Population geneticists have demonstrated that species evolve within groups of organisms called populations. Genes in populations do not just float around willy-nilly as they pass from parents to offspring. There are specific rules and regularities between such things as population size and natural selection that determine how the population will actually evolve. If a population shrinks down to a very few breeding individuals, for example, a lot of genetic diversity will be lost, and the population will pass through a so-called bottleneck. Mathematical relationships of population size and genetic diversity have been worked out for many species. Calculations suggest that in a simplistic model of a founding band of hominids to account for all living humans, with the level of molecular change that we see in Homo sapiens, world populations would have been only about ten thousand people. This is much too small a number and it strongly suggests that something major is wrong with the population models for human evolution.
Geneticist Elise Eller of the University of Colorado has hypothesized that a pattern of successive population extinctions and recolonizations could explain such a molecular pattern in human evolution.19 Her results do not support predictions made by the multiregional hypothesis but they do provide a basis for understanding the apparent discrepancy between what must have been large population sizes ("census size") of early Homo (that we deduce from the geographic spread of fossils) and the small "effective population size" (that is indicated by the genetic data). Genetic diversity as judged by rapidly evolving biomolecules was lost as local populations went extinct, giving an artificially low estimate of past population size. A neighboring and related population, carrying a number of genes of the extinct population through its clinal genetic connections, then moves into the region and occupies it. If such a pattern of evolutionary change were repeated for thousands, tens of thousands, and then hundreds of thousands of years, we would see the resulting short branches of rapid evolutionary molecular change.
Eller's observations are profoundly important, but they leave us in a quandary: How do we make sense of the new and compelling population genetics perspective, and at the same time account for the apparent synchrony of evolutionary change as seen in the fossils? We need a model that can explain both the relatedness of widespread past hominid populations, as we see at 1.9 million years ago in Africa and Eurasia, and a mechanism to explain the short histories of biomolecules in human ancestry.
We propose a hypothesis that explains both our independent observations on the fossil hominids and many geneticists' observations of the molecular evolutionary data. Homo erectus ergaster evolved into Homo erectus erectus in both Asia and Africa at the same time because of genetic connections among populations, but data of rapidly evolving biomolecules from modern human populations do not record this relatively ancient event. The rapid mode of molecular evolutionary change in Homo sapiens has served to overprint prior genetic change. We believe that this overprinting has conspired with a more complete fossil record for later phases of human evolution to give the impression of total replacement of species, as hypothesized by "Recent African Origin," "Out of Africa," and "Mitochondrial Eve" replacement theorists.20 Our model, we believe, explains observations of both Replacement and Multiregionalist camps and makes sense of the fossil evidence in a defensible genetic and populational model.
The extant human species exists as clines—geographically defined populations with different gene frequencies (and physical traits) that intergrade at the edges. In the past this broad geographic clustering of physical characteristics and gene frequencies has been termed "race." Race has become an unpopular term because of its long history of association with human rights abuses and genocide (as in Nazi-era "racial purity," U.S. "segregation," South African "apartheid," and Serbo-Croatian "ethnic cleansing," to name just a few examples). But biological anthropologists have recognized and studied geographically circumscribed human variation for a century and a half. It exists. Geneticists measure it and forensic anthropologists can recognize it in the bones of murder victims. "Race" is biological variation with a geographic component. It is frequently confused with but is distinct from "ethnic" variation. An "ethnic group" is a culturally defined unit also originally circumscribed by geography but primarily defined by
Clinal replacement. An idealized population spread across 12 regions evolving for 19 generations was modeled by population geneticist Sewall Wright (1940). Stylized population-growth curves increase, and then connect forward to a new generation or connect to an adjacent region by gene flow. A cline is a related group of populations within a species that show traits varying along a gradient of geography. If we walked from region 1 to region 12 at generation 9 in this figure, we would traverse a cline. There would be slightly greater genetic discontinuity when crossing from region 5 to 6, and from region 8 to 9, because of decreased gene flow between these regions. At certain times, such as at generation 6 in region 12, a cline goes extinct. Related but distinct populations from adjacent regions then spread into and repopulate the region. It is this mechanism that we hypothesize ultimately accounted for the appearance of new species in human evolution, as when Homo erectus evolved from Homo habilis, or when Homo sapiens evolved from Homo heidelbergensis. Clinal replacement explains how such species transitions can show continuity in traits within regions while at the same time accounting for the appearance of new species in evolution.
language, religion, custom, dress, and other learned behaviors. Ethnicity and culture coexist with and exert many important influences on the genetics of populations, but they are not the same thing.
Geneticists today are fond of saying that human races do not exist because the variation found within a population equals or exceeds that found between and among populations. This statement is both profoundly important from the standpoint of understanding gene flow in human populations, and virtually meaningless from the standpoint of whether or not we use the term "race." It is profoundly important because human groups tend to outbreed, that is, members of a group find attractive, mate with, and reproduce with members outside their immediate groups. This tendency to outbreed is termed "exogamy"-— marrying outside." Exogamy is a culturally mediated behavior and it acts to blur the edges of geographi cally delimited differences in populations. Exogamy is counteracted by geographic distance and physical barriers. The farther apart two people are the less likely they are to get together to have children. Similarly, if mountains, rivers, chasms, deserts, oceans, glaciers, or other physical barriers separate them, they are not likely ever to meet each other. The meaningless part of the above statement comes in when we choose a "population" that is based on political, tribal, or linguistic criteria but that spans large tracts of territory and crosses multiple physical barriers. Of course we will find such a contrived population to be characterized by as much or more biological variation as an adjacent population with less diverse geography and fewer geographic barriers. And we have not even taken into consideration large-scale migration, which greatly confuses patterns of biological variation. But our goal here is to understand how human populations are organized and have evolved, not to quibble about terminology. Let's just say that in general we will find a geographic cluster of biological characteristics in a human population group and that there will usually be a gradient of change of those characteristics into surrounding groups. This is the definition of a "cline."
