Africa would appear to have the longest record of human habitation in the world. The first hominins are likely to have emerged 6-7 million years ago, and among the earliest anatomically modern human skulls were discovered at Omo Kibish, south Ethiopia. There is potentially older fossils linked to Homo sapiens found at a site in Jebel Irhoud, Morocco. When the site was occupied by early humans, it would have been a cave; the covering rock and much sediment was removed by work in the 1960s.

Perhaps, it has been suggested, the Jebel Irhoud humans “were an ‘archaic’ species that survived in North Africa until H. sapiens from south of the Sahara replaced them. East Africa is where most scientists place our species’ origins: two of the oldest known H. sapiens fossils — 196,000 and 160,000-year-old skulls – come from Ethiopia, and DNA studies of present-day populations around the globe point to an African origin some 200,000 years ago……..What we think is before 300,000 years ago, there was a dispersal of our species — or at least the most primitive version of our species — throughout Africa,” Hublin says. Around this time, the Sahara was green and filled with lakes and rivers. Animals that roamed the East African savanna, including gazelles, wildebeest and lions, also lived near Jebel Irhoud, suggesting that these environments were once linked.”(see http://www.nature.com/news/oldest-homo-sapiens-fossil-claim-rewrites-our-species-history-1.22114)

Researchers in genetics have been tracing Homo Sapiens back to Africa; for example studies of Chinese populations show that 97.4% of their genetic make-up is from ancestral modern humans from Africa, with the rest coming from extinct forms such as Neanderthals and Denisovans. But in China we may yet find early hominins began to emerge before the African. Many Western scientists tend to see Asian fossils and artefacts through the prism of what was happening in Africa and Europe,” says Wu. Those other continents have historically drawn more attention in studies of human evolution because of the antiquity of fossil finds there, and because they are closer to major palaeoanthropology research institutions, he says. “But it’s increasingly clear that many Asian materials cannot fit into the traditional narrative of human evolution.” Chris Stringer, a palaeoanthropologist at the Natural History Museum in London, agrees. “Asia has been a forgotten continent,” he says. “Its role in human evolution may have been largely under-appreciated.”

But we are one human race, wherever and however we evolved into Homo Sapiens, we descend from the same single ancestor as the chimpanzee.

Between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of East Africa. They not only had to cope with more intense sunlight but had to develop a better cooling system. It was harder to get food in the hot savannas and as mammalian brains are prone to overheating then 5 or 6 °C rise in temperature can lead to heatstroke – so there was a need for the development of better heat regulation. The solution was sweating and loss of body hair. If the skin was white it burned, so some early hominins may have stayed under canopies of forests, whilst others ventured gradually into the burning heat and developed skin protection, turning their skin pigmentation darker.

When human life evolved in the equatorial region it was bombarded by UVB and still today the pattern of distribution of UVB is most strongly influenced by latitude because of atmospheric scattering and absorption. Africa receives high and uniform amounts, whereas northern Eurasia receives negligible amounts. The darker skinned could stay longer in the UVB rays, the lighter skinned would move away from those burning rays.

The bacterial sludge from which life emerged came, in the case of our ancestors, to use mitochondrial DNA, (mtDNA) being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today’s eukaryotic (true nucleus) cells. And humans are still categorically eukaryotic organisms. This means that all human cells—including those found in the brain, the heart, the muscles, and so on—are also eukaryotic.

In most multicellular organisms, mtDNA is inherited from the mother.

Mothers transmit vitamin D to their foetus in the womb. Therefore pregnant mothers must have sufficient to pass on to help the developing baby grow normally. Another important requirement for producing a healthy baby is folic acid, this the body does not make and can only be obtained from the right diet.

Folic acid is a B vitamin which is vital for the formation of red blood cells. The form of folic acid occurring naturally in food is termed ‘folate’. Folic acid is essential for the body to make DNA, RNA, and metabolise amino acids which are required for cell division. As humans cannot make folic acid, it is required from the diet, making it an essential vitamin. Somehow early humans found the right foods to maintain folic acid levels in order to produce healthy offspring.

A 1978 study examined the effect of sunlight on folate – a vitamin B complex – levels. The study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. Thus, the light skinned early humans would protect themselves by dwelling in shaded areas whilst the sun was at its most dangerous to their skin.

Nina Jablonski has suggested the interference with folic acid synthesis occurs when excessive UV radiation penetrates deep into the dermis. The end result of this is reduced folate levels, which in pregnant females often causes neural tube abnormalities. Any impact on pregnancy success is an extremely powerful selective force. In this model the dark skin of humans naturally arose because women who were darker skinned carried more normal fetuses to term than those who were light skinned. 

To remain in Equatorial Africa we could not have survived unless our skin evolved protection from the UVB rays. That protection came from melanin.

Melanin is derivative of the amino acid tyrosine. Eumelanin is the dominant form of melanin found in human skin. Eumelanin protects tissues and DNA from radiation damage of UV light. Melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis. Melanin is produced inside small membrane-bound packages called melanosomes. People with naturally occurring dark skin have melanosomes which are clumped, large, and full of eumelanin. A four-fold difference in naturally occurring dark skin gives seven to eightfold protection against DNA damage, but even the darkest skin colour cannot protect against all damage to DNA. From  Wikipedia 

From Nina Jablonski:

Cooling by evaporation of eccrine sweat is impeded by thick body hair (9); the primary selective pressure promoting the evolution of hair loss in humans was thermoregulation. The loss of body hair in humans was accompanied by enhanced barrier functions of the stratum corneum (10, 11), including the evolution of other epidermal keratins (12, 13), which reduced the skin’s permeability and improved its abilities to resist abrasion and microbial attack. The rapid divergence of genes responsible for epidermal differentiation was one of the most significant results to emerge from the initial comparison of human and chimpanzee genomes (12). Changes in skin pigmentation also accompanied loss of body hair, and multiple lines of evidence indicate that permanent, dark, eumelanin-based pigmentation evolved soon after the emergence of the genus Homo in Africa (7, 14). See http://www.pnas.org/content/107/Supplement_2/8962

Variation exists within all populations of organisms. This occurs partly because random mutations arise in the genome of an individual organism, and offspring can inherit such mutations. Throughout the lives of the individuals, their genomes interact with their environments to cause variations in traits. The environment of a genome includes the molecular biology in the cell, other cells, other individuals, populations, species, as well as the abiotic environment. Because individuals with certain variants of the trait tend to survive and reproduce more than individuals with other, less successful, variants, the population evolves. Other factors affecting reproductive success include sexual selection (now often included in natural selection) and fecundity selection.

Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, but the genetic (heritable) basis of any phenotype that gives that phenotype a reproductive advantage may become more common in a population. Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species, macroevolution). In other words, natural selection is a key process in the evolution of a population.

Image of young chimpanzee (note the pale skin beneath the fur)

Homosapiens descend from the same single ancestor as chimpanzees. The earliest hominid of presumably primitive bipedalism, is considered to be either Sahelanthropus or Orrorin, both of which arose some 6 to 7 million years ago. These were probably the last single ancestor before the ancestral tree split.

When our ancestral branch split, hominids were still covered in fur and this continued until Homo erectus developed the skill of moving fast through walking and running.

Homo erectus (meaning “upright man”) is an extinct species of archaic humans that lived throughout most of the Pleistocene geological epoch. Its earliest fossil evidence dates to 1.9 million years ago. It likely originated in East Africa and spread from there, beginning 1.8 million years ago, migrating throughout Eurasia.

That vital development occurred over thousands of years of hominids mostly living in tropical forests and beginning to venture out into open spaces. The hunter became more efficient and effective once the body became capable of walking and particularly, running.

The body needed to have strong bones to carry the body mass as it changed to a shape more suited to fast movement. The UV rays from the sun produced the vital Vitamin D in the body to maintain circulation and calcium density. The body required magnesium to help absorb the essential vitamin D, and Calcium can only reach its full bone-building potential if the body has enough vitamin D. Calcium and vitamin D work together to protect bones—calcium helps build and maintain bones, while vitamin D helps the body effectively absorb calcium. Sources of food providing magnesium include nuts, dark leafy green vegetables, legumes, whole grains and fish.

Image of changed body shape for the effective running machine.

Seeking foods which provided good nutrition for the body were understood by the human over thousands of years of experience. When humans migrated, they had to seek out essential foods for their bodily needs.

For example, by the time the Aztecs existed they had found a widely nutritious seed, chia. This gave them the energy to get through their days.

Chia seeds provide:
*  Fiber: 11 grams.

* Protein: 4 grams.

* Fat: 9 grams (5 of which are Omega-3s).

* Calcium: 18% of the RDA.

* Manganese: 30% of the RDA.

* Magnesium: 30% of the RDA.

* Phosphorus: 27% of the RDA.

* They also contain a decent amount of Zinc, Vitamin B3 (Niacin), Potassium, Vitamin B1 (Thiamine) and Vitamin B2.

In the Amazon, the tribes who had made it there understood how to work with the environment to have the nutrition they needed and yet establish a healthy ecosystem around them. Here Anne Roosevelt stated:

“If you like,” she said. “You could go [along the river] where you wanted and homestead— the forest gives you all kinds of fruit and animals, the river gives you fish and plants. That was very important to societies like Marajó. They had to be much less coercive, much more hang-loose, much more socially fluid, or people wouldn’t stay there.” Compared with much of the rest of the world at that time, people in the Amazon “were freer, they were healthier, they were living in a really wonderful civilization.”

Anne Roosevelt, Archaeologist

But as light skinned people evolved away from the harmful UVB rays, vitamin D was harder to come by.

Our bodies manufacture vitamin D when the sun’s ultraviolet B (UVB) rays interact with 7-dehydrocholesterol (7-DHC) present in the skin. “However, we can produce only a limited amount of vitamin D from UVB. A few minutes at midday are sufficient for many Caucasians,” says Roy Geronemus, MD, clinical professor of dermatology at New York University Medical Center and director of the Skin/Laser Division at the New York Eye & Ear Infirmary. “After reaching the production limit, further exposure actually destroys the vitamin, decreasing vitamin D levels.”

Here the crude word ‘caucasian’ is used. The term “Caucasian race” was coined by the German philosopher Christoph Meiners in his The Outline of History of Mankind (1785). Meiners’ term was given wider circulation in the 1790s by Johann Friedrich Blumenbach, a German professor of medicine and a member of the British Royal Society, who is considered one of the founders of the discipline of anthropology.

It is more accurate to speak of ‘light skinned’ which is the most vulnerable to UVB rays. Light skinned people need to cover themselves or sit in shade to avoid depletion of folate and vitamin D. Dark skinned people need more exposure to UVB to ensure full vitamin D and folate development for strong bones and for pregnant women to pass on to their foetus to avoid harm to their offspring.

Skin color is determined genetically. Genes tell the body how much of the two types of melanin, the pigment that helps to determine the skin color, to produce. Pheomelanin causes reddish yellow pigments, and eumelanin gives deep brown coloring. Sunlight exposure causes the optic nerve to signal the pituitary gland to release more melanin. The skin will then tan.

The next, rather complex genetic finding is worth quoting as it shows we are getting closer to understanding the genetic complexities which cause skin pigmentation.

See https://en.m.wikipedia.org/wiki/SLC45A2

Since researchers began to sequence the genome of ancient populations recently, it has been discovered that Europeans today are the product of hunter gatherers and farmers of at least three ancient populations having mixed together during their migration to the continent over the past 8,000 years………

……..SLC24A5 appears to have played a key role in the evolution of light skin in humans of European ancestry. The gene’s function in pigmentation was discovered in zebrafish as a result of the positional cloning of the gene responsible for the “golden” variety of this common pet store fish. Evidence in the International HapMap Project database of genetic variation in human populations showed that Europeans, represented by the “CEU” population, had two primary alleles differing by only one nucleotide, changing the 111th amino acid from alanine to threonine, abbreviated “A111T”…….

………By comparing key parts of DNA across the genomes of 83 ancient humans from European archaeological sites with recent ones from the 1000 Genomes Project, Iain Matheison of Harvard University’s lab of population, and geneticist David Reich, discovered the genes linked to skin pigmentation that had survived the natural selection process across Europe……..

……SLC45A2 is a transporter protein that mediates melanin synthesis……..

………Sodium/potassium/calcium exchanger 5 (NCKX5), also known as solute carrier family 24 member 5 (SLC24A5), is a protein that in humans is encoded by the SLC24A5 gene that has a major influence on natural skin colour variation. The NCKX5 protein is a member of the potassium-dependent sodium/calcium exchanger family. Sequence variation in the SLC24A5 gene, particularly a non-synonymous SNP changing the amino acid at position 111 in NCKX5 from alanine to threonine, has been associated with differences in skin pigmentation……..

……….The SLC24A5 gene’s derived threonine or Ala111Thr allele (rs1426654) has been shown to be a major factor in the light skin tone of Europeans compared to Africans, and is believed to represent as much as 25–40% of the average skin tone difference between Europeans and West Africans. It has been the subject of recent selection in Europe, and is fixed in European populations……

These few paragraphs illustrate how the understanding of skin pigmentation is tied to processes within the human body and changes in the chemistry.

If a human has access to plenty of quality vitamin d, calcium and magnesium but lives in the higher northern hemisphere where UVB heat never arrives then the skin pigmentation is not paler despite lack of UVB rays. The Inuit have a bronze skin. They are also, due to their fish dominated diet, healthier than most people of the western world.