Humans

Yesterday I spoke about how humans are classified down to the order and suborder level (our order is primates and our suborder is anthropoids). In this post I'm going to talk about some features that separate us from the other primates, particularly those in the same superfamily as us.

Our superfamily is called Hominoidea. This is then broken down into several families, including the hominidae (which includes humans), the hylobatidae (which includes gibbons) and pongidae (which includes the great apes). Here are some of the ways in which we differ from other members of our superfamily:

• Our hands and feet are different. Our hands allow us to use not only the power grip involved in holding objects, but also the precision grip involved in fine movements like writing or sewing. Our feet don't allow us to grasp onto stuff, but they do allow us to walk on two feet, which leads me to...
• Humans stand erect (upright). We are bipedal (walk on two legs), as opposed to gorillas and chimpanzees which are quadrupedal (walk on four legs). There are a lot of differences between us and the apes which allow us to walk on two legs- more on these later.
• Our brains are larger than those of the apes. (Our brains are not the largest, however- many larger animals have bigger brains than us.)
• Our canine teeth are shorter and do not interlock with one another, and the distance between the front of our mouth and the back of our mouth is shorter than that of the apes.
• We can talk. There have been some gorillas that have been taught how to use sign language though (one famous example is Koko the gorilla).
• We have less hair than the apes.
• Human females have bigger boobs. No other female primate has such well-developed breasts. (Seriously!)
• Humans grow slower than the other primates.

Now to go into these points in a bit more detail!

Hands and Feet

• Our hands are shorter and broader than that of the apes. Our fingers are short and straight, while our thumbs are long and strong by comparison. This means that our thumbs are very opposable (see my previous post on the primates for more info).
• Another factor contributing to opposability of our thumbs is the saddle joint between the metacarpal in the thumb and the bone in the wrist. This allows the thumb to undergo flexion (bending), extension (straightening), abduction (move outwards), adduction (move inwards) and opposition (reach across the palm and touch the tip of the other fingers).
• There are also lots of muscles controlling the thumb and finger. Some are long, with their bellies in the forearm and tendons stretching across the wrist, and others are short, located entirely in the hand.
• As stated previously, we're able to use the precision grip (unlike other primates). This is due to the short muscles in the thumb and fingers and the opposability of our thumbs.
• Another kind of grip that we can use is the power grip, which can also be used by other primates. It requires the long muscles of the forearm, and exerts considerable strength.
• Hands and fingers can also do a helluva lot of other stuff, like twist things and form a cup-shape.
• Regions of the human brain that control fine hand movements are well developed.

Being Bipedal

Being a biped requires loads of special adaptations, mainly in the position of the head, the shape of the vertebral column, and in the structure of the pelvis, knee and foot. Other important adaptations involve our muscles and sense organs. 

• Head- Our heads balance on top of the vertebral column, while the skull of quadrupeds like gorillas and chimpanzees is at an angle to the vertebral column and is held in place with large neck muscles. To allow for this, the foramen magnum (the hole in the skull where the brain joins onto the spinal cord) is at the base of the cranium in humans, while it is towards the back of the skull in quadrupeds. Another relevant adaptation is that the human jaw bone is much smaller than that of the quadrupeds.
• Vertebral column- The vertebral column has a double curvature, making it S-shaped. Wedge-shaped vertebrae in the lower, or lumbar, region form a forward-jutting curve, improving body balance and allowing the head to balance on top of the neck. The cervical curve of the neck brings the vertebral column under the skull.
• Pelvis- The pelvis is broader and shorter from top to bottom than that of the apes. It supports the abdominal organs and gives attachment to muscles that allow the arms and legs to move and keep the upper body erect. The shape of the pelvis also results in the hip joint being directly under the head, allowing weight to be readily transferred from the pelvis to the legs.
• Legs and knees- The broad pelvis results in the hip sockets being wide apart. However, the thigh bones converge towards the knees, resulting in weight distribution remaining centred when walking, which in turn results in greater stability and our striding gait (as opposed to swaying from side to side like a gorilla walking on two legs).
  Weight is then transmitted down the outside of the femur (as opposed to the inside, like the apes) to the knees. The centre of gravity tends to be just in front of the knees, which creates a force that tries to bend the knee backwards, but this is resisted by the super strong ligaments of our knee joints. Yay!
  Another important part about legs is that our legs are longer than the arms (as opposed to the apes, which have longer arms than legs). Also, our legs take up a greater percentage of our total height- around 1/2 of our height is legs, compared to 1/3 in the apes. This gives us a lower centre of gravity, which in turn makes us more stable.
• Feet- Weight from the knee joint then goes to the tibia (the larger and stronger of the two lower leg bones) and then down to the ankle and foot. Weight goes through the talus to the tarsals, then the metatarsals and phalanges. Our feet, unlike that of the apes, lacks grasping ability, or prehensility, and our big toes are not opposable. Our feet, however, do have some special characteristics. We have two arches: a longitudinal arch (front to back) and a transverse arch (side to side), the latter of which is unique to us.
• Muscles- A lot of muscles are required to resist the pull of gravity. Muscle tone, the partial contraction of skeletal muscles, is important in keeping us upright.
• Sense organs- A variety of sense organs, or receptors, are involved in giving the brain input regarding balance so that we can adjust our stance if necessary. (Most of the work involved in adjusting stance is done without conscious thought- these are called postural reflexes.) Aside from the eyes, we also have balance organs in the inner ears, pressure sensations in the feet and stretch receptors located in the joints, ligaments and tendons.