There are many examples of clines of human genetic and physical traits. A gene that causes dry ear wax, for example, is very common in the Far East, and as one moves westward it becomes progressively less common, until it virtually disappears in populations in the British Isles. The cline may exist because of the exogamy of innumerable small populations over millennia—populations passing the genes along like runners handing off batons in a relay race. Alternatively or additionally, the spread of this gene may have been facilitated by mass population movements east to west, such as the Mongol invasion of the thirteenth century. Physical traits that show clinal variation in humans include skin color, which tends to be light in high latitudes and darker near the equator, and body form, which tends to be slender and linear near the equator and compact and rounded in colder climates. There are many exceptions to these generalities, but they are broadly true. We know that early African Homo erectus (the "Turkana Boy") at Nariokotome had the same linear proportions of limbs and trunk that Africans who live there, near the equator, still have.21 We would predict, but we cannot yet demonstrate, that the hominid populations occupying higher latitudes in Eurasia would have relatively shorter limbs and stockier builds. These bodily characteristics would have changed gradually as one moved away from the equator, as would be predicted by the clinal replacement model.
In discussing genetic change as measured by molecular evolutionists investigating human origins, it is important to remember that they are looking at genes and their protein products that evolve rapidly, that is, that change on the order of every few thousand years. Most of the remainder of our genes evolve quite slowly, and because they do not gauge the time frame for recent events in human evolution, they have not come into the discussion. But in our discussion of clines the majority of the human genome, not just the small, rapidly evolving portion, is important. We must understand that when a clinal neighbor replaces an extinct population, most of its genes will be the same. This is quite obvious with a moment's reflection. We share basic characteristics too numerous to list with life forms ranging from the single cell to primates, and all of these physical and physiological traits are controlled by homologous genes. These basic, ground plan genes that control much of our biological formation are not replaced when one clinally related population of a species replaces another. There is continuity in most gene lines and we consequently see this continuity in anatomical traits through time. All too often in debates between molecular-based and fossil-based theorists, a false equation is made between lineages of rapidly evolving genes or biomolecules and lineages of populations of a species. They are in fact very different.
Some of the confusion surrounding the interpretation of molecular and fossil data in human evolution probably originates within genetics itself, spawned by unresolved and divergent viewpoints from biochemical genetics and population genetics. When Rebecca Cann, a fellow graduate student of ours at Berkeley in the 1970s, was developing the "Mitochondrial Eve" hypothesis, she did so in collaboration with Allan Wilson, a professor of biochemistry. Wilson had earlier collaborated with anthropology professor Vince Sarich, himself an anthropologically trained biochemist, to research the ape—human split. The theoretical underpinning of this revolutionary approach to understanding human evolution came from the seminal understanding of the "molecular clock" discovered by biochemists Emile Zuckerkandl and Linus Pauling.22 Cann, as had Sarich before her, took their internally consistent and tightly argued molecular conclusions and applied them directly to interpretations of the fossil record. Paleoanthro-pologists, who for the most part do not accept data that they cannot see with the naked eye, were left scrambling for resolution. We believe that the missing perspective is population genetics, first integrated with hominid fossils in the evolutionary hypotheses of Franz Weidenreich via his contact with and citing of the work of geneticist Theodosius Dobzhansky, discussed earlier.
Population genetics in general is undertaken by a very different group of scientists than is molecular genetics. Mathematics plays an important part in the theoretical formulations of population genetics; also, work with living and breeding organisms, such as fruit flies, has traditionally formed a central part of the experimental work in this discipline. Some of the great names of population genetics are Dobzhansky, Sewall Wright, J. B. S. Hal-dane, Ronald Fisher, and Maynard Smith. None of their works or ideas, however, played a major part in the biochemical genetic formulations of recent Replacement models in human evolution. Yet population genetics occupies a central place in the "synthetic theory of evolution"— an emergent perspective after World War II, and it affected paleoanthropology dramatically through the "new physical anthropology" of Sherwood Washburn.23 The population perspective of clinal replacement, we suggest, will allow distinctions between which molecules, genes, and physical traits are being compared, and it will facilitate determining the appropriate time frame and geography for discussions of human evolutionary dynamics. Eller's recent paper, mentioned above, is a good start.
The structures of past hominid populations are important to understand if we are to deal with past clinal connections among them. Population density among mammals is related to their body size. As body size increased from Homo habilis to Homo erectus, population density and home-range size also increased. This happens because larger bodies require more food resources.24 If the species is an omnivorous or carnivorous one its home range tends to cover a much larger territory than if it is a herbivorous species. As it spreads out to obtain more food, it will come into contact, and probably conflict, with neighboring populations. We believe this model explains much of the overland movements of early Homo populations. But once humans were in Eurasia, geographic barriers, such as bodies of water, which affect how and where populations can move, exerted important influences on subsequent evolution.
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