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Genetics concerns the scientific study of inheritance and variations in living organisms. While it has long been known that “ancestors” passed on certain traits to their “offspring,” from around the mid-nineteenth century it became evident that organisms inherit these traits via distinct units, now known as genes. Genetic change occurs over time through mutations and natural selection or eveolution.
The most recent common ancestor (MRCA) to humans and to the nearest primates, chimpanzees, lived in Africa, most probably in the tropical forest, 5 million years ago, a date that some prefer to set somewhat earlier, by 1 or 2 million years. The forest is still the environment where chimpanzees and earlier ancestors like gorillas and orangutans lead arboreal lives. Because bone conservation is not good in a forest environment, almost every known intermediate ancestor in the direct line to humans comes from drier environments in the Rift Valley, extending from East Africa to South Africa. This is where almost all the earliest human fossils were found.
Two major evolutionary changes were observed in the human line but not in the simian lines: the transition to the erect, bipedal posture, which favored greater speed on the ground and also freed the hands for the making and use of tools, and an apparently continuous increase in brain size. The latter, and the development of phonation organs, may have facilitated the genesis of articulated speech, which is not present in primates, although they can be taught the meaning of hundreds of words, using special teaching techniques. The capacity to form complete sentences makes the transmission of complex thoughts possible.
The capacity to communicate using language that is typical in the human species is a real source of human superiority to all other vertebrates, probably the most important one. It must have been the major skill that made it possible for a relatively small group of East Africans to grow and expand across the whole Earth, starting about a hundred thousand years ago. The information about this comes from paleoanthropological and archeological observations that offer dates and places. The evolution from the MRCA to modern humans was, however, long and slow, with several culs-de-sac, but a common evolutionary theme throughout the human line is an increase in brain size, by a factor of four times, as judged from the size of the brain case.
One particularly well-dated woman, Australopithecus afarensis (Lucy), 3.2 million years old, may be at the origin of the bifurcation into an extinct line leading to several Australopithecine species, and that leading to Homo sapiens sapiens (the species to which we all belong). The genus Homo is given an age of about 2.5 million years and is characterized by the first use of rough stone tools, hence the first species of the human genus is called Homo habilis. Some improvement in shape and increased variation of shapes and uses of tools is noticeable at the time of Homo erectus (dated about 2 million years ago). There is currently a tendency to rename these fossils, but we retain the simpler, earlier terms. H. erectus is the human who, starting 1.7 million years ago from Africa (where the Rift Valley is still the only source of finds) relatively quickly settled most of Europe and Asia.
The slow increase in brain size stopped around 300 kya (300,000 years ago). For a long period, between 500 kya and 40–30 kya, two types of humans in different parts of the world were particularly successful and left a greater number of fossil proofs of their existence. One type, Neanderthal, lived in Europe and extended to western Asia, reaching the Middle East between 80 and 60 kya. According to some anthropologists, Neanderthal was the ancestor of modern Europeans, but recent genetic work on fossil remains has shown that it became almost certainly completely extinct. According to others, the branch leading to all modern humans developed in Africa, in the Rift Valley. The only fossil predecessors of modern humans were found in Africa. Two recent finds, an erectus of 1 mya found in Eritrea, and a 150 ky-old almost perfectly modern skull found not very far away in Ethiopia, filled important gaps and have helped to show a continuous evolution in East Africa of modern humans from the earliest types.
Genetic Analysis and Human Migration
Genetic analysis of human evolution was started very soon after the discovery of genetic variation. At the beginning the only frequent (polymorphic) genetically variable traits inherited in a clear-cut way according to Mendel’s laws were blood groups ABO, RH, and others. These traits are inherited equally by both parents and could be used to prove the influence of the four standard factors of evolution and to follow their effects quantitatively thanks to the mathematical theory of evolution, developed in the 1920s and 1930s by Ronald Aylmer Fisher, John Burdon Sanderson Haldane, and Sewall Wright.