I will discuss walking in greater depth later.

Our Brains

Humans have relatively large brains compared to that of the apes. Our brains are 900-2200 cm^3 (average is 1350 cm^3), while apes' brains are around 400-500 cm^3. Most of this increase in brain size is related to the cerebrum, which is divided into the left and right hemispheres by a longitudinal (front to back) fissure). The outer portion is called the cortex, which has a lot of folds, called convolutions, which increase the surface area of the brain. The front part of the cerebrum, known as the frontal lobe, which is involved in the higher functions of thinking, reasoning, planning and processing, is relatively large in human brains. In humans, the frontal lobe makes up 47% of the total surface of the cortex, while in apes it only makes up 33%.

In order to make way for our big brains, we have a larger cranium (brain case). In humans, more of the skull is used in housing the brain, resulting in a vertical brow and a lack of prominent brow ridges (the latter feature is possessed by the apes). We do, however, have a more prominent nose than other primates, though our faces are still relatively flat compared to animals with snouts.

Teeth

Humans have smaller teeth than apes, and thus smaller, weaker jaws and jaw muscles. Our canines are the same length as the other teeth, as opposed to those of the apes, who have relatively long canines compared to other teeth. The teeth of apes also interlock with each other, while those of humans do not. The shape of the tooth row is also different between humans and apes: humans have a shorter tooth row that is shaped like a parabola, while apes have a longer tooth row which is U-shaped.

Speech

Our prominent chin and shortened jaw provide our tongue with more forward attachment, allowing for greater freedom and thus also allowing for the production of more complex sounds. Our larynx is also directly below the tongue and soft palate, and when air passes over its vocal cords, sounds are made, which can be modulated by the tongue, hard and soft palate, teeth and lips. Aside from these structural features that allow for speech, a large portion of our cerebrum is devoted to speech.

Hair (or lack of it)

Humans are relatively hairless compared to other primates. We are, however, covered by lots of fine hairs that we don't often notice, plus we grow hairs in plenty of places during adolescence (particularly males). Some scientists believe that the relative lack of hair allows us to cool down more efficiently- we cool down when our sweat evaporates, and hair would impede the evaporation of sweat. To compensate for the increased susceptibility to cold, we have a layer of fat just below the skin to help retain body heat.

Breasts

As stated previously, female humans have well-developed breasts, which other primates lack. My book* says that "the large, conspicuous breasts of the female may have a sexual as well as a maternal function" to which I just thought, "NO SHIT SHERLOCK!!"

* Newton, T. J. and Joyce, A. P. (2003). Human Perspectives. 4th ed. Australia: McGraw Hill. 390.

Growth and Development

Humans have a relatively long period of development after birth. (Actually, all primates have a relatively long developmental period compared to other animals, but humans have the slowest of all.) One of the slowest periods of development is that between weaning (moving onto solid foods) and puberty.

Now, as promised, back to the subject of walking...

Walking

Walking is quite complex, involving loads of muscles. Each leg has 60 muscles, and most are involved in walking. That's not counting all the muscles in the rest of our body that keep us upright while walking. I'm now going to present a simplified explanation of walking- which is still kinda complex, if you ask me.

Walking has two phases: the supporting leg, or stance, phase, and the swinging leg phase.

During the supporting leg phase, the leg starts to support the body weight. Weight is transmitted from the heel progressively forward over the foot: along the outside to the ball and across the ball to the big toe, which eventually propels the whole weight of the body forward (pretty damn amazing for a humble toe, if you ask me). Calf muscles help to stabilise the foot as it takes the weight of the body.

While one leg is in the supporting leg phase, the other is in the swinging leg phase. The gluteal muscles contract to maintain stability while the hamstrings contract and the quadriceps relax in order to bend the knee. The leg swings forward until the heel touches the ground, before the hip is bent and the knee straightens (the latter is done via contraction of the quadriceps). The anterior tibialis muscle then contracts, bending the foot at the ankle and allowing weight to fall on the heel.

The pelvis also plays a part in walking. At each step, the pelvis tilts downward slightly over the supporting foot, centring the body weight over the supporting foot. This is more prominent in women than in men.

During walking, each leg is in the stance phase for about 60% of the time and in swing phase the other 40% of the time. Both feet are on the ground around 25% of the time, but this percentage decreases the faster you walk. If at any time both feet are off the ground, you're no longer walking, but running.

Aaaaaaaaaaaaaaaaaaaand we've gotten through year 11 human bio!! Yay!!!!!!

(Now I need to find something else to do with my life...)