Humans as mammals

Humans as mammals
Our species belongs to category of mammals known as primates. Nearly all primates are tree-dwellers, and most are confined to the belt of warm forest that girdles our planet between the tropics. Many aspects of human anatomy – for example, our grasping hands, forward-facing eyes, and color vision – are typical primate features, vestiges of millions of generations spent clambering through trees. And we are unmistakably primates in our social lives and behavior: like many monkeys and apes, we live in complex, hierarchical societies where survival depends on navigating an ever-changing web of social relationships.
Being human
It is common knowledge that we share nearly all our genes with chimpanzees. Yet the tiny fraction that makes us different has a profound effect on our anatomy., so much so that, for centuries, scientists had difficulty believing we had evolved from apes at all. Unlike other apes, we lack the opposable (flexible) big toes needed for climbing, and we get around by walking on outsized hind limbs with our short forelimbs dangling in the air. Compared to apes, our skin is almost naked, our skeleton is bent out of shape, our head is swollen like a balloon, and our breasts and penis are bizarrely large. Even so, if alien scientists were to land on Earth and study Homo sapiens, they would have very little difficulty in recognizing us as apes. Physically unusual we may be, but the mountain of similarities between us and the other apes would soon become apparent.
If those visitors were to study human mind, however, they might think that we too had arrived from another planet. In intellectual terms, a yawning chasm separates Homo sapiens from every other species on Earth. We have languages of astonishing complexity. We build cities, cars spaceships. We invented morality, religion, trade, science, world wars. We can think symbolically, create art, plan for the future, and solve problems using our imagination. And, perhaps even better than other primates, we can read each other’s minds and intentions from only the slightest inflection of the voice or dart of the eyes.
Flexible shoulders
Many of the features that make humans special are unique to us, but others are shared with our close relatives. Like other apes, we have flatter chests and longer, more powerful arms than monkeys. These differences evolved because of the way that apes move through the trees. Instead of scurrying along branches on all fours as all monkeys do, apes haul themselves up trees by grasping the trunk in their arms. The shoulder blade of apes are at the back of the chest rather than the sides, which frees up the shoulder joint and allows the arm to reach overhead and swing around. In humans, the flexible shoulders that evolved from climbing are put to other uses. Uniquely, we can throw objects with force and accuracy – an ability that proved enormously useful when our hunting ancestors invented throwable weapons. Flexible shoulders also make our grasping hands more useful. If our shoulders were less mobile, we would find it much harder to carry and manipulate objects.
On two feet
The ability to walk on two feet not only sets human apart from other apes but also marks a major divide between us and the rest of mammals. No other mammal can match our ability to stand, walk and run on two legs. Of course, many other animals are capable of twolegged-or bipedal-movement, including ostriches, kangaroos (although they hop), and penguins. However unlike ostriches and kangaroos, which are counterweighted by long necks and tails that act like a tightrope walker’s pole, humans stay vertical largely because our nervous system is so beautifully coordinated. Our sense of balance also enables us to learn how to ice skate, ski, and even perform feats such as walking on our hands. One price we pay for moving in such a strange manner is that walking takes time for us to master – human baby cannot walk for about a year.
Although other apes cannot walk or run as we can, they show hints of our ability. Apes have more upright posture than monkeys. In trees, they frequently stand on their hind limbs while holding branches with their arms. Chimps and gibbons can even walk – or waddle – a short distance on the ground on two feet, an ability that comes in handy when they need to wade across the river, but one that requires great effort.
Bipedal motion is easy for humans as a result of changes in the skeleton of our ancestors. We can extend our legs fully to form a vertical column that supports the body-s weight, and the knees lock to prevent overextension of the lower leg. In contrast, apes can only partially extend the lower leg, forcing them to stand with their knees bent and rely on muscle power alone to stay upright, which is tiring.
Seen from the front, the human femur (thighbone) slopes inward from the hip to the knee, ensuring that the knees and feet are directly below the body’s center of gravity. Apes’ legs are more splayed, resulting in an awkward waddling gait. Human legs are much larger than our arms, giving our body a relatively low center of gravity that aids stability. Our center of gravity falls between two hips, giving us stable, vertical posture. In contrast, apes have large, muscular arms, short legs, and a high center of gravity results in an unstable, crooked posture when upright.
Our feet are arched so that the heel and the ball of the foot carry our weight as we move. In contrast, apes stand with the whole length of the foot on the ground. Apes have opposable big toes for climbing, but in humans the big toe is aligned with the others. When we walk, weight is transmitted from the heel to the ball of the foot and on to the big toe, which is the last point of contact as the foot pushes off the ground. This is a more efficient way of walking.
The human skull is balanced on top of a vertical spine rather than being held by muscles in front of horizontal spine. As a result, the hole through which our spinal cord passes (the foramen magnum) is shifted forward relative to that of apes, so that it lies directly under the brain. The spine is curved into an undulating S-shape that acts as a spring, absorbing shocks during movement.
To accommodate the enormous muscles that we need for walking- especially the gluteus maximus (buttock) –the human pelvis is much shorter and broader than that of other apes. It is also bowl-shaped to support the abdominal organs cradled above it.
There are drawbacks to being bipedal. One is that childbirth is more painful and protracted in humans than in other mammals. Another is that lower back pain and injury are more common in humans. The vertebrae of the lower back have to carry all the weight of the upper body, yet they must be small to preserve the spine’s flexibility. When we bend over to lift a heavy weight, the lower back is subjected to a force much greater than our body weight, and this force can rupture one of the shock-absorbing disks between the bones (called a slipped disk).

Source: HUMAN (DK, SMITHSONIAN INSTITUTION – Robert Winston, Dr. Don E. Wilson, 2006.)