The first factor, mutation, is the source of all inherited differences, which we now know are changes in DNA: a linear molecule formed by a chain of four simple substances called nucleotides or bases—A, C, G, T—attached one to the other, that are reproduced and passed to descendants in the same order in which they were in the parents. Human DNA is made of 3.1 billion bases divided in twenty-three filaments of unequal length, the chromosomes. Mutation is a transmissible change in DNA: the smallest and most frequent one is the replacement of one of the nucleotides by another of the four. It happens spontaneously at a very low rate. A mutant (an individual carrying a new mutation) may increase in relative frequency over the generations because of natural selection, when it has an advantage over the parental type, either because it survives some disease or other unfavorable environmental condition better and/or is more fertile. When mutation is more disadvantageous or even deleterious than advantageous, carriers tend to be eliminated by natural selection and then the mutation disappears. Most mutations, however, are neither advantageous nor disadvantageous (i.e., are selectively “neutral”), and their permanence in the population, their decrease, increase, or even fixation at the end, are determined exclusively by chance. Like natural selection, this phenomenon, called random genetic drift, causes changes in the frequencies with which all the types of genes caused by mutation are found in populations in successive generations. Both selection and drift tend to differentiate a population from all others. Natural selection causes adaptation to the environmental conditions in which an organism is living that change with time or place, while drift acts only according to the laws of probability. Drift therefore causes changes of greater relative magnitude in populations that are of smaller size.
Migration of individuals from one population to another tends instead to homogenize populations whose genetic differences have been caused by selection or drift or both. But when a group migrates to a previously unsettled area, it may expand demographically and create a new population that, if successfully settled, will eventually grow and differentiate from the original one. Under these conditions migration of a group to a new area may cause differentiation, and it enhances drift effects, especially if the migrating group is small.
Genetic considerations lead us to conclude that modern humans derive from one small population of perhaps one or a few thousand individuals in East Africa that started expanding slowly across Africa about 100 kya. Members of this small group, a tribe, must have almost by definition spoken a single language, which was probably as sophisticated as any language existing today. Like practically all humans until then, they were hunter-gatherers (also called foragers), living in small groups at very low density (on the order of 0.1 inhabitants per square kilometer), and roving semi-nomadically within their hunting territories.
Around 50 kya, a group in a very similar area started expanding more rapidly, demographically and geographically. They were probably helped by some innovations, like rough navigation means, and a new more-sophisticated stone tool set, in addition to modern language, which they most probably shared with those ancestors who were responsible for the first slower expansion of 100 kya. They may have traveled along the southern coast of Asia, using rough navigation means; evidence for this comes mostly from the fact that from Southeast Asia they reached New Guinea and Australia and had to cross various long tracts of sea. They also reached Central Asia from which they moved out in all directions: east to East Asia, north to Siberia and America via the Bering Strait (probably all emerged at the time of the crossing, before 10 kya), and west to Europe. The expansion from Central Asia was especially rapid. After more than 10 kya, all the land inhabited today, except Polynesia, had been settled (it was settled between 6 and 1 kya, using sophisticated boats and navigation systems, starting from Taiwan or the Philippines).
After 13 to 12 kya, the last glaciation ended and there were changes in the flora and fauna as a result of the weather change. In many parts of the world, especially in temperate areas like the Middle East and the Nile Valley, in China and on the Mexican highlands, people started living on cereals (wheat and barley in the first region, rice in southern China and millet in the north, and in Mexico corn and many other vegetables). In areas in which cereals became the major food source, it became convenient to practice agriculture, which brought crops closer to home and made it useful to build more permanent houses. Nomadic habits were largely replaced by domestic ones. In some regions wild animals were easily domesticated. In the Middle East it was possible to domesticate sheep and goats, pigs, cattle, and later horses (north of the Black Sea and the Caucasus). In northern Syria, a mixed agro-pastoral economy was already developed by 11,500 years BP (before the present). It is possible that Europe owes its presently flourishing economy to having been able to develop the most effective domestic animals.
The story of the recent peopling of the world was first modeled by reconstructing trees of populations tested for standard genetic markers and using genetic distances between population pairs calculated from the frequencies of genetic traits in pair members. One way to reconstruct a tree is to start from the nearest populations (having the smallest genetic distance) and continue bringing all the population pairs together in order of increasing distance until the root of the tree is reached. The best way to reconstruct the root involves using remote ancestors for traits comparable to those used in the analysis of the group of populations. It became clear that trees reconstructed were the same irrespective of the genetic traits used, provided adequate numbers of traits are employed. The independence of conclusions, irrespective of the type of genetic traits studied, is an important guarantee that we can trust them.
