The trends you need to understand relate to the following:
Skeletal changes linked to bipedalism |
Changes in skull and endocranial features |
Changes in the manipulative ability of the hand |
1. Skeletal changes linked to bipedalism
A defining moment in hominin history is the day a quadruped began walking on two legs, instead of four. We don't know when this was, or who exactly this quadruped was, but they paved the way for modern humans. Without bipedalism, we wouldn't have evolved into what we are today.
How did bipedalism even happen?
Whoever it was that began walking on two legs must have become advantaged in one way or another, otherwise this characteristic would not prevail to this day.
Benefits of bipedalism
Bipedalism increased the reproductive success of early hominins who walked on two legs. Some benefits are outlined below.
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Survival
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Energy
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Resource gathering
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Regardless of habitat, walking upright gives a greater field of view and means those who were bipedal could see and respond to predators better than those who walked on all fours. By spotting predators, hominins could retreat back into the trees or hide from them - this increases their chances of survival. This leads to increased reproductive success, passing on the genes that supported effective bipedal movement. Keep in mind that those who were not effective bipeds may not have lived long enough to reproduce - only those who could stand up tall enough would be surviving and reproducing.
Therefore bipedalism = increased survival and increased reproductive success. |
Those who walked upright were less susceptible to heat exhaustion, as less surface area is directly under the sun. Walking upright allowed only the head to absorb the full heat of the sun, rather than the head and back, like in quadrupeds. Walking upright also allows faster cooling as more surface area is exposed to the wind. This meant there were less hominin deaths due to heat exhaustion etc. and thus more offspring produced by those who walked bipedally. When hominins were moving out in the open savannah, walking upright was much more beneficial than walking on all fours. Those who walked on all fours in this environment may not have survived and reproduced.
Therefore bipedalism = increased survival and increased reproductive success. |
Bipedalism allowed the individual to cover more distance with less energy, compared to quadrupedalism. This was because a bipedal gait is slightly longer than a quadurpedal gait - so if they were to take the same number of steps, the biped would cover more distance. The toes evolved over time to help with the thrust movement during the walking and running gait. Those who walked most effeciently could spend their time doing other things, which aided their survival (i.e. gathered more food) and thus were more reproductively successful and passed on these traits to their numerous offspring.
Therefore bipedalism = increased survival and increased reproductive success. |
Bipedalism allowed the individual to see further (as previously stated) - allowing them to see possible sources of food and make decisions about which direction to go. This saved energy, as walking on four legs in the long grass provides little information about where the food is - but by seeing over the grass there is more chance that food and other resources can be found faster, allowing the energy saved to be used for other things that made their survival more likely.
Therefore bipedalism = increased survival and reproductive success. |
More food was able to be accessed as a result of bipedalism - particularly to those who maintained agility in the trees. Those who could access more food and nutrition were able to survive for longer, or were more reproductively successful and thus passed on those genes to reinforce the bipedalism traits.
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Walking on two legs, instead of four, freed up the front limbs (arms/hands) which became useful for other tasks such as gathering resources, making tools, etc. which helped individuals exploit more resources and live more successfully. Hominins could locate food and then carry large amounts back to the home group, because their arms were free to do so.
Therefore bipedalism = increased survival and increased reproductive success. |
Comparative anatomy
to see trends relating to bipedalism
We can compare the anatomy of humans and Australopithecus afarensis (also known as Lucy, who lived nearly 4mya) to see what parts of our bodies have changed over time in repsonse to different selective pressures. In other words, by looking at A. afarensis and looking at ourselves, we can see how we have evolved over time. We can also compare our anatomy with apes, to see how different we have become over time. The common ancestor of humans and apes was more ape-like than human-like.
The diagram below compares H. erectus with A. afarensis - this help to see the changes that have occurred. H. sapiens reseble H. erectus but in a more refined form - our bodies are better at walking bipedally!
Skeletal changes resulting directly from bipedalism:
You need to be able to explain why each feature changed over time - what is the benefit that each feature provides a biped? How does the feature assist bipedal locomotion? None of this 'they didn't need their arms for walking so they got smaller' business - that's not WHY they got smaller - what is the benefit of smaller arms?
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Spine
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Pelvis
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Skull
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Legs/arms
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Feet
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Hair
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Quadrupeds have a curved spine - which supports their type of locomotion. By having a curved spine, their centre of gravity is further forward, so they can balance when walking on all fours. Over time, hominins developed a more S-shaped spine. This keeps our centre of gravity over our legs so that we can maintain our balance when walking upright. It also helps to absorb the shock that is created when walking bipedally. However, we are more prone to back-aches because of our spine shape!
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Quadrupeds have a very long and narrow pelvis that extends quite far up the back. In apes, this allows for the large muscles needed for swinging in tree branches to attach. With this shaped pelvis, chimps etc. can walk/run on two legs for short distances but kinds of waddle from side to side. It's not very effective and isn't good for travelling for long distances. In humans, the pelvis is angled backward to help push our spine into the S-shape that is so good for bipedal motion. To walk efficiently, our pelvis also allows our hip joints (and therefore legs) to be closer to gether and our centre of gravity over our legs. Having a shorter and bowl-shaped pelvis also helps reduce the stress that the upper body weight puts on the hips, assisting in bipedal locomotion.
