Surfacin9: Out of Africa: The Origin of Our Species (Part 8)

The environment we adapted to

Note: I started reading Africa: A Biography of the Continent by John Reader. The following posts will basically summarize what I find interesting in the book as I’m reading it. None of the ideas or thoughts are of my own.

Figure 1 - Upright bipedal gait

Our early ancestors had an advantage: bipedalism. Our body cooling system was an adaptive response to the environmental stresses of tropical Africa; the upright bipedal gait is otherwise inefficient when it comes to moving fast (Figure 1).

Quadruped animals were able to run much faster than our ancestors of the time. But the upright stance was better at cooling the body, especially at noon, when the sun was directly overhead. Only 7% of the biped’s body surface was exposed to the sun, while the quadruped had 20% of their bodies exposed. Furthermore, the cooling effect of a biped as a result of standing tall from the ground compared to an other animal standing on the same location is that heat is lost 33% faster. The quadruped may sweat just as much, but it’s that much harder to lose the sweat to the wind when the length of the body is close to the ground. The air at ground level is more humid because of the vegetation which makes it hard for the quadruped to ventilate.

A baboon roaming the fields of Tarangire
(Tanzania, Feb 2012)

Warthogs (Tarangire, Tanzania, Feb 2012)

Hair is another factor. Fur serves savanna quadrupeds as a shield, reflecting and re-radiating heat before it reaches the skin. If quadrupeds lost their fur, melanin would’ve protected them from harmful UV-B radiation, but this would’ve decreased the reflectiveness of the skin, causing a greater energy gain throughout the exposed body to the sun rays. But a biped could get rid of most of the hair and make it easier to sweat. A shield is only needed where sun rays would hit most severely: the head.

Humans have has many hairs has the chimpanzee per square centimeter but they’re shorter and finer. This nakedness, along with well developed sweat glands, enable us to lose heat at a rate of 700 watts per square meter of skin, a rate not even close to any other living mammal.

Figure 2 - Look at the location of the jugula sinus

We wouldn’t be what we are today, Homo sapien sapiens (i.e., bipedal with large brains), as opposed to something more like a chimpanzee, if it wasn’t for the necessary evolution of our cooling system. This is not to say that temperature controlled strategies caused the evolution of a large brain, merely that they removed certain physiological constraints and thus made the enlargement of the brain possible. As functional as the long muzzle and the carotid rete are at keeping a quadruped’s head cooler than its body, it has its physiological limits. The system depends on the amount of blood that the carotid rete can cool to the critical temperature before it flows to the brain. However, large brains need a larger blood supply and therefore a larger carotid rete. Since the rete functions by pumping blood through an ever-finer network of blood vessels, its maximum size is determined by the the pressure and volume that can be accommodated. It was calculated that if the blood supplying the modern human brain was cooled through a carotid rete, the jugular sinus (Figure 2) in which it was situated would fill the entire diameter of the neck! Clearly, the whole body cooling system supported by the upright stance and naked skin was a more feasible strategy.

The ability to develop a large brain doesn’t come without an expense: providing the energy to keep it functional.

Brain is “expensive tissue.” The brain uses energy nine times faster than is average for the human body as a whole. Skeletal muscle, for example, where energy is more obviously expended, consumes less than 15% of the body’s budget even though it makes up 41.5% of average body weight. The brain makes up 2% of an average body weight, but consumes over 16% of the energy. And since the brain has no means of storing energy for future use, it must be continuously supplied with hight levels of fuel and oxygen.

You’d expect with an increase of brain size a corresponding increase in the size of the digestive tract because of the needed fuel, however, that’s not what happened. In fact, the human gut is almost exactly half the size that would be predicted to match the enlarged brain. This is achieved by learning to seek out nutritious foods that only need to be consumed in small amounts, which comes in contrast with other primates that process large quantities of leaves and grass (see Part 3 - Australopithecus (robust vs. gracile)). The guts of our ancestors filled the environmental niche that allowed them to consume high quality foods such as seeds, nuts, eggs, and bone marrow, in addition to meat.

Bigger brains seek more nutritious foods, and more nutritious foods make bigger brains.

A few more pictures from our trip to Tanzania in February 2012: