Biological Anthropology

I

Introduction

Biological Anthropology, area of study concerned with human evolution and human adaptation, formerly known as physical anthropology. Its main components are human palaeontology, the study of our fossil records, and human genetics, which examines the ways in which human beings differ from each other. Also embraced are aspects of human ecology, ethnology, demography, nutrition, and environmental physiology. Biological anthropology is often grouped or classified with social anthropology and both are frequently taught together in university undergraduate courses as the study of human beings, especially in North America. Social anthropology, however, concerned as it is with social form and structure, identifies with the humanities and in aims and methods is very different from the biological sciences. The two fields have tended to develop at the research level quite independently throughout the 20th century, with biological anthropology being progressively influenced by evolutionary biology and experimental physiology, while social anthropology has aligned itself with philosophy and history. There are signs now, however, of some coming together, especially in ecology, where biological anthropologists recognize the central importance of culture, and in sociobiology, which hypothesizes that social behaviour and social institutions have been, at least to some extent, determined by biological pressures for reproductive success.

II

Human Evolution

One of the key issues being studied in anthropology is the evolution of the human genome, as a key to understanding human evolution as a whole. Biologically, what makes us human lies in the nature of our DNA (deoxyribonucleic acid), which is present in the chromosomes in all our body cells at some stage, at least, of their life. DNA forms our genes and provides a blueprint for the synthesis of innumerable proteins which, interacting with each other, direct our development from the fertilized egg to adulthood. In recent years a great deal has been discovered about the detailed structure of DNA, and individual human genes have been characterized in terms of the chemical composition of their DNA. Much is also now known about the DNA of other animals, and it is possible to compare levels of similarity between species. The closest living relatives of human beings are the African apes—the gorilla and the chimpanzee, particularly the latter, with which we share around 98 per cent of our DNA. The distinctiveness of human beings lies in the remaining 2 per cent, but that includes many hundreds of genes.

There is evidence that at least some parts of DNA evolve over time at a more or less constant rate. When this rate of change can be calibrated with events in the fossil record, it provides what is known as a “molecular clock” for dating the common ancestry of different species. Although there is still much debate about the accuracy of such clocks, most researchers consider that the evolutionary paths of human beings and chimpanzees separated some 5 to 7 million years ago.

The fossil evidence itself is in broad agreement. Anatomically, human beings differ from apes in three major regards: brain size, jaw size and shape, and mechanisms of locomotion. Human beings tend to have brains about three to four times larger than those of chimpanzees; they have smaller, less projecting jaws with small teeth and non-projecting canines, and the bones of the limbs, and especially the hind limbs, are quite distinctive as they are adapted for bipedal walking and running.

The earliest well-known fossil on the human lineage is Australopithecus afarensis, which has been found at various sites in eastern Africa and dates from around 3.5 million years ago. Key names in this field are Mary Leakey, Yves Coppens, and Don Johanson. Recently, earlier fossils have been found, also in eastern Africa, which are even more ape-like but could well be ancestral to A. afarensis. One is known as A. anamensis and dates from over 4 million years ago: very close to the split of human and chimpanzee lineages as dictated by molecular evidence.

Later australopiths have been found in many other sites in eastern Africa and South Africa. One form, A. africanus, best known from specimens recovered from the Sterkfontein limestone caves, in Gauteng province, South Africa, shows many similarities with A. afarensis but is more advanced. It probably dates from 2 to 3 million years ago. Factors such as teeth and skeleton size provide valuable insights into the lifestyle of a species, such as its diet or method of locomotion. Raymond Dart and Robert Broom were both particularly important in this field of study.

At around 2 million years ago, a new kind of human being appears in the fossil record and is especially well known from remains at Olduvai Gorge, Tanzania, and around Lake Turkana, Kenya, but also seems to have occurred in South Africa. Named Homo habilis, it is regarded as the first species of our own genus. Louis Leakey and Mary Leakey were crucial to the eastern African discoveries.

Biological anthropologists address the controversial issue of whether all the fossils of Homo at this time can be attributed to this single species, but there can be little doubt that among them were the ancestors of a subsequent species Homo erectus. This is first known from geological deposits around Lake Turkana dating from at least 1.5 million years ago, but unlike all previous human fossils it is also found in many other parts of the Old World, though at later dates of between 1 and 0.4 million years ago. Best known are the remains from Java (found by Eugene Dubois and previously known as Pithecanthropus erectus) and China (Sinanthropus pekinensis), but representatives have also been found in India and Europe (for example, Petralona). Homo erectus not only spread out of Africa but also out of the tropics.

Undoubtedly, one descent line from Homo erectus led to the Neanderthals. The fossils of this group are confined to Europe, northern Africa, and the Near East and are associated with a particular stone tool industry known as the Mousterian. The first Neanderthal skeleton was reconstructed by Marcellin Boule. Neanderthalers in their classic form occurred at the time of the first part of the last ice age 75,000 to 30,000 years ago, but forms resembling them in some features occurred earlier: to about 200,000 years ago and in warmer conditions.

