The second phase of human migrationEdit
The time period between 250,000 and 50,000 years ago is commonly called the Middle Paleolithic.
At the same time that Neanderthals occupied Europe and Western Asia, other kinds of people lived in the Far East and Africa, and those in Africa were significantly more modern than the Neanderthals.
These Africans are thus more plausible ancestors for living humans, and it appears increasingly likely that Neanderthals were an evolutionary dead end, contributing few if any genes to historic populations.
Topics to be covered in this chapter:
- Summary of the fossil evidence for both the Neanderthals and some of their contemporaries;
- Second phase of human migration ("Out-of-Africa 2" Debate)
History of ResearchEdit
In 1856, a strange skeleton was discovered in Feldhofer Cave in the Neander Valley ("thal" = valley) near Dusseldorf, Germany. The skull cap was as large as that of a present-day human but very different in shape. Initially this skeleton was interpreted as that of a congenital idiot.
The Forbes Quarry (Gibraltar) female cranium (now also considered as Neanderthal) was discovered in 1848, eight years before the Feldhofer find, but its distinctive features were not recognized at that time.
Subsequently, numerous Neanderthal remains were found in Belgium, Croatia, France, Spain, Italy, Israel and Central Asia.
Anthropologists have been debating for 150 years whether Neanderthals were a distinct species or an ancestor of Homo sapiens sapiens. In 1997, DNA analysis from the Feldhofer Cave specimen showed decisively that Neanderthals were a distinct lineage.
These data imply that Neanderthals and Homo sapiens sapiens were separate lineages with a common ancestor, Homo heidelbergensis, about 600,000 years ago.
Unlike earlier hominids (with some rare exceptions), Neanderthals are represented by many complete or nearly complete skeletons. Neanderthals provide the best hominid fossil record of the Plio-Pleistocene, with about 500 individuals. About half the skeletons were children. Typical cranial and dental features are present in the young individuals, indicating Neanderthal features were inherited, not acquired.
Morphologically the Neanderthals are a remarkably coherent group. Therefore they are easier to characterize than most earlier human types.
Neanderthal skull has a low forehead, prominent brow ridges and occipital bones. It is long and low, but relatively thin walled. The back of the skull has a characteristic rounded bulge, and does not come to a point at the back.
Cranial capacity is relatively large, ranging from 1,245 to 1,740 cc and averaging about 1,520 cc. It overlaps or even exceeds average for Homo sapiens sapiens. The robust face with a broad nasal region projects out from the braincase. By contrast, the face of modern Homo sapiens sapiens is tucked under the brain box, the forehead is high, the occipital region rounded, and the chin prominent.
Neanderthals have small back teeth (molars), but incisors are relatively large and show very heavy wear.
Neanderthal short legs and arms are characteristic of a body type that conserves heat. They were strong, rugged and built for cold weather. Large elbow, hip, knee joints, and robust bones suggest great muscularity. Pelvis had longer and thinner pubic bone than modern humans.
All adult skeletons exhibit some kind of disease or injury. Healed fractures and severe arthritis show that they had a hard life, and individuals rarely lived past 40 years old.
Neanderthals lived from about 250,000 to 30,000 years ago in Eurasia.
The earlier ones, like at Atapuerca (Sima de Los Huesos), were more generalized. The later ones are the more specialized, "classic" Neanderthals.
The last Neanderthals lived in Southwest France, Portugal, Spain, Croatia, and the Caucasus as recently as 27,000 years ago.
The distribution of Neanderthals extended from Uzbekistan in the east to the Iberian peninsula in the west, from the margins of the Ice Age glaciers in the north to the shores of the Mediterranean sea in the south.
South-West France (Dordogne region) is among the richest in Neanderthal cave shelters:
- La Chapelle-aux-Saints;
- La Ferrassie;
- Saint-Césaire (which is one of the younger sites at 36,000).
Other sites include:
- Krapina in Croatia;
- Saccopastore in Italy;
- Shanidar in Iraq;
- Teshik-Tash (Uzbekistan). The 9-year-old hominid from this site lies at the most easterly known part of their range.
No Neanderthal remains have been discovered in Africa or East Asia.
