There are two basic models that researchers use to explain the development of modern homo sapiens: The Multi-regional Hypothesis  and The Population Replacement Hypothesis
 

Wolpoff

Multiregional Hypothesis  1. Modern homo sapiens evolved in a number of different geographic areas. 2. Human evolution throughout most of the Pleistocene is a story of change within a single evolving species 3. Homo sapiens evolved in a number of geographically separate areas. 4. Connected with sufficient gene flow and population migrations to maintain biological homogeneity.

Stringer

Population Replacement Hypothesis 1. Modern Homo sapiens evolved in a limited geographic area and migrated out from there 2. Physically replaced the Archaic Homo sapiens who already inhabited the rest of Africa, Europe and Asia. 3. Pre-modern and modern Homo existed at the same time for some time. 4. Genetically, there are differences of a degree that would indicate that the species were separate.

It is suggested that this replacement was facilitated by their more highly adaptive traits, which included: greater intelligence, an increased ability to communicate, better hunting abilities, and better technology. With these advantages, the proponents of the replacement theory argue that modern homo sapiens out-competed Archaic Homo Sapiens. In evolutionary terms, this is another example of the "Law of Competitive Exclusion".

Assessing the Models
If we are to determine which model is correct, we must first be able to identify the differences between Archaic and Modern Homo sapiens, only then can we look for evidence of a transitional stage.

African Evidence
Some of the earliest evidence for Modern Homo sapiens comes from portions of southern and eastern Africa. As such, many believe Africa to be the geographic source of anatomically modern humans. Two important sites in Africa where we have found the earliest evidence for modern humans are Klasies River Mouth (90,000 BP) and Border Cave (100,000 BP). Analysis of the material from KLM shows modern traits, lightly built mandible that has a modern looking chin. The Border Cave specimens are more complete and show a very modern looking individuals, as shown in this slide. In addition to these early modern specimens, there are some hominid fossils from Africa that seem to represent intermediate forms, that is, individuals that have both Archaic and Modern traits. These are known as the "African Transitional Group". The presence of these fossils represent some of the best physical evidence for the evolution of modern humans from Archaic forms in Africa.

The next question, is whether these "intermediate forms" can be found in Europe and Asia?

European Evidence
The oldest modern humans have been found in Germany and date to only 36,000 BP. The Cro-Magnon sites in France date to about 30,000 BP and are quite modern looking, with very high foreheads and small brow ridges. In contrast to the Neanderthals who had lived in Europe before them, these modern Homo sapiens were more gracile and were physically proportioned in ways that would suggest that they had adapted in warmer climates. This would suggest that they had only recently moved into the colder climate of Europe.

The fact that neanderthals disappear in Europe, about the same time as modern Homo sapiens appear, could be used as evidence to support the replacement hypothesis. However, not all the evidence, however, supports the Replacement Hypothesis in Europe. In 1998 at the Lagar Velho site in Portugal, the remains of a child, aged 3-1/2 to 5 years was found that apparently shows a "mixing" of Neanderthal and Modern human traits. The skull of this specimen has a pronounced chin, which is a hallmark of modern homo, however, post-cranially, the boy's bones were quite robust, and he was barrel chested and had short lower limbs, all of which are cold adaptations which characterized Neanderthal.

Some Neanderthal specialists, including Erik Trinkaus regard this boy as the offspring of Neanderthal and human parents. Others, like Ian Tattersal are more skeptical, and suggest that there is sufficient variation within modern homo to account for the boy's appearance. Likewise, Christopher Stringer notes that even if the boy was a mix of human and neanderthal, that this occurred so infrequently, that it had no evolutionary impact.

Middle East Evidence

Sites in Middle East, such as Amud, Shanidar, Kebra, Tabun and Teshik Tash have all produced typical neanderthal remains. However, at the nearby sites of Skhul and Qafzeh, something quite different can be found. Like modern skulls, the specimens from these sites have rounded and have higher foreheads and smaller faces, and chins, again, traits of modern Homo. However, the specimens from these sites are robust with pronounced brow ridges by modern standards. By all accounts, these specimens seem to be transitional between modern humans and Neanderthals, except for the fact that they predate many of the classic Neanderthal specimens found in the region!

