Out of Africa: The Origin of Our Species (Part 9)

Early Human Migration

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.

The African fossils found so far are up to 100,000 years old, while the non-African counterparts have all been significantly younger. This tells us that anatomically modern humans from Africa were ancestral to all non-African populations and their modern descendants. Gunter Brauer, a German anthropologist, published his "Afro-European sapiens hypothesis" in 1984, in which he concluded that anatomically modern humans had evolved in East Africa from the pre-existing hominid stock not less than 150,000 years ago. From there the early human species, homo sapien sapien, spread rapidly throughout the African continent to the Nile valley reaching the Delta, the Mediterranean, the Middle East, and from there to the continents of Europe, Asia, Australia, and the Far East.

Map showing early human migration (Wiki). Numbers accompanied by arrows indicate how many million years ago the migration of homo sapiens happened.
Red: Homo Sapiens Yellow: Neanderthals Green: Earlier hominid

The fossil record shows that modern humans were in the Middle East by 100,000 years ago. Populations that migrated north from that point on were established in Europe 40,000 years ago. And those that turned east reached Australia by 35,000 - 50,000 years ago at the latest, and those who reached China did so before 30,000 years ago. From Asia, when the sea levels were low, groups of modern humans crossed the Bering Straits into North America between 30,000 and 15,000 years ago and from there to the tip of South America by 12,000 years ago.

In addition to fossil records, the past history of human populations can be constructed from genes. A group of geneticists at the University of California at Berkley analyzed the mitochondrial DNA of different groups of people around the world, and found that more mutations had occurred among Africans than among or between any other groups. The mitochondrial DNA molecules are identical in every cell of an individual. Mitochondria reproduces by cloning; asexually, by division. But the mitochondria are only inherited from the female parent because in the sperm cells they disintegrate at fertilization. Clonal reproduction and female inheritance leave mtDNA unaffected by the combination of genes that occurs in the reproduction of nuclear DNA.

Mitochondria in a cell

The Berkley geneticists found the greatest degree of variation in mtDNA among indigenous people in Africa, and significantly less variations among non-Africans. In fact, the mtDNA of an individual born in England and another born in New Guinea was more alike than the mtDNA of two individuals from Nigeria. This shows that a greater time-depth of mutation was preserved among people in Africa, while everyone else shared a predominance of mutations which had accumulated in the relatively recent past. From this, the geneticists concluded that the entire population of the modern world was descended from a relatively small group of people that left Africa about 100,000 years ago.

In addition to that, the geneticists found that every human being alive today carries the mtDNA of just one African woman who lived more than 10,000 generations ago. She wasn't the only woman alive at the time (i.e. she wasn't the Biblical Eve). It just happened that her particular mtDNA steadily became dominant as some maternal lineages disappeared with each succeeding generation. Not every woman produces daughters who would go on carrying the mtDNA. The geneticists referred to this ancestor as "our common mother," but later she became known as "The African Eve."

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:

Where there's water there is life

Cooling off

Amazing colors

Impalas in the shade

Male impala

Baboons

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:

Impalas cooling off in the shade (Tarangire, Tanzania. Feb 2012)

Dik dik (Tarangire, Tanzania. Feb 2012)

Impalas in the morning mist (Tarangire, Tanzania. Feb 2012)

Landscape (Tarangire, Tanzania. Feb 2012)

Great migration (Serengeti National Park, Tanzania. Feb 2012)

Landscape (Serengeti National Park, Tanzania. Feb 2012)

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

Out of Africa: The Origin of Our Species (Part 6)

Homo Sapiens

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.

The source of raw materials used by Homo Habilis (Part 4) to make stone tools was never much more than 4km away, but Homo Erectus (Part 5), 900,000 years ago, was making tools from stone found more than 8km away. And 600,000 years ago, some of the material for stone tools was brought from 15-20km away. These distances weren’t easily covered within a day, which indicates that Homo Erectus must have been planning activities several days ahead for a journey to obtain raw materials. Also, the materials may have not been carried over the entire distance by members of a single group; the distribution pattern could indicate that some form of barter or trade was taking place among groups camped closest to the sources of the materials.