Since the 1990s or so it has become possible to reconstruct trees of markers that are found only in males (Y chromosomes) or are found in both sexes but are transmitted only by females (mitochondrial DNA or mtDNA). These can be studied in single individuals rather than in populations, and the trees reconstructed in this way are true genealogies. For technical reasons they are particularly accurate and free from doubt for the Y chromosome, but there also is significant agreement between genealogies of mtDNA and Y chromosomes, apart from well-understood exceptional behavior in specific populations that have specific marriage customs. These studies have confirmed results obtained on gene frequencies of populations for markers carried in the usual chromosomes and have contributed sharper conclusions on dates and places of major events in human prehistory.
The Importance of Agriculture
The introduction of agriculture was probably a reaction to need: the hunting-gathering economy practiced until then was not sufficient to guarantee enough food, making food production necessary. It inevitably started a continuous raise of population density, in spite of fluctuations due to accidental severe disturbances. In the 90,000 years prior to the beginnings of agriculture, the population had increased from the initial one or few thousand at the start of growth, by a factor of at least 1,000. In the last 10,000 years, agriculture raised population numbers by another factor of 1,000.
The spread of farming around the places of origin is an unavoidable effect of the increase in the number of farmers made possible by the increase of available food that farming generated. When local population growth rapidly reached saturation, it caused migration to nearby areas in search of new fields. This search also was promoted by the rapid exhaustion of the soil caused by primitive agricultural methods. There were continuous technological improvements like plowing and irrigation over long periods of time, which also created new problems, for example, desertification due to soil salinization in the Middle East or overgrazing in the Sahel. Nevertheless, farming expanded from the place of origin over long periods at a constant rate, estimated at 1 kilometer per year toward Europe (where a better archeological coverage permits an accurate average estimate of the rate of spread). The expansion was determined both by the higher growth rate of the number of farmers (called demic diffusion) and by technological adaptation by local hunter-gatherers (cultural diffusion of the technology).
Genetic studies show different results for the relative role of the genetic/cultural contribution of original farmers to the genetic constitution of European men and women. The problem gave rise to an intense scientific debate, still not completely settled, but the results indicate from 50 to 60 percent demic diffusion for males, and 20 percent for females, or an average for the two sexes that is around one-third genetic and two-thirds cultural. Another example of demic diffusion due to agriculture is found in the Bantu expansion from Cameroon to South Africa, which started about 3,000 years ago and only recently came to an end. Here also, there is a less well investigated but clear male-female difference in the same direction. Hunter-gatherers are everywhere considered socially inferior, and passage to the social class of “farmer” ordinarily was permitted only to females through marriage to a member of the higher class (hypergamy). Male polygamy (polygyny), probably more frequent among farmers, also helped in the same direction.
Farming was initiated everywhere while tools were still made of stone. In Europe (but not in Japan), the term neolithic (as opposed to paleolithic or foraging) refers to the presence of agriculture, which caused a change. The European neolithic adopted ceramic around 3,000 years after the inception of farming, while in Japan ceramics developed much earlier (almost 12 kya), but agriculture entered from Korea only about 2,000 years ago. Technological developments have driven genetic, cultural, and social evolution of modern humans from the very beginning. The next major innovations after farming were the introduction of metals, first bronze, beginning some 5 kya, probably in eastern Europe, followed by iron 3,500 years ago. The spread of these two major technical developments to the rest of the world was much more rapid than that of agriculture. Metal technology contributed greatly to the development of war. Its beginnings tend to be similar in several areas to that of writing (and therefore of recorded history).
Benefits of Genetic Variation
The recent origin of all modern humans from a single, small population living in a narrow area explains why human living populations show small difference: There was not enough time for the generation of much genetic differentiation among the populations that dispersed over the Earth. We can, however, guess the geographic origin of individuals on the basis of some external traits like skin color and facial and body traits that represent the adaptation to a highly varied set of environments, and that differ by climate, flora, and fauna. But each population, even if small, harbors considerable genetic variation, which ensures its survival in the face of new and unpredictable challenges.
In general, high genetic variation is beneficial to a population. Pure races are a fiction, and even if they were possible, they would be truly undesirable. By contrast, cultural differentiation, for instance, the linguistic one, tends to be greater between than within populations. Culture—intended as the knowledge, customs, habits, and values accumulated over generations—needs to be sufficiently homogenous for the harmonious life of a society, in the same way that language spoken by individuals living together must be similar enough for it to achieve its major purpose, mutual understanding.
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