East Asia and Australia
East Asia and Australia area perhaps the regions that provide the strongest arguments in support of the multi-regional hypothesis. In fact, certain skeletal traits indicate a continuity that may stretch back as far as Homo erectus. These traits include: shovel shaped incisors, extra cranial sutures, and a ridge inside the lower jaw. It has been argued that there is a clear development of intermediate stages between Archaic and modern homo. For example, the Sangiran, Willandra Lakes and Kow Swamp specimens show no outside influence from more modern looking "Out of Africa" specimens such as this Border Cave skull.

Alternatively, it has been argued that these in-situ regional developments are based on early traits that predate the local populations. It's said that these traits were also are ancestral to early homo in Africa and Europe, and that were only maintained in Asia.

The Genetic Evidence
In general, as modern humans, we have very little genetic variation within our species as compared to say, a chimpanzee, which has 10 times as much genetic variation within its species. This "lack" of genetic variation suggests that we have a relatively recent common source of genetic material, which seems to support the population replacement hypothesis. Likewise, however, the multi-regional hypothesis has also accounted for this lack of genetic variability within homo through gene flow and migration of populations between regions.

Despite the fact that there is little genetic variation amongst our species, comparisons between regions have provided some interesting patterns. Using the idea of a molecular clock, which measures genetic change over time, we shown that:  the world's populations tend to cluster into two groups, sub-Sahara Africa and everywhere else; that the genetic variability amongst humans is greater in Africa than anywhere else and that Africa was the birthplace of our species.

Again, this evidence supports the replacement hypothesis, since we would expect that African populations should have the greatest genetic variability, and that all populations elsewhere should have less genetic variability. Although these sorts of studies show that modern homo first evolved in Africa, they do little to say anything definite about how populations of modern humans came to be elsewhere, and how the interactions of populations affected the development of our species.

Cultural Evidence

If Homo developed in Africa first, that we should see a corresponding sophistication in the material culture. Some of the earliest most sophisticated stone tools are these linear blades that date to about 240,000BP and were found in Kenya at a site near Lake Baringo. These blades represent a very sophisticated form of stone tool production, which requires the manufacture of a carefully prepared core. It's said that this type of core preparation requires a high level of abstraction and planning that probably marks the intellectual advances of hominids from Archaic to modern homo.

A collection of preserved bone artifacts recovered from Katanda, Zaire, and dating to about 90,000BP demonstrates the high degree of intelligence and technological advancement of modern humans at that time. Now, if the replacement hypothesis is correct, we should see evidence for the spread of these kinds of technologies as modern populations expanded out of Africa. Likewise, we should see less developed tools associated with pre-moderns.

However, when the artifacts are compared from early modern and late pre-modern homo, there are few, if any notable differences. It is not until the beginning of the Upper Palaeolithic and the development of the Aurignacian tradition, that we see big advancements in tool sophistication.

Implications on the Notion of "Race"
It is a well documented fact that there are no meaningful differences within our species. As a result, the multi-regional hypothesis has one fatal problem: It suggests that regional differences within our species are deep-rooted. This implies that modern so-called racial groups are very ancient and thus are profoundly different. Early attempts at explaining the origin of the "races", suggested that five subspecies of Homo erectus evolved into five major races of Homo sapiens. It was also suggested that different groups reached the finish line before others. This supposedly accounted for differences in levels of cultural and technological evolution that were encountered around the world.

The present Multi-regional model is quite different than this. It does not say that populations show continuity, but rather, that certain traits within these populations do. As such, it is now recognized that the so-called racial groups were never isolated in ways that were genetically significant, as recent DNA tests attest to. Rather, there has been sufficient migration and gene flow to maintain a species identity. How we got this way doesn't change who we are, and either model could account for our present state.

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