eat most meat

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hyenas come and nibble the rest of the meat

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then humans approach the carcass and find nothing left to eat, yet you use your stone tools to break open the bones and eat the marrow, since no other animal had the ability to get the marrow, this is your specialty characteristic of survival in the savanna

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for 2 million years, humans were somewhere in the middle of the food chain, hunting small animals and eating only the marrow of larger animals

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400,000 years ago several species of humans such as the Neanderthal would form groups to hunt larger animals

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only in the 100,000 years did humans jump from the middle to the top of the food chain

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humans were not used to being at the top of the food chain and were ill-adapted to this role, it evolutionary time it was a sudden change