However, over evolutionary time the human skull and brain has increased in size; the wider pelvis allows offspring with proportionaltely larger heads to still be born. Brain (and skull) size has increased over time and a wider pelvis supports this evolutionary trend. |
Foramen magnum
In apes, the foramen magnum (hole where the spinal cord threads through to the brain) is towards the back of the skull. This is because apes walk on all fours, and their head sticks out in front of their body. To allow the head to move around adequately, the foramen magnum is at the base of the skull. In humans, who walk on just two legs, the foramen magnum is position in a more central location on the skull. This helps the head to balance on top of the spinal cord, reducing the need for large neck muscles, and allows the head to rotate around quite far, giving us improved vision - helpful when we're walking out in the open where predators could spot us! |
Nuchal crest
In apes, the nuchal crest is more prominant as larger neck muscles are needed to support and hold up the head. As the head sticks out in front of the body, larger and stronger muscles are needed to hold the head up and to rotate the head. Becuase the human skull can balance relatively effortlessly on top of the spinal column, large neck muscles are not required to support the head - only small ones. This means the nuchal crest, where neck muscles attach, can also be reduced in size, and the energy better spent elsewhere, such as increasing brain size. |
Valgus angle
In apes, the femur is not angled at all - it points straight down - allowing for effective swinging motion when moving through the trees. The shape of the knee also means apes cannot fully extend their legs, like we can. Humans can take longer strides as a result of being able to fully extend their legs from the knee joint. Humans have a longer femur which is angled in towards the middle. This creates the 'knock-kneed' stance we have. This angle helps to bring our knees almost directly under our pelvis, allowing our weight to be centred in a way that allows for efficient bipedal movement. This allows for greater balance during walking/running. |
Arm-to-leg ratio
Apes have long arms and shorter legs - this is to support their mode of locomotion; braciation. They need big, strong arms for swinging in the trees and only use their legs for grasping the branches and walking (on all fours) across them. Humans have long legs and shorter arms - this is to support bipedalism. The need for longer arms disappeared and over time lost bone and muscle mass due to them not being used for locomotion - instead, the energy saved was spent on making the legs longer and stronger to support upright walking. |
In apes, the big toe is opposable; spreading outwards from the foot. This helps the apes to grip on to branches while moving through the trees, as well as other things. Apes also have curved toes which aids their movement through the trees. They also have no arch in their foot. Humans have a forward-pointing big toe which is not opposable. It aids the forward thrust needed for bipedal locomotion. There is an arch in the foot which acts as a shock absorber when walking and running (eg. away from predators) and helps humans walk over long distances.
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Over time, we have become a lot less hairy! Initially this was thought to be related to the change of climate over time but there are more theories coming to light. The following ideas regarding hair loss remain true regardless of the prevailing theory of why hair was lost over time.
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2. Changes in skull and endocranial features
The change from quadrupedalism to bipedalism allowed the diets of hominins to change. This change was exaggerated by the use of fire for cooking food. This allowed for some more skeletal changes - mainly to the skull, as a response to the change in diet that came from bipedalism. These trends are indirectly caused by bipedalism.
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Brow ridges
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Sagittal crest
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Brain size
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Jaw/teeth/zygomatic arch
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Occipital bun & condoyles
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Apes and very early hominins have large brow ridges. This helps to protect the face and, particularly, the eyes. As the jaw and jaw muscles are so huge, without large brow ridges, the face could cave in on itself. Humans have smaller jaws and jaw muscles, leaving the risk of a facial collape very small. Over time, the brow ridges have reduced in size and this has given space for frontal lobe expansion in the brain.
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Apes have a large sagittal crest (ridge on top of the skull). This is because apes have huge jaws and the large muscles that control them need somewhere to attach. Jaw muscles wrap around the head and the sagittal crest helps anchor the muscles to the skull support the large jaw needed to eat vegetation. Humans have an incredibly reduced (basically absent) sagittal crest, as our jaws are a lot smaller. When humans began cooking their food, the need for large grinding molars became very small and more energy was spent on increasing brain size. Smaller jaws need less space to attach to the skull, thus the sagittal crest is completely gone.
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Over millions of years brain size has increased. As brain size increased language, communication, spirituality, forward planning, co-operation, empathy and many other complex cognitive functions have developed as a result. The more intelligent the individual, the more they can adapt and outsmart preditors and competitors. Larger brains meant increased chance of survival, and increased reproductive success. Luckily the pelvis shape that developed over time to support bipedal locomotion can also fit such a big skull through it! Here's an article that talks more about this.
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The jaw of apes is large, strong, and full of big teeth. There is obvious differences between the teeth types; large grinding molars (for vegeative diet), large canines for agressive displays, a diastema to fit these canines in. The zygomatic arch in apes is large to acommodate the large jaw muscles which feed through and attach to the top of the skull. The jaw of humans is much smaller, as are the teeth size. The teeth are much the same size and shape, compared to ape teeth, as vegetation is not the primary diet of humans - cooked food meant teeth did not have to be so large! Humans do not have large canines so do not need a diastema.
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3. Changes in the manipulative ability of the hand
- now this didn't help bipedalism, but it was a direct result of bipedalism!
By walking on two legs, the hands were freed up and were able to be used for numerous things which gave bipeds an advantage over quadrupeds.
Longer fingers not needed as brachiation no longer the type of locomotion used. Over time fingers became shorter, the grip became more precise, the thumb became larger and more opposable.
With more manipulative ability of the hand came
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