Figure 1 - Stone blade knife

Homo Erectus was the longest surviving and most widely dispersed of the ancestral toolmakers. The species disappeared from the fossil record around 200,000 years ago, and the biface technology it introduced cease to dominate the archeological record at roughly the same time. Instead, small flake tools, scrapers, and blades took over, which signify another drastic advance in toolmaking technology (Figure 1). It is as though the mind’s eye had looked beyond the primary needs of life and seen a multitude of secondary needs. Animals were no longer simply food, but also a source of skin that could be fashioned into into bags and coverings; trees supplied bark from which string could be twisted, and gum for fixing stoned blades to wooden shafts. But is there a variation in species in our fossil records that might accompany this development in toolmaking? Yes, at about 400,000 years ago a variation on the Homo Erectus appears, which is very similar overall to the existing species, but with differences in the configuration of the skull and face that are related to a significant increase in brain size. The brain averages 1,250cc, which is very close to the modern human average, and so the newcomer is called “archaic Homo Sapiens.”

Time (millions of years) vs. brain capacity (cc)

The archaic Homo Sapiens remain until a third hominid makes an appearance about 130,000 years ago. The skeleton of this third hominid is tall and slender, the face is short and tucked in under the skull, there is a high forehead but no brow ridges, and the brain measures between 1,200 - 1,700cc. This is the first evidence of ourselves, anatomically modern humans, Homo Sapiens Sapiens.

Where do we go from here?

Watch: The Journey of Man by Spencer Wells.

Out of Africa: The Origin of Our Species (Part 5)

Homo Erectus

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.

The distinctive characters of Homo Erectus (Figure 1) compared to the earlier Homo Habilis is its larger body size, which enabled the species to cover wider planes of land. The species is known from sites in South Africa, Tanzania, Kenya, Ethiopia, Morocco and Algeria. Some representatives of the species even took the human line out of Africa for the first time. Their fossil remains have been found in Europe, Java (an island of Indonesia), and China. The oldest dates from 1.8 million years ago (in Java). 

Figure 1 - Reconstruction of a Homo Erectus at the Smithsonian's National Museum of Natural History 

Homo Erectus is a prime candidate for the immediate ancestry of the Neanderthals (Homo Neanderthalensis), who populated Europe from about 120,000 years ago, and who became extinct about 30,000 years ago (Figure 2). The large body gave a greater mobility, but it also required larger portions of food. A larger brain gives a higher capacity to think and work out solutions to problems that the species might face, but a larger brain need more energy to think. Even thought the brain constitutes only 2% of body weight in modern humans, it consumes 16% of the body’s energy.

Figure 2 - Reconstruction of a Neanderthal by National Geographic

1.5 million years ago the brain of the Homo Erectus measured about 1,000cc, which is on the edge of human range for adults (1,000cc to 2,000cc), and about 3/4 of the modern average human adult (1,330cc). Brain size is not an absolute measure of intelligence, but it is reasonable to assume that Homo Erectus depended on its brain capacity to distinguish itself from other species of the time (Figure 3). Only humans depend for their survival on elaborate systems of culture and social interactions. Even chimpanzees, which whom humans share 99% genetic identity, are far less able to assess problems and formulate solutions.

Figure 3 - Hominid Skulls

The earliest known manifestation of humanity becoming specialists in advanced toolmaking dates from around 1.4 million years ago. The manufacturing style of these tools are different from the Oldowan tools found 2.4 million years ago as used by Homo Habilis (explained in Part 4). These later tools are known as “bifaces”, and they introduce the key factors of preconception and planned manufacture to human technology. The earlier Homo Habilis made tools according to an arbitrary “least effort” principle: by knocking off some flakes from a cobble and use whatever is produced, whereas Homo Erectus made bifacial cutting tools according to a predetermined pattern.

The toolmakers selected suitable pieces of stone, knocked flakes from both sides and consistently produced tools that were longer than they were broad, pointed at one end and rounded at the other (Figure 4). There was nothing arbitrary about this manufacturing process. The long axis, the cutting edge, the point and the symmetry of the carefully controlled curves were imposed on the stone, they did not happen by chance. The process of making this kind of a tool could only happen by constant comparison of the result with an image of the finished product that is fixed “in the mind’s eye.”

Figure 4 - The biface tool

This capacity to visualize things that do not yet exist has been seen as the fundamental hallmark of culture. Imagination combined with memories of the past and experience in the present enables people to plan for the future.

To be continued...