1. Classifying human populations

Processes which have taken place in the past have shaped the human populations found in the world today. The most fundamental process has been genetic, namely the shaping of human gene-pools through natural selection (Harrison et al., 1988). In addition, human population migration has served to isolate different groups across sufficiently long time-spans to allow between-population genetic variation. Fig. 1 shows the dispersal of anatomically modern humans out of Africa, starting about 70,000 years ago.

Our knowledge of the pattern of human dispersal gives us clues about how human populations might best be classified: (1) since evidence points to humans having a common point of origin, with dispersion only coming late in evolutionary time, human populations, regardless of where they live, have tremendous genetic similarity; (2) the migratory route suggests that variation in any trait is likely to be a continuum across populations, and that any classification is to some extent arbitrary; and (3) prior to the onset of agriculture, some 12,000 years ago, humans lived as hunter-gatherers at low population densities often in isolation from each other, leading to the possibility of the development of regional population genetic differences.

Fig. 1. Dispersal of anatomically modern humans from Africa (approximate dates in thousands of years).

Migrations taking place after the onset of agriculture served to create larger, more genetically homogeneous populations across wide areas, with genetically isolated populations left in less-hospitable ecological niches. Genetically isolated populations to be found in the world today include tribal groups of hunter-gatherers in Africa, Latin America, and Asia. Large-scale colonisation of the Pacific Islands took place at this time. Later migrations, during colonial times, include the migrations of (1) Europeans to the Americas, Australasia and parts of Africa; (2) Africans, mostly of Bantu origin, to the Americas and the Caribbean; and (3) Asiatics, Chinese in particular, and Indo-Mediterraneans, largely South Asians, to most parts of the tropical world and to parts of the New World. Migrations in the post-colonial period are largely related to economics and urbanisation. Examples include the migrations of Mexicans and Hispanics to the USA, and of South Asians and Afro-Caribbeans to Britain. In addition, there is the global trend of rural to urban migration in post-colonial times.

In summary, therefore, human populations have great homogeneity for most genetically determined characteristics, between-population variation having taken place in the recent evolutionary past. Migrations and population expansions have created several larger population groupings within which genetically isolated groups may be found.

Different human populations are known to vary from each other in a large and ever-growing list of genetic markers, growth patterns, body size and composition are polygenic in character, and the genetic component of human growth in different populations is a debated but poorly resolved issue (Schmitt & Harrison, 1988; Harrison & Schmitt, 1989; Bogin, 1991). It is possible to classify populations around the world in a number of different ways; Table 1 gives a modification of the Eveleth & Tanner (1990) classification.

Table 1. Classification of population types

Classification	Definition	Other terms used
European and European origin	Living in Europe or elsewhere, of European ancestry	Caucasian, White, Anglo-American, European-American
African and African origin	Living in Africa or elsewhere, of African ancestry	Negro, Black, Black British, Afro-Caribbean, Afro-American, African-American
Asiatic	Living in Asia or elsewhere, of Asian ancestry	Asian, Indian (American)
Indo-Mediterranean	Peoples of the Near East, North Africa, the Indian subcontinent and their descendants	Indian, Asian, Indo-Pakistani, Arab, South Asian
Australian Aborigines and Pacific Island peoples		Melanesian, Polynesian, Micronesian

This typology is rather rigid and simplistic. It does not comfortably include mixed populations such as Spanish-Indians in the Americas, European-Africans in the Caribbean, United States and Britain. Further, it aggregates populations that have been shown to have clear differences in growth pattern. For example, the term 'African' includes the majority of Bantu-descended populations of Africa, as well as distinctively short-statured hunter-gatherer groups such as the Mbuti of Zaire and the!Kung bushmen of Namibia. Furthermore, the term 'Australian Aborigines and Pacific Islanders' covers populations with considerable genetic heterogeneity. However clumsy the classification is, it is the best that is currently available, and is used in this article in considering broad population growth characteristics.