What is a human? Everyone thinks they know the answer, but nobody can define the term. Curiously, systematists have not defined a ‘type specimen’ for humans, in contrast to other species. Recent attempts to provide a definition for our species, so-called ‘anatomically modern humans’, have suffered from the embarrassment that exceptions to such definitions inevitably arise – so are these exceptional people then not ‘human’? Anyway, in comparison with our closest-living relatives, chimpanzees, and in light of the fossil record, the following trends have been discerned in the evolution of modern humans: increase in brain size; decrease in skeletal robusticity; decrease in size of dentition; a shift to bipedal locomotion; a longer period of childhood growth and dependency; increase in lifespan; and increase in reliance on culture and technology. The traditional classification of humans as Homo sapiens, with our very own separate family (Hominidae) goes back to Linnaeus. Recently, the controversial suggestion has been made of lumping humans and chimpanzees together into at least the same family, if not the same genus, based on the fact that they are 98–99% identical at the nucleotide sequence level. DNA sequence similarity is not the only basis for classification, however: it has also been proposed that, in a classification based on cognitive/mental abilities, humans would merit their own separate kingdom, the Psychozoa (which does have a nice ring to it). As for sub-categories, or ‘races’, of humans, in his Systema Naturae of 1758 Linnaeus recognized four principal geographic varieties or subspecies of humans: Americanus, Europaeus, Asiaticus, and Afer (Africans). He defined two other categories: Monstrosus, mostly hairy men with tails and other fanciful creatures, but also including some existing groups such as Patagonians; and Ferus, or ‘wild boys’, thought to be raised by animals, but actually retarded or mentally ill children that had been abandoned by their parents. In his scheme of 1795, Johann Blumenbach added a fifth category, Malay, including Polynesians, Melanesians and Australians. Blumenbach is also responsible for using the term ‘Caucasian’ to refer in general to Europeans, which he chose on the basis of physical appearance. He thought Europeans had the greatest physical beauty of all humans – not surprising, as he was of course European himself – and amongst Europeans he thought those from around Mount Caucasus the most beautiful. Hence, he named the ‘most beautiful race’ of people after their supposedly most beautiful variety – a good reason to avoid using the term ‘Caucasian’ to refer to people of generic European origin (another is to avoid confusion with the specific meaning of ‘Caucasian’, namely people from the Caucasus). The extent to which racial classifications of humans reflect any underlying biological reality is highly controversial; we merely note here that proponents of racial classification schemes have been unable to agree on the number of races (proposals range from 3 to more than 100), let alone how specific populations should be classified, which would seem to greatly undermine the utility of any such racial classification. Moreover, it seems to us that the goal of investigating human biological diversity is to ask how such diversity is patterned and how it came to be the way that it is, rather than how to classify populations into discrete ‘races’. Human origins. All of the action in human evolution, beginning with our divergence from our common ancestor with chimpanzees some 5–6 million years ago, up to our initial exodus somewhere around 1.8 million years ago, took place in Africa. The earliest recognizable direct ancestors of humans, known as hominids, will for the sake of convenience be referred to here as australopithecines, although recently other genera have been proposed. Australopithecines are characterized by bipedality, with increasing brain size coming later. In fact, although anthropologists understandably focus on traits that eventually result in humans, and consequently emphasize the human characteristics of australopithecines, a more impartial consideration suggests that early australopithecines were basically chimpanzees that had a funny way of walking. When Homo first emerges is a matter of controversy and definition, with early definitions stipulating a cranial capacity of at least 900mL, in between the cranial capacities of chimpanzees (about 400mL) and modern humans (a hefty 1350mL). Louis Leakey thus created quite a stir in 1964 by calling his finds at Olduvai ‘Homo habilis’, even though the inferred cranial capacity was only about 600ml – after all, it sounds a lot better to say that you've discovered the first members of our own genus, rather than the last members of some other genus. But Leakey's re-definition has come to be accepted, as even 600ml is still outside the range of cranial capacities of australopithecines, a mere 400–500mL. Homo now encompasses several species which existed over the past two million years or so, with little agreement as to exactly how many there were, and which (if any) were ancestral to modern humans. Anatomically modern humans made their first appearance, in Africa, about 130,000–160,000 years ago, and began spreading out of Africa between 50,000 and 100,000 years ago. Modern humans are in Australia by at least 50,000 years ago, in the New World at least 13,500 years ago, and reached the most distant Polynesian islands by about 1,500 years ago; by this time humans were distributed across some 70% of the Earth's land mass, exceeding the range size of any other terrestrial mammal by at least an order of magnitude. What made this vast expanse in range size possible was, of course, culture. It has been claimed that many of the behaviors associated with modern humans – art, body decoration, microlithic tools and so forth – appear suddenly in the archaeological record, around 50,000 years ago, and thus a ‘revolution’ in human cognitive ability, possibly associated with language, occurred at this time. But this has been criticized as a Eurocentric view of the archaeological record, as much of this archaeological evidence shows up earlier in Africa; certainly, when compared to how much is known about Europe, archaeologists have barely scratched the surface of Africa. Humans by numbers. There are about 6.3 billion humans on the planet, with another 4.5 individuals born every second. A rough estimate of the total number of individuals who have lived over the past 5 million years or so is about 150 billion (give or take a few billion). With a mutation rate of about 7 × 10–5 mutations per gene per person (in a diploid genome), on the order of 10.5 million different alleles could have appeared at every gene during human evolution. This would seem ample opportunity for producing lots of genetic diversity, yet the amount of genetic variation in contemporary humans is quite low, suggesting a long-term ‘effective population size’ of only about 10,000 individuals. Compare this with chimpanzees, who have an estimated census size of about 100,000, but an effective population size (again, based on genetic variation in contemporary populations) of at least 30,000. Based on relative amounts of genetic variation, humans are more endangered than chimps! The paucity of genetic variation in our species is consistent with the recent African origin of humans from a fairly small population. Not only is genetic variation reduced in humans, but the total number of genes also falls short of expectations. The current estimate is about 21,500 genes; by comparison, the lowly fruit fly Drosophila has 13,525 genes – more than half the human total. We may find some consolation in the fact that, while humans can do things that fruit flies can't – such as writing a quick guide for Current Biology – fruit flies can also do things that humans can't – such as flying or walking upside down on the ceiling. But rice beats both fruit flies and humans, with 62,435 genes, and it is hard to argue that rice are 3–5 times as complex as humans or fruit flies. So if the sheer number of genes doesn't account for differences in organismal complexity, what does? The answer is that we don't know, but a likely candidate is differences in the regulation of gene expression. In other words, it's not so much what you have, but what you do with what you have, that matters. Humans and their environment. No other species has had as profound an impact on the environment as humans, and this is not just because there are so many of us – after all, the total biomass of humans is only about the same as that of ants. Annual carbon dioxide production by human populations is five orders of magnitude greater than that of other mammals of similar body size. Consumption of many resources by humans similarly is orders of magnitude greater than that of other species. The extent to which prehistoric humans had a significant impact on their environment – for example, the extinction of Pleistocene megafauna following the colonization of the New World – is a matter of debate, but there is no denying our current impact on the environment. Rates of extinction of other species and of resource depletion are at an all-time high, and if we do not take steps to alter our behavior, the answer to the next question will be moot. Are humans still evolving? At first glance, the answer to this question would seem to be no. After all, one of the defining characteristics of humans is our dependence on culture and technology, so any change in our environment that might lead to biological evolution will instead lead to a cultural/technological response. For example, if the ozone layer disappears, increasing ultraviolet light exposure and thus the risk of skin cancer, we will most likely not respond by evolving thicker skins, but by developing protective clothing, skin creams, and so on. But the fact is that humans are still evolving, both because culture is inadequate to deal with some biological challenges, and because cultural changes themselves lead to biological change. An example of the former is a deletion in the CCR5 receptor gene that seems to be increasing in frequency because it is associated with resistance to HIV infection. Infectious disease has been, and undoubtedly will continue to be, a powerful selective force in human evolution – even in this modern age, 3000 people die every day from malaria, and the best documented examples of adaptation via natural selection in humans involve genes that confer resistance to malaria. A probable example of cultural evolution leading to biological evolution is the ongoing loss of olfactory receptor genes in humans, possibly because cultural developments have reduced the dependence of humans on their sense of smell for survival. While it is unlikely that the extremely large-headed humans that are commonly depicted in science fiction will ever evolve – after all, as any woman who has ever given birth can tell you, the size of a newborn's head is quite large enough, thank you – evolution is, and will continue to be, a part of human existence. Where can I find out more?