Neanderthalers disappeared from the fossil record in Europe fairly abruptly at around 30,000 years ago to be replaced by human beings very much like ourselves anatomically. They were essentially Homo sapiens. There has been much debate among biological anthropologists as to whether they interbred with Neanderthalers. In April 1999 the discovery was announced of a child's skeleton which displayed features of both Neanderthals and modern human beings. These findings constitute the strongest evidence yet that interbreeding did occur. Debate about the origins of modern human beings has led to the so-called “Out of Africa” (or “Centre of Origin”) hypothesis, which postulates an origin for Homo sapiens in Africa around 200,000 years ago and a spread of some groups to the rest of the Old World about 50,000 years ago. Fossil records and studies of DNA support this. Recent finds in Ethiopia offer compelling evidence for the “Out of Africa” theory of human evolution. The almost complete skulls of two adults and a child were presented to the world by scientists in 2003. Unearthed at Herto, Ethiopia, they are said to date back 160,000 years: the oldest record of Homo sapiens. However, due to the lack of fossil discoveries on the immense African continent, the question of whether early modern man spread outwards from East Africa or was multi-regional in Africa remains unresolved.

“Out of Africa” is now the most favoured explanation, but there is an alternative view of merit, the “multi-regional hypothesis”. This argues that many populations from all over the Old World contributed to our origins, and that Homo erectus was slowly transformed into Homo sapiens. This view requires a more or less regular exchange of genes between the co-evolving groups, but there is good evidence that the hunter-gatherer lifestyle of these late human beings involved substantial migrations. The strongest piece of evidence for this hypothesis is that distinctive characteristics found today in different geographical regions are also found in Homo erectus populations of the same region. Milford Wolpoff has done much to promote this hypothesis.

Although human beings who were indistinguishable anatomically from ourselves arose many thousands of years ago, the species was a rare one for most of this time. It probably numbered no more than a few tens of thousands at any one time. This all began to change about 10,000 years ago when the population increase, which we are still witnessing, began. This was thanks mainly to the domestication of animals and cultivation of plants, which made human groups largely independent of natural food resources.

Human beings’ success, therefore, in terms of number is not directly attributable to biology but rather to technology and culture. By contrast, our beginnings as australopiths were profoundly affected by a single biological event: the adoption of an erect bipedal mode of locomotion. The reasons for this are far from clear; bipedalism is not a particularly effective method of movement. It does, however, totally free the forelimb for other purposes, for example, tool use and toolmaking, and carrying materials and food to a home base. Especially important may have been the capacity to carry the immature and dependent human infant over considerable distances. It has also been pointed out that the erect posture greatly reduces the level of solar heat load, which would have been very significant for daylight collecting and scavenging in tropical savannahs. Sherwood Washburn has done much important research in this area.

III

Evolutionary Processes in Action

The fossil record provides information about the course of human evolution, but to understand the processes involved it is necessary to examine contemporary populations. Essentially, evolution involves changes in the genetic composition of populations. Three main processes are involved: mutation, which produces new genes; natural selection; and genetic drift, which determines what happens to these new genes. Natural selection occurs when alternative genes affect the probability of survival and/or reproduction. Genetic drift occurs as change by chance when alternative genes are neutral, that is, they make no difference to a person’s chances of survival. There is much debate about the relative importance of these last two processes, but there is no doubt that many protein-coding genes are not neutral. One of the best examples concerns resistance to one particular form of malaria in early childhood. Malaria is particularly dangerous for children in many tropical countries. Numerous genes that are now known to afford protection are found in high frequencies in malarial regions. One of the best known is the sickle-cell haemoglobin gene. Another gene leads to the continuing protection, beyond infancy, of the enzyme that breaks down milk sugar. It is found in populations that rely heavily on fresh milk products as a source of food. The geographical distribution of genes determining skin colour is strongly related to levels of incidental ultraviolet radiation from the Sun. Many other examples occur and together they at least partly account for the differences in the genetic composition of different human populations. This is the material of what is sometimes called “racial” variation, but racial groups are very poorly defined when all the genetic variation in humankind is considered.

One of the most remarkable features of human beings is their adaptability or plasticity. Notwithstanding genetic differences owing to environment, practically all people when exposed to hot climates alter their physiology so as to be better adapted to heat (heat acclimatization). Cold and altitude acclimatization also occur. Individuals can also adapt to food changes and exposure to infectious disease. These are species characteristics produced by natural selection and are most strikingly and importantly expressed in human behaviour. Human beings can through technology make their own environment, and it is this power that has made them a worldwide species. By studying biological evolution and the physiological adaptations of human beings, it is hoped that we will be better able to understand the origin of the species and the factors that have contributed to its success.