Compared to the Neandertals and other archaic Homo sapiens, modern humans generally have more delicate skeletons. Their skulls that are more rounded. Their brow ridges are less protruding. They also have relatively high foreheads and pointed chins.
The first fossils of early Homo sapiens sapiens to be identified were found in 1868 in a 28,000 year old rock shelter site near the village of Les Eyzies in southwestern France. They were subsequently named the Cro-Magnon people. They were very similar in appearance to modern Europeans. Males were 5 feet 4 inches to 6 feet tall. Their skeletons generally were lighter than the Neandertals. The Cro-Magnon had broad, small faces with pointed chins and high foreheads. Their cranial capacities were up to 1590 cm3, which is relatively large even for people today.
Origins of Modern Humans
Current data suggest that Homo sapiens sapiens very likely evolved from archaic Homo sapiens relatively rapidly in Africa and/or the Southwest Asia. They have been dated to 115,000-96,000 years ago at Qafzeh Cave in Israel. In South Africa, they have been found at Klasses River Mouth and Border Cave sites dating to 120,000-100,000 years ago. Since these time ranges overlap, it is not clear which area was the earliest to have modern people. However, it was not until 50,000-40,000 years ago that they began to appear in Europe and East Asia. This was during a short temperate period in the midst of the last ice age. It would seem from these dates that the location of initial modern human evolution and the direction of their dispersion from that area is obvious. That is not the case. Since the early 1980's, there have been two leading contradictory models that attempt to explain Homo sapiens sapiens evolution--the replacement model and the regional continuity model.
The replacement model of Christopher Stringer and Peter Andrews proposes that modern humans evolved from archaic Homo sapiens 200,000-100,000 years ago in Africa and then some of them migrated into the rest of the Old World replacing all of the Neandertals and other late archaic Homo sapiens. If this interpretation of the fossil record is correct, all modern people share relatively modern African ancestry. All other lines of humans that had descended from Homo erectus presumably became extinct. From this view, the regional anatomical differences that we see among humans today are recent developments--evolving only in the last 50,000-40,000 years. This hypothesis is also referred to as the out of Africa and the Noah's ark model.
The regional continuity (or multiregional) model of Milford Wolpoff at the University of Michigan proposes that modern humans evolved more or less simultaneously in all major regions of the Old World from local archaic Homo sapiens populations. For example, modern Chinese are seen as having evolved from Chinese archaic Homo sapiens and ultimately from Chinese Homo erectus. This would mean that the Chinese and some other peoples in the Old World have great antiquity in place. Advocates of this model believe that the ultimate common ancestor of all humans was Homo erectus in Africa more than a million years ago. Since then, however, it is proposed that there was sufficient gene flow between Europe, Africa, and Asia to prevent reproductive isolation and the subsequent evolution of distinct regional species. It is argued that intermittent contact between people of these distant areas would have kept the human line a single species at any one time. However, regional varieties, or sub-species, of humans are expected to have existed.
Replacement Model Arguments
There are two lines of evidence supporting the replacement model--the fossil record and DNA. So far, the earliest finds of modern Homo sapiens skeletons come from Africa and bordering areas of southwest Asia. Elsewhere in the Old World they appear 60,000-50,000 years later. Unless modern human remains dating to around 100,000 years ago are found in Europe and/or East Asia, it would seem that the replacement model better explains the data.
Geneticists at the University of California have argued that the geographic region in which modern people have lived the longest should have the greatest amount of genetic diversity. They base this on the premise that the rate of mutation is more or less constant everywhere. Through comparisons of mitochondrial DNA from living people throughout the world, it was concluded that Africa has the greatest genetic diversity and therefore must be the homeland of all Homo sapiens sapiens. Assuming a specific rate of mutation, it was suggested that the common ancestor of modern humans was a woman living 200,000-100,000 years ago. She has been dubbed "mitochondrial Eve."
Critics of this genetic argument say that the rate of mutation is not necessarily constant and that there were flaws in the computer program that was used to construct the human family trees. The results varied with the order in which the data were entered. Further genetic studies carried out since the mid 1990's have both supported and undermined an African origin for modern humans. John Relethford, of the State University of New York College at Oneonta, has pointed out that Africa could have had the greatest diversity in mtDNA simply because there were more people living there during the last several hundred thousand years. Researchers from the University of Chicago and Yale University have discovered that variations in the DNA of the Y chromosome and chromosome 12 have the greatest diversity among Africans. This is consistent with the replacement model. However, geneticists from Oxford University have found that the human betaglobin gene is widely distributed in Asia but not in Africa. Since this gene is thought to have originated more than 200,000 years ago, it undercuts the claim that an African population of Homo sapiens sapiens replaced East Asian archaic Homo sapiens.
