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The Evolutionary Origins of Human Population Variation. As we have seen, there is a geographically based pattern to human biological variation. Later on, we will explore more of the varying biological features that have been studied.
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The Evolutionary Origins of Human Population Variation As we have seen, there is a geographically based pattern to human biological variation. Later on, we will explore more of the varying biological features that have been studied. Today, our task is to trace the evolutionary origins of modern human population variation: How did it arise? What are the evolutionary and other processes that have brought it about?
Evidence for Human Evolution • Comparative Anatomy The examination of the gross anatomical features of humans and apes reveals their close similarity and evolutionary relatedness. • Comparative Genetics Comparisons of the genetic materials of the African apes and humans documents that these creatures are more closely related to each other than to any other living animal. • Fossil Evidence
Fossil Evidence • The direct evidence of our extinct ancestors. • Other evidence documents relationships; only fossil evidence provides data on the biology and adaptation of our actual ancestors. • Because of the nature of the fossilization process, this evidence is often difficult to fully understand and interpret.
What’s in a name? Although there is much debate about what terms to use when referring to our extinct ancestors, for our purposes, we will call them hominids, and speak of human evolution as the evolution of the hominids (Using this term places ourselves and our extinct bipedal ancestors in a separate family: Hominidae) . Thus, we can say that hominid evolution seems to have begun in Africa, and it is only much later in our evolutionary history that we find signs of our ancestors outside the African continent. We are,more specifically, members in the genus Homo, of which there are a number of now extinct species, and we are the only living member: Homo sapiens.
Human Origins humans chimpanzees gorillas Hominid evolution 5 - 8 myr 10 myr. common ancestry of humans and African apes An evolutionary diagram of human and African ape relationships, based on various genetic studies
The Human Fossil Record • With recently found fossil discoveries from western Kenya and Chad, the human evolutionary line may now begin as early as about 6 million years ago. • The earliest evidence is of animals that possessed some biological traits like those of the modern humans, some that resembled the apes, and many unique traits (these are nothumans in fur suits, or bipedal apes, but a wholly extinct set of species with their own biology). • Later-in-time members of our line look more and more like living people. • A number of genera and species have been proposed. • For us, this afternoon, we will be interested in the last 200,000 years of human evolution and the emergence of modern people.
Human Evolution: Beginnings • Our earliest ancestors, mainly of the group known as the australopithecines, are characterized by a combination of ape-like and human traits. They were bipeds, with small, non-projecting canines, but they had small, ape-sized brains in a skull that was very ape-like. Uniquely, they had massive back chewing teeth and huge jaw muscles. There were a number of species of this group. We know virtually nothing about their adaptation, diet, social organization or general behavior. • By about 2 million years ago, members of our own genus, Homo, appear on the scene, probably evolving from one of the later australopithecines. They had bigger brains and smaller back teeth, but were still quite different from living humans. The first stone tools appear in the fossil record about two million years ago, as well as indications from scratch marks on animal bones that meat eating was occurring, but from hunting or scavenging is not known.
Human Evolution:First Out of Africa • Early members of the modern human genus Homo are found in Africa between 2 - 1.8 million years ago. There is evidence of these early humans on the island of Java sometime after 1.8 million years ago Thus, about this time, there is a spread out of Africa, and into Eurasia. No one knows why this expansion of range occurred. • From this point in human evolution on, the Old World is more or less populated by human ancestors. Keep in mind that the glacials, or ice ages, periodically descended in the northern hemisphere, making large areas uninhabitable. • After our ancestors spread out of Africa, there begins a time of human evolution which will culminate with the appearance of modern humans in various parts of the world. How they evolved and the precise evolutionary pathways, are much in dispute.
The First Europeans • Some archaeologists claim that the very earliest sites in Europe are in southern Spain and central France and are dated to more than one million years ago. • The earliest well documented site in Europe, however, is in central Spain, near Burgos, where human fossils are dated to about 780,000 years. There is no agreement as to the species of these fossils. • This is considerably later than Homo expansion into Asia, and was probably dictated by glacial activity and the limited routes into Europe. • Other sites in Europe, in Germany, France, Italy and Greece are all probably 500,000 years or later.
Theories of Modern Human Origins • Two major theories attempt to explain the latter phases of human evolution and the development of modern human population variation (human ’races’) • They view human origins very differently, with the differences based primarily on how isolated hominid populations were after spreading out from Africa around 1.8myr. • Both theories have long histories, and in one guise or another, have been around since the recognition of the essential non-modern human qualities of the neandertals in the middle of the 19th century
Competing Models of Human Origins The two competing models are known as: 1. The Multi Regional Evolutionary Model. 2. The Single Origins Model (usually called “Out of Africa”).
Multi Regional Evolution I • With expansion of early Homo into Eurasia, hominid populations moved into new environments and began to evolve biological features for life in those places. • In this model, hominid populations were continuously distributed over the continents, and were in more or less constant contact with other populations, thus sharing genes. • This gene flow insured that the hominids remained one evolving species. • By about 700,-400,00 years ago, archaic members of H. sapiens had appeared.
Multi Regional Evolution II • These archaic H. sapiens populationsin the different areas eventually evolve into living human regional populations (“races”). • Thus, human races have a long antiquity in their local environments, having evolved from earlier archaic sapiens, and before that, from the local early Homo populations. • Multi regional evolution stresses the ebb and flow of gene flow as a crucial factor in human evolution and in modern human origins.
Single Origins Theory I • Begins in the same fashion as multi regional evolution with the spread of early Homo out of Africa into Eurasia. Hominid populations move into new environments and begin to evolve biological features for life in those places. • In this theory, hominids lived in small, isolated populations and, lacking genetic contact, evolved into a number of new species. • In Europe, this new species will eventually evolve into the neandertals, who become extinct toward the end of human evolution.
Single Origins Theory II While in Europe these now isolated hominids evolve into a new species, the Neandertals, In Africa and Asia, other species of Homo were also evolving. Like the Neandertals in Europe, they also possess low sloping brain cases, and large projecting faces lacking a chin. They had large brains, often within the range of living humans.
Single Origins Theory III • Between about 200,-100,000 years ago, modern humans, Homo sapiens, evolved from an earlier Homo ancestor. • This evolutionary origin apparently took place in one locale, most probably somewhere in sub- Saharan Africa. • Soon after this origin, these modern humans begin to expand out of Africa, marking a second expansion out of Africa. • These modern humans move into all parts of the Old World, replacing earlier species of Homo, like the Neandertals,in those areas.
Single Origins Theory IV • Thus, in this theory, modern humans, Homo sapiens, evolve relatively recently in one locale and spread out from there. • Modern human races all have a relatively recent origin in Africa. • Earlier humans in other parts of the Old World were separate species from modern humans. They were not part of the ancestry of modern humans but an extinct side branch, replaced by these newcomers who moved ‘out of Africa’.
Modern Human Origins • Thus, two different theories: 1) Multi Regional Evolution 2) Single Origins : “Out of Africa” • Because they are amongst the most numerous of fossils, much of the emphasis of both theories centers on the Neandertals.
Neandertal Discoveries • Earliest of the fossil hominids to be found and identified. • First recognized in 1856 from a quarry find in the Neander Valley of Germany. • Because it was the first discovery of a fossil hominid, and because this find came just 3 years before the publication of The Origin of Species, it quickly became part of the controversy surrounding Darwinian evolution.
The Neandertals • Fossil hominids who occupied Europe and the Middle East from about 150,000-30,000 years ago, when they disappear from the scene. • Some view them as a subspecies of Homo sapiens, while others place them in their own species: H. neanderthalensis. • Morphologically, they possessed large brain cases, with low and long skulls and large projecting faces lacking a chin. • They were capable of very complex behavior, including the deliberate burial of their dead.
The Neandertals Said one English anatomist of the first find in the Neander Valley: “It may have been one of those wild men, half-crazed, half idiotic, cruel and strong, who are always more or less to be found living on the outskirts of barbarous tribes, and who now and then appear in civilized communities to be consigned perhaps to the penitentiary or the gallows, when their murderous propensities manifest themselves”.
Neandertal Discoveries II • After the initial discovery, many other fossils similar to the original Neander Valley find were excavated in many parts of Western and Eastern Europe, in the Middle East and as far East as Iraqi Kurdistan and Uzbekistan . • Early on, they became stereotypes of the brutish, primitive, bestial ancestors of humans....and the term Congresspeople often hurl at each other when they wish to convey their opponents lack of humanity (did the Neandertals, one wonders, call their adversaries “Congresspeople”?).
La Chappelle Skull The Skull of a Neandertal from France Large brain case with a brain often larger than those of living humans No chin
Modern Human Origins • So, what are the relationships between the Neandertals (and their contemporaries in other parts of the Old World) and living humans? • Multi Regional evolution and Single Origins theories rely on different sorts of evidence. 1) Multi regional evolution relies primarily on fossil evidence from Asia. 2) Single Origins emphasizes fossil evidence from Africa and comparative genetic evidence from living human populations.
Single Origins Theory: Fossil Evidence This theory would be acceptable if fossils were found that were modern human in form, but dated earlier in time than Neandertals. Clearly, it would be difficult to support multi-regional evolution if modern humans were around either before or at the same time as their presumed ancestors, the Neandertals.
Single Origins Theory: The Fossil Evidence • Three sites in Africa appear to have modern human fossil remains, and each seems dated to the time of the Neandertals, or earlier. • Unfortunately, each of these sites has problems associated with either the dates or the interpretation of the anatomy. • The Three sites are located in: 1) southern Ethiopia 2) on the border between South Africa and Swaziland 3) on the very southern most coast of South Africa
Problems: Fossil Evidence for Single Origins Theory • Unfortunately, as these three sites in Africa all have a variety of difficulties associated with their total acceptance as evidence for the early appearance of modern humans in Africa, the evidence has to be carefully evaluated. • There are other sites, however, not in Africa, but in Israel, in the Middle East, which would appear to offer much better evidence for the very early appearance of modern humans. • We will examine this fossil evidence later on.
Single Origins Theory: Genetic Evidence • At the moment, this is the strongest evidence for a recent origin of modern humans in Africa. • It is based on the analysis of DNA, but not primarily the DNA found on the chromosomes in the neucleus. Other genetic material is found in structures called mitochondria (known as mtDNA). • Mitochondria (singular: mitochondrion) are cell structures responsible for carrying out the conversion of the sugar glucose into a form usable to the cell for energy.
mtDNA Results • Comparisons based on segments of the mtDNA from a number of human populations: 1) Documents a greater amount of mtDNA variation in Africans in comparison to human populations in other parts of the world. 2) Discovered unique variations in Africa. • Conclusions drawn from this data: 1) Modern humans originated in Africa. 2) There was a subsequent spread to other parts of the Old World, replacing earlier hominid populations.
Debates about mtDNA Results • Many scientists believe that these results are simplistic and do not reflect the realities of human origins. • Some suggest that because Africa was an optimal environment for earlier hominids, population size was always larger there than elsewhere; thus there was a greater number of mutations, and more variability. • Others argue that if there was significant evolutionary selection on the mtDNA genes, then it would be very difficult to predict the nature of this evolution.
Multi Regional Evolution: Fossil Evidence • The evidence for multi regional evolution is primarily centered on a number of fossils from Asia. • In China, for example, there is fossil evidence that the distinctive facial features of living Asian peoples had already appeared early in Asian human evolution, before any possibility of modern human migration out of Africa.
