A great deal has been written on the development of the race concept and how scientists have attempted to define different human races over the past two hundred plus years. Study of variation within the human species dates back to the first expeditions that encountered native (indigenous) populations. Toward the end of the eighteenth century, the first cohesive scientific theories were produced that classified and named different races. The problem with many of these classifications is that they used observed characteristics and personalities to place different populations or races on a hierarchical ladder. This ladder implied that the highest rung, incidentally the European race, was ideal and the other races occupying lower rungs were less evolved or degenerate from the ideal. It is the ascription of value that disqualifies the term race in the realm of modern science.

Attempts to create categories of biological races have centered on phenotypic differences. A person's phenotype is the entirety of traits that individual possesses, including external characteristics such as eye color and shape, body size and shape, hair color and texture, and skin color. In recent years attempts have also been made to evaluate genotypic differences to justify biological races. (Genotype refers to a person's genetic makeup.) These attempts have sought to define clusters of characteristics in one population that are lacking in other populations. These clusters supposedly would enable different populations to be divided into distinct races. Such attempts have failed, however, and what researchers have found is that biological variations exist on a cline rather than in delimited geographic clusters with gaps in between. A cline, as defined by anthropologists, means a gradual change of a trait and its frequency from one place to another within a species or population. The change usually corresponds to some transition in environment across the geographic range of a species. Any boundary line drawn along the continuum is therefore arbitrary. So, the idea of distinct races defined by hard-and-fast differences has fallen apart as anthropologists have studied the genetic and physical characteristics of human populations.

Although anthropologists thus no longer classify populations in terms of races, they do recognize that human populations exhibit diverse phenotypes. Different traits are, for example, very useful in the field of forensic anthropology. A forensic anthropologist must extract as much information as possible to assist in the identification of an individual. Part of that job requires identifying that individual's ancestral phenotype. Ancestral phenotypes are suites of traits that are associated with geographic populations. At first, this sounds a lot like a synonym for race; however, the difference lies in the lack of distinct divisions. The task simply relies on the idea that any given individual may have characteristics known to be common in a particular geographic area. Determining ancestry comes from familiarity with the clinal distribution of phenotypic characteristics.

So, when in the past did phenotypic diversity of the sort found in modern humans evolve? To answer this question, anthropologists usually look to correlations that exist between external traits and environmental variations. The evolution of varied skin color offers a good example. At some point in human evolution the amount and length of body hair commonly seen as a thick coat in other primates was greatly reduced. Only a fine coat of body hair was retained, which means that the skin of modern people is much more exposed to the elements than in other primates. Increases and decreases in skin pigmentation became a crucial way of adapting to the diverse climate zones that humans, especially modern humans, came to inhabit.

The results of a recent study demonstrate that skin coloration is strongly correlated with ultraviolet (UV) radiation levels and latitude. The color of a person's skin is the result of the amount of melanin pigment in the skin. More melanin means darker skin. Melanin absorbs UV radiation from the sun. UV radiation aids the body in vitamin D synthesis; however, it also causes nutrient photolysis (chemical decomposition caused by radiant energy) particularly the photolysis of folate, a converted form of folic acid that is crucial to reproductive success.

Due to small amounts of melanin beneath their thick hair, other primates tend to have light colored skin over most of their body. Exposed areas such as the face and hands, though, have varying degrees of melanin. So, for example, common chimpanzees often have light facial skin, while other apes also closely related to humans have darkly-pigmented faces, even though the skin beneath their thick coats is often light in color. Using other primates as a guide, then, our ancestors could have had light colored skin over parts of their bodies prior to the time when hairiness was greatly reduced. However, it is simply impossible to prove ancestral skin pigmentation.

Lacking any preserved paleo-skin to go on, paleoanthropologists have looked to skeletal evidence for some clues. With the evolution of the species Homo ergaster, there was an important shift in body proportions. H. ergaster, beginning about 1.7 million years ago, was the first tall human species we know of. Its body was strong and powerful, which suggests that this species was susceptible to heat stress as it moved around during the day. The best-known specimen of this species, the "Turkana Boy" (KNM WT15000) from northern Kenya, had a narrow pelvis and a skeleton reminiscent of the long, linear bodies evident in modern peoples from hot equatorial environments. Looking at the nasal bones of the face, H. ergaster may also have been the oldest human to evolve an enlarged nasal cavity effective in moisturizing inhaled air and precipitating the moisture on to the nasal membranes during exhalation. This matter of moistening dry air but not exhaling the moisture is a critical factor in preventing water loss in dry environments during the process of breathing.

This combination of evidence points to the possibility that Homo ergaster was the first hominin to be strongly affected by a hot, dry environment. It's in this type of habitat that sweat glands similar to our own and skin exposure due to hair reduction are likely to have evolved. These developments enabled the evaporative cooling of sweaty skin - a novel and effective way of regulating body temperature in a hot setting. Since H. ergaster originated in Africa, UV radiation levels would have been high, perhaps similar to their levels in historic times. Low melanin levels would have allowed a great deal of UV radiation to penetrate the skin and enter the subepidermal tissues. To protect these delicate tissues, melanization (increase in the density of melanocytes in the epidermis and the production of darker skin) would have proven very advantageous.

Nutrient photolysis of folate, mentioned above, is thought to have played an important role in natural selection and the evolution of darker skin colors. Folic acid is required for normal DNA biosynthesis, and folate (a conjugated form of folic acid) is required for bone marrow maturation and red blood cell development. Research has also shown a causal relationship between neural tube defects and folate lysis. In addition, tests on lab mice and rats have shown that folate deficiency can cause male infertility (by arresting spermatogenesis). Putting all of these clues together, it's reasonable to hypothesize that H. ergaster individuals that had greater protection against UV radiation were likely to produce more offspring than those with lower concentrations of melanin, eventually leading to darker skin colors associated with tropical environments.

As populations spread from Africa, it seems likely that dark skin color was less well suited to environments with lower UV radiation levels in the temperate zone. While dangerous in excess, UV radiation is essential for the synthesis of previtamin D3, which is needed for calcium absorption and normal skeletal development. In tropical areas, there is no problem receiving enough UV light for D3 synthesis. In higher latitudes, though, where exposure to UV light is significantly less, a high concentration of melanin may hinder the passage of enough radiation to synthesize the necessary amount of the vitamin precursor. Medical records show that people with darker skin living in the higher latitudes are at greater risk for vitamin D3 deficiency (which can trigger the onset of various bone density diseases that can result in immobilization, deformities, and death). For this reason, it is believed that as populations moved north, natural selection favored lighter shades of skin. The point is that by understanding the biological benefits of traits, it is possible to understand the evolution of them.

In light of all this information, phenotypic diversity - the expression of different external traits in different geographic regions - probably has deep roots in human ancestry. Yet the historical pattern of geographic variation evident in modern humans has been around for only a very brief time. Humans have spread extensively over the past 50,000 to 100,000 years, much more so than in any prior period of human evolution. And as populations have extended to new continents, islands, and more extreme environments, humans groups have adapted in new ways and have interbred. Phenotypic differences among humans are thus many and diverse, and have been shaped and reshaped over the past 50,000 years - less than 1% of the evolutionary history all people have in common.

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