Chronology and GeographyEdit
The time and place of Homo sapiens origin has preoccupied anthropologists for more than a century. For the longest time, many assumed their origin was in South-West Asia. But in 1987, anthropologist Rebecca Cann and colleagues compared DNA of Africans, Asians, Caucasians, Australians, and New Guineans. Their findings were striking in two respects:
- the variability observed within each population was greatest by far in Africans, which implied the African population was oldest and thus ancestral to the Asians and Caucasians;
- there was very little variability between populations which indicated that our species originated quite recently.
The human within-species variability was only 1/25th as much as the average difference between human and chimpanzee DNA. The human and chimpanzee lineages diverged about 5 million years ago. 1/25th of 5 million is 200,000. Cann therefore concluded that Homo sapiens originated in Africa about 200,000 years ago. Much additional molecular data and hominid remains further support a recent African origin of Homo sapiens, now estimated to be around 160,000-150,000 years ago.
The Dmanisi evidence suggests early Europeans developed in Asia and migrated to Europe creating modern Europeans with minor interaction with African Homo types. July 5 2002 issue of the journal Science and is the subject of the cover story of the August issue of National Geographic magazine. New Asian finds are significant, they say, especially the 1.75 million-year-old small-brained early-human fossils found in Dmanisi, Georgia, and the 18,000-year-old "hobbit" fossils (Homo floresiensis) discovered on the island of Flores in Indonesia.
Such finds suggest that Asia's earliest human ancestors may be older by hundreds of thousands of years than previously believed, the scientists say. Robin Dennell, of the University of Sheffield in England, and Wil Roebroeks, of Leiden University in the Netherlands, describe their ideas in the December 22, 2005 issue of Nature. The fossil and archaeological finds characteristic of early modern humans are represented at various sites in East and South Africa, which date between 160,000 and 77,000 years ago.
Herto (Middle Awash, Ethiopia)Edit
In June 2003, publication of hominid remains of a new subspecies: Homo sapiens idaltu. Three skulls (two adults, one juvenile) are interpreted as the earliest near-modern humans: 160,000-154,000 BP. They exhibit some modern traits (very large cranium; high, round skull; flat face without browridge), but also retain archaic features (heavy browridge; widely spaced eyes). Their anatomy and antiquity link earlier archaic African forms to later fully modern ones, providing strong evidence that East Africa was the birthplace of Homo sapiens.
In 1967, Richard Leakey and his team uncovered a partial hominid skeleton (Omo I), which had the features of Homo sapiens. Another partial fragment of a skull (Omo II) revealed a cranial capacity over 1,400 cc. Dating of shells from the same level gave a date of 130,000 years.
Ngaloba, Laetoli area (Tanzania)Edit
A nearly complete skull (LH 18) was found in Upper Ngaloba Beds. Its morphology is largely modern, yet it retains some archaic features such as prominent brow ridges and a receding forehead. Dated at about 120,000 years ago.
Border Cave (South Africa)Edit
Remains of four individuals (a partial cranium, 2 lower jaws, and a tiny buried infant) were found in a layer dated to at least 90,000 years ago. Although fragmentary, these fossils appeared modern.
Klasies River (South Africa)Edit
Site occupied from 120,000 to 60,000 years ago. Most human fossils come from a layer dated to around 90,000 years ago. They are fragmentary: cranial, mandibular, and postcranial pieces. They appear modern, especially a fragmentary frontal bone that lacks a brow ridge. Chin and tooth size also have a modern aspect.
Blombos Cave (South Africa)Edit
A layer dated to 77,000 BCE yielded 9 human teeth or dental fragments, representing five to seven individuals, of modern appearance.
African skulls have reduced browridges and small faces. They tend to be higher, more rounded than classic Neanderthal skulls, and some approach or equal modern skulls in basic vault shape. Where cranial capacity can be estimated, the African skulls range between 1,370 and 1,510 cc, comfortably within the range of both the Neanderthals and anatomically modern people.
Mandibles tend to have significantly shorter and flatter faces than did the Neanderthals.
Postcranial parts indicate people who were robust, particularly in their legs, but who were fully modern in form.
Out-of-Africa 2: The debateEdit
Most anthropologists agree that a dramatic shift in hominid morphology occurred during the last glacial epoch. About 150,000 years ago the world was inhabited by a morphologically heterogeneous collection of hominids: Neanderthals in Europe; less robust archaic Homo sapiens in East Asia; and somewhat more modern humans in East Africa (Ethiopia) and also SW Asia. By 30,000 years ago, much of this diversity had disappeared. Anatomically modern humans occupied all of the Old World.
In order to understand how this transition occurred, we need to answer two questions:
- Did the genes that give rise to modern human morphology arise in one region, or in many different parts of the globe?
- Did the genes spread from one part of the world to another by gene flow, or through the movement and replacement of one group of people by another?
Unfortunately, genes don't fossilize, and we cannot study the genetic composition of ancient hominid populations directly. However, there is a considerable amount of evidence that we can bring to bear on these questions through the anatomical study of the fossil record and the molecular biology of living populations. The shapes of teeth from a number of hominid species suggest that arrivals from Asia played a greater role in colonizing Europe than hominids direct from Africa, a new analysis of more than 5,000 ancient teeth suggests. (Proceedings of the National Academy of Sciences Aug 2007)
Two opposing hypotheses for the transition to modern humans have been promulgated over the last decades:
- the "multi-regional model" sees the process as a localized speciation event;
- the "out-of-Africa model" sees the process as the result of widespread phyletic transformation.
The "Multi-regional" modelEdit
This model proposes that ancestral Homo erectus populations throughout the world gradually and independently evolved first through archaic Homo sapiens, then to fully modern humans. In this case, the Neanderthals are seen as European versions of archaic sapiens.
Recent advocates of the model have emphasized the importance of gene flow among different geographic populations, making their move toward modernity not independent but tied together as a genetic network over large geographical regions and over long periods of time. Since these populations were separated by great distances and experienced different kinds of environmental conditions, there was considerable regional variation in morphology among them.
One consequence of this widespread phyletic transformation would be that modern geographic populations would have very deep genetic roots, having begun to separate from each other a very long time ago, perhaps as much as a million years.
This model essentially sees multiple origins of Homo sapiens, and no necessary migrations.
The "Out-of-Africa"/"Replacement" modelEdit
This second hypothesis considers a geographically discrete origin, followed by migration throughout the rest of the Old World. By contrast with the first hypothesis, here we have a single origin and extensive migration.
Modern geographic populations would have shallow genetic roots, having derived from a speciation event in relatively recent times. Hominid populations were genetically isolated from each other during the Middle Pleistocene. As a result, different populations of Homo erectus and archaic Homo sapiens evolved independently, perhaps forming several hominid species. Then, between 200,000 and 100,000 years ago, anatomically modern humans arose someplace in Africa and spread out, replacing other archaic sapiens including Neanderthals. The replacement model does not specify how anatomically modern humans usurped local populations. However, the model posits that there was little or no gene flow between hominid groups.
If the "Multi-regional Model" were correct, then it should be possible to see in modern populations echoes of anatomical features that stretch way back into prehistory: this is known as regional continuity. In addition, the appearance in the fossil record of advanced humans might be expected to occur more or less simultaneously throughout the Old World. By contrast, the "Out-of-Africa Model" predicts little regional continuity and the appearance of modern humans in one locality before they spread into others.
Out-of-Africa 2: The evidenceEdit
Until relatively recently, there was a strong sentiment among anthropologists in favor of extensive regional continuity. In addition, Western Europe tended to dominate the discussions. Evidence has expanded considerably in recent years, and now includes molecular biology data as well as fossils. Now there is a distinct shift in favor of some version of the "Out-of-Africa Model".
Discussion based on detailed examination of fossil record and mitochondrial DNA needs to address criteria for identifying:
- regional continuity;
- earliest geographical evidence (center of origin);
- chronology of appearance of modern humans.
The fossil evidence most immediately relevant to the origin of modern humans is to be found throughout Europe, Asia, Australasia, and Africa, and goes back in time as far as 300,000 years ago.
Most fossils are crania of varying degrees of incompleteness. They look like a mosaic of Homo erectus and Homo sapiens, and are generally termed archaic sapiens. It is among such fossils that signs of regional continuity are sought, being traced through to modern populations.
For example, some scholars (Alan Thorne) argue for such regional anatomical continuities among Australasian populations and among Chinese populations. In the same way, some others believe a good case can be made for regional continuity in Central Europe and perhaps North Africa.
By contrast, proponents of a replacement model argue that, for most of the fossil record, the anatomical characters being cited as indicating regional continuity are primitive, and therefore cannot be used uniquely to link specific geographic populations through time.
The equatorial anatomy of the first modern humans in Europe presumably is a clue to their origin: Africa. There are sites from the north, east and south of the African continent with specimens of anatomical modernity. One of the most accepted is Klasies River in South Africa. The recent discovery of remains of H. sapiens idaltu at Herto (Ethiopia) confirms this evidence. Does this mean that modern Homo sapiens arose as a speciation event in Eastern Africa (Ethiopia), populations migrating north, eventually to enter Eurasia? This is a clear possibility.
The earlier appearance of anatomically moderns humans in Africa than in Europe and in Asia too supports the "Out-of-Africa Model".
Just as molecular evidence had played a major role in understanding the beginnings of the hominid family, so too could it be applied to the later history, in principle.
However, because that later history inevitably covers a shorter period of time - no more than the past 1 million years - conventional genetic data would be less useful than they had been for pinpointing the time of divergence between hominids and apes, at least 5 million years ago. Genes in cell nuclei accumulate mutations rather slowly. Therefore trying to infer the recent history of populations based on such mutations is difficult, because of the relative paucity of information. DNA that accumulates mutations at a much higher rate would, however, provide adequate information for reading recent population history. That is precisely what mitochondrial DNA (mtDNA) offers.
MtDNA is a relatively new technique to reconstruct family trees. Unlike the DNA in the cell nucleus, mtDNA is located elsewhere in the cell, in compartments that produce the energy needed to keep cells alive. Unlike an individual's nuclear genes, which are a combination of genes from both parents, the mitochondrial genome comes only from the mother. Because of this maternal mode of inheritance, there is no recombination of maternal and paternal genes, which sometimes blurs the history of the genome as read by geneticists. Potentially, therefore, mtDNA offers a powerful way of inferring population history.
MtDNA can yield two major conclusions relevant for our topic: the first addresses the depth of our genetic routes, the second the possible location of the origin of anatomically modern humans.
- extensive genetic variation, implying an ancient origin, going back at least a million years (certainly around 1.8 million years ago);
- no population would have significantly more variation than any other. Any extra variation the African population might have had as the home of Homo erectus would have been swamped by the subsequent million years of further mutation.
- limited variation in modern mtDNA, implying a recent origin;
- African population would display most variation.
- If modern populations derive from a process of long regional continuity, then mtDNA should reflect the establishment of those local populations, after 1.8 million years ago, when populations of Homo erectus first left Africa and moved into the rest of the Old World. Yet the absence of ancient mtDNA in any modern living population gives a different picture. The amount of genetic variation throughout all modern human populations is surprisingly small, and implies therefore a recent origin for the common ancestor of us all.
- Although genetic variation among the world's population is small overall, it is greatest in African populations, implying they are the longest established.
- If modern humans really did evolve recently in Africa, and then move into the rest of the Old World where they mated with established archaic sapiens, the resulting population would contain a mixture of old and new mtDNA, with a bias toward the old because of the relative numbers of newcomers to archaic sapiens. Yet the evidence does not seem to support this view.
The argument that genetic variation among widely separated populations has been homogenized by gene flow (interbreeding) is not tenable any more, according to population geneticists.
Although these two hypotheses dominate the debate over the origins of modern humans, they represent extremes; and there is also room for several intermediate models.
- One hypothesis holds that there might have been a single geographic origin as predicted by replacement model, but followed by migrations in which newcomers interbred with locally established groups of archaic sapiens. Thus, some of genes of Neanderthals and archaic H. sapiens may still exist in modern populations;
- Another hypothesis suggests that there could have been more extensive gene flow between different geographic populations than is allowed for in the multi-regional model, producing closer genetic continuity between populations. Anatomically modern humans evolved in Africa, and then their genes diffused to the rest of the world by gene flow, not by migration of anatomically modern humans and replacement of local peoples.
In any case the result would be a much less clearcut signal in the fossil record.
Neanderthal fossils have been found in Israel at several sites: Kebara, Tabun, and Amud. For many years there were no reliable absolute dates. Recently, these sites were securely dated. The Neanderthals occupied Tabun around 110,000 years ago. However, the Neanderthals at Kebara and Amud lived 55,000 to 60,000 years ago. By contrast, at Qafzeh Cave, located nearby, remains currently interpreted as of anatomically modern humans have been found in a layer dated to 90,000 years ago.
These new dates lead to the surprising conclusion that Neanderthals and anatomically modern humans overlapped - if not directly coexisted - in this part of the world for a very long time (at least 30,000 years). Yet the anatomical evidence of the Qafzeh hominid skeletons reveals features reminiscent of Neanderthals. Although their faces and bodies are large and heavily built by today's standards, they are nonetheless claimed to be within the range of living peoples. Yet, a recent statistical study comparing a number of measurements among Qafzeh, Upper Paleolithic and Neanderthal skulls found those from Qafzeh to fall in between the Upper Paleolithic and Neanderthal norms, though slightly closer to the Neanderthals.
The Lagar Velho 1 remains, found in a rockshelter in Portugal dated to 24,500 years ago, correspond to the complete skeleton of a four-year-old child.
This skeleton has anatomical features characteristic of early modern Europeans:
- prominent chin and certain other details of the mandible;
- small front teeth;
- characteristic proportions and muscle markings on the thumb;
- narrowness of the front of pelvis;
- several aspects of shoulder and forearm bones.
Yet, intriguingly, a number of features also suggest Neanderthal affinities:
- the front of the mandible which slopes backward despite the chin;
- details of the incisor teeth;
- pectoral muscle markings;
- knee proportions and short, strong lower-leg bones.
Thus, the Lagar Velho child appears to exhibit a complex mosaic of Neanderthal and early modern human features. This combination can only have resulted from a mixed ancestry; something that had not been previously documented for Western Europe. The Lagar Velho child is interpreted as the result of interbreeding between indigenous Iberian Neanderthals and early modern humans dispersing throughout Iberia sometime after 30,000 years ago. Because the child lived several millennia after Neanderthals were thought to have disappeared, its anatomy probably reflects a true mixing of these populations during the period when they coexisted and not a rare chance mating between a Neanderthal and an early modern human.
Population dispersal into Australia/OceaniaEdit
Based on current data (and conventional view), the evidence for the earliest colonization of Australia would be as follows:
- archaeologists have generally agreed that modern humans arrived on Australia and its continental islands, New Guinea and Tasmania, about 35,000 to 40,000 years ago, a time range that is consistent with evidence of their appearance elsewhere in the Old World well outside Africa;
- all hominids known from Greater Australia are anatomically modern Homo sapiens;
- emerging picture begins to suggest purposeful voyaging by groups possessed of surprisingly sophisticated boat-building and navigation skills;
- the only major feature of early Greater Australia archaeology that does NOT fit comfortably with a consensus model of modern human population expansion in the mid-Upper Pleistocene is the lithic technology, which has a pronounced Middle, rather than Upper, Paleolithic cast.
Over the past decade, however, this consensus has been eroded by the discovery and dating of several sites:
- Malakunanja II and Nauwalabila I, located in Arnhem Land, would be 50,000 to 60,000 years old;
- Jinmium yielded dates of 116,000 to 176,000 years ago.
Yet these early dates reveal numerous problems related to stratigraphic considerations and dating methods. Therefore, many scholars are skeptical of their value.
If accurate, these dates require significant changes in current ideas, not just about the initial colonization of Australia, but about the entire chronology of human evolution in the early Upper Pleistocene. Either fully modern humans were present well outside Africa at a surprisingly early date or the behavioral capabilities long thought to be uniquely theirs were also associated, at least to some degree, with other hominids.
As a major challenge, the journey from Southeast Asia and Indonesia to Australia, Tasmania and New Guinea would have required sea voyages, even with sea levels at their lowest during glacial maxima. So far, there is no archaeological evidence from Australian sites of vessels that could have made such a journey. However, what were coastal sites during the Ice Age are mostly now submerged beneath the sea.
Overall the evidence suggested by mitochondrial DNA is the following:
- the amount of genetic variation in human mitochondrial DNA is small and implies a recent origin for modern humans;
- the African population displays the greatest amount of variation; this too is most reasonably interpreted as suggesting an African origin.