Regional Continuity Model Arguments
Fossil evidence is used to support the regional continuity model. Its advocates claim that there has been a continuity of some anatomical traits from archaic Homo sapiens to modern humans in Europe and Asia. In other words, the Asian and European physical characteristics have antiquity in these regions going back over 100,000 years. They point to the fact that many Europeans have relatively heavy brow ridges reminiscent of Neandertals. Similarly, it is claimed that Chinese facial characteristics can be seen in Asian archaic Homo sapiens dating to 200,000 years ago. Like Homo erectus, East Asians today commonly have shovel-shaped incisors while Africans and Europeans rarely do. This supports the contention of direct genetic links between Asian Homo erectus and modern Asians. Alan Thorne of the Australian National University believes that Australian aborigines share key skeletal and dental traits with people who inhabited Indonesia at least 100,000 years ago. The implication is that there was no replacement by modern humans from Africa 60,000-50,000 years ago. However, the evidence does not rule out gene flow from African populations to Europe and Asia at that time and before. David Frayer of the University of Kansas believes that a number of European fossils from the last 50,000 years have characteristics that are the result of archaic and modern Homo sapiens interbreeding.
Middle Ground Arguments
It is apparent that both the complete replacement and the regional continuity models have difficulty accounting for all of the fossil and genetic data. What is now emerging are new models that incorporate both of the old ones. One suggestion has been a partial replacement model. Günter Bräuer of the University of Hamburg in Germany proposes that the first modern humans did evolve in Africa, but when they migrated into other regions they did not simply replace existing human populations. Rather, they interbred to a limited degree with late archaic Homo sapiens resulting in hybrid populations. In Europe, for instance, the first modern humans appear in the archaeological record rather suddenly around 40,000 years ago. The abruptness of the appearance of these Cro-Magnon people could be explained by their migrating into the region from another area, possibly Southwest Asia or North Africa. They apparently shared Europe with Neandertals for another 10,000 years or more. During this long time period, it is argued that interbreeding occurred and that the partially hybridized predominantly Cro-Magnon population ultimately became the modern Europeans.
Expansion Out of the Old World
The world population of modern Homo sapiens began to grow rapidly after 50,000-40,000 years ago. It was around this time they expanded their territory by migrating into new regions. Their movement into northern areas coincided with the end of a long cold period that had begun about 75,000 years ago. By 60,000 years ago, modern humans apparently moved into Australia for the first time. Around 35,000-30,000 years ago, they moved into Northeastern Siberia. Possibly as early as 30,000 years ago and certainly by 11,500 years ago, they migrated into North America via the Bering Plain (or Beringia). That intercontinental land connection appeared between Siberia and Alaska as a result of sea levels dropping more than 300 feet during the last ice age. Until that time, all human evolution had occurred in the Old World. The rate of human population growth has continued to accelerate until now. The current world population is over six billion and intercontinental migration and gene flow are at higher levels than ever before.
A tragic consequence of human migrations into new regions of the world has been the extinction of many animal species indigenous to those areas. By 11,000 years ago, human hunters in the New World apparently had wiped out 135 species of mammals, including 3/4 of the larger ones. Most of these extinctions apparently occurred within a few hundred years. It is likely that the changing climate at the end of the last ice age was also a contributing factor. However, the same cannot be said for the animal extinctions that occurred following the arrival of aboriginal people in Australia and Polynesians in New Zealand. In both cases, humans were instrumental in wiping out easily hunted species. Vulnerable marsupials were the main victims in Australia. In New Zealand, it was mostly large flightless birds that were driven to extinction by hunters.
It is sobering to realize that the rate of animal and plant extinction has once again accelerated dramatically. During the last century and a half, the explosion in our global human population and our rapid technological development has allowed us to move into and over exploit most areas of the planet. That exploitation has usually involved cutting down forests, changing the courses of rivers, pushing wild animals and plants out of farm and urban areas, polluting wetlands with pesticides and other man-made chemicals, and industrial-scale hunting of large land animals, whales, and fish. During the early 19th century, there were at least 40,000,000 bison roaming the Great Plains of North America. By the end of that century, there were only a few hundred remaining. They had been hunted to near extinction with guns. The same fate came to the African elephant and rhinoceros during the 20th century. Likewise, commercial fisherman have depleted one species of fish after another during the last half century. Governments have had to step in to try to stem the tide of these human population effects on other species. However, they have been only marginally successful.
Breaking News: Part
of the mitochondrial DNA was extracted recently from the bones of a
60,000 year old modern Homo sapiens skeleton found in 1974 on the shores
of Lake Mungo in Southeastern Australia. This is the oldest DNA
that has been extracted from a human so far. Comparison of this
DNA with that of nine other ancient Australian skeletons, 2 Neandertals,
and 3,453 contemporary people from around the world indicates that
"Mungo Man" had a unique genetic marker. This indicates
that a now lost genetic line of modern Homo sapiens existed in Australia
prior to the arrival of later Australian Aborigines. This evidence
provides significant support for rejecting the "out of Africa"
complete replacement model of modern Homo sapiens evolution.
More Late Breaking News: In a March 7, 2002 article in Nature , Alan Templeton, a geneticist at Washington University, reported that a new computer based analysis of 10 different human DNA sequences indicate that there has been interbreeding between people living in Asia, Europe, and Africa for at least 600,000 years. These data suggest that the complete replacement model of Homo sapiens origin is incorrect. According to Templeton, "humans expanded again and again out of Africa, but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world." This view is gaining support among paleoanthropologists, but critics say that Templeton's sample is still too small to be conclusive.