Surfacin9: Out of Africa: The Origin of Our Species (Part 7)

Water & Sweat

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.

People, like our planet, contain more than 70% liquid. Much of the human body consists of “plumbing” parts, such as blood vessels, lymph and urinary tracts. These parts are vital for transporting nutrients and eliminating waste. The water content of a 65kg human is nearly 50 liters. An average human, in average temperature conditions needs to take in and eliminate 2.5 liters of liquid each day, and no matter how much the intake varies the body’s total water weight remains relatively consistent, deviating less than 1% from what is normal. A minimum of about half a liter of urine output is essential to get rid of toxic wastes. Water is so important for human bodies that about 5% loss impairs many functions such as brain’s ability to process information, and death will result from a loss of anything more than 5%, which amounts to about one day’s liquid intake: 2.5 liters.

Walking with a bush guide in Tarangire, Tanzania.
How our ancestors lived in this environment
without weapons, gadgets, clothing or bottles of
water, I don't know. (Feb, 2012)

In tropical conditions, humans need more water than normal to keep up with the rate of sweating, which keeps the body at lower temperatures compared to the environment. Our nervous system is sensitive to changes in temperature. Normal functioning of the brain becomes progressively impaired at temperatures above 37^oC, and even a temperature of 40.5^oC can be fatal. Every drop of sweat has to be replaced. Failure to do so within a 24-hour cycle produces serious dehydration. Death is inevitable if a second day of high heat load is experienced.

The temperatures of the East African savannas 200,000 years ago, in which human ancestors lived, neared 30^oC during the early hours of the day, and peaked at 45^oC. At such temperatures humans would’ve had to have a continuous supply of water within a 24-hour walking vicinity.

Landscape of Serengeti National Park, Tanzania. (Feb, 2012)

Humans are not the only animals that depended on water so greatly in their daily lives. Grazing animals spend their days exposed to the full impact of the sun. Their muzzles however are effective in keeping their heads’ temperatures lower than their body temperatures. The length of the muzzle loses heat by evaporating water from moist linings of the nasal chambers. The evaporation removes heat from blood flowing beneath the nasal membrane, and the cool blood travels to the base of the cranium. The brain is supplied by the carotid arteries, which pass through the sinus, where they branch into a net of fine blood vessels called the carotid rete. As arterial blood flows through the carotid rete, excess heat is transferred to the cooler venous blood in the sinus, and the brain receives the blood at the critical temperature.

Impalas (Tarangire, Tanzania. Feb, 2012)

Humans don’t have this natural “radiator”, and the closest living primate to us today, chimpanzees, don’t have it either. During the course of primate evolution the face broadened, and the jaw shrank as a result of dietary changes. Chimpanzees, for example, are very poorly equipped to adapt to high temperatures. Anything above 40^oC causes considerable distress. They sweat and breath heavily, yet the only relief they get is once evening comes.

So if our early ancestors, the Australopithecus Afarensis (Part 2), lived in the East African savannahs, and had to hunt for food during the day (to avoid other predators that hunt during the night), how did they manage?

To be continued...

A few more pictures from our trip to Tanzania: