The big picture...
The time estimates garnered from the fossil record (approximately 200 000 years ago, given that AMH must have had time to migrate to both South and North Africa), the mtDNA (approximately 200 000 years ago), the Y chromosome (approximately 200 000 years ago), and the nuclear genome (approximately 300 000 years ago) are all relatively equivalent when understanding that precursors were around for millions of years. These dates, however, are not as precise to one another as would be liked, and perhaps explanations are available.
The data from the Y chromosome may be influenced by polygamy. In polygamy one male has several wives while another (usually) has no wives. This multiplies one man's contribution to the next gene pool and eliminates another's. This may cause a MRCA to appear earlier than expected (Mitchell).
Also, researchers must be very wary of population sizes. If evolution is inferred from genetic distances, population size is very important, and, unfortunately, has not always been corrected for in the past. This is a problem because small population sizes cause genetic drift to have larger effects. Also, smaller populations tend to be less diversified because there are less carriers for diversity. This leads to poor cladistic diagrams with disproportionate arm lengths (Relethford et al). Population size also leads to the next problem: bottlenecks. Bottlenecks occur when the population balloons to a level much larger than it previously was at. This can be due either to rapid expansion or normal expansion following drastic population reduction. This is a problem because the bottleneck effect causes those people who were living at the time of the bottleneck to have a much larger effect on future genomes than their immediate precursors, whose contributions may be diluted by the new, larger population or wiped out completely. Any study must be cautious of these problem, or it could calculate a date much older than correct. These bottlenecks did occur, possibly one 30 000 years ago and one 80 000 years ago (Sherry et al). Fortunately however, ways of circumventing these problems are being investigated and used, so hopefully these problems can be avoided and a complete picture of human evolution can be established.
Those arguments, then, settle all problems available as to the agreement of ages of appearance of AMH. What about, however, the expansion of AMH? The Out of Africa hypothesis does not account for human expansion very well, saying only that humankind spread out of Africa. Furthermore, the Out of Africa hypothesis does not discuss the foundation of human diversity, a primary selling point of MRE.
Fortunately, there are several additions to the Out of Africa hypothesis that have been proposed. The one most relevant to this paper and to the argument presented in it is the Multiple Dispersals Model (Lahr). This model holds that differentiation outside Africa arose due to differentiation inside Africa that was spread to the rest of the world through several waves of migration through two distinct paths out of Africa. There are two pieces of evidence that support this model. The idea that geographic distance created differentiation is evident in cultural areas. As noted previously, H. sapien sapien tools included a large variety of types. This cultural differentiation could be indicative of a relative lack of interpopulational communication, which could be both culturally and genetically, which would lead to differentiation. Secondly, every fossil of AMH found outside of Africa appears to be already differentiated (Lahr). This occurrence is expected by MRE but not by the general Out of Africa hypothesis. The Multiple Dispersals Model, however, accounts for it.
Africa represents approximately one-third of the Old World in terms of area, and for a large stretch of time, at least 60 000 years, was the only part of the world occupied by modern man. It is not difficult to assume that, given these conditions and relative isolation, some differentiation may have arose as separate populations formed within the African continent.
From this assumption, it is simple to believe that the population nearest the Middle East would be the population most likely to populate Southern Asia from this point. This population would carry its specific differentiation with it into the new territory. At the same time, however, a second population could migrate through the Horn of Africa into Europe, carrying its own differentiation with it. This introduces two distinct populations outside of Africa, each with possibly different characteristics which, given time, could differentiate to create the situation at present. This picture is further added to by the possibility of multiple waves leaving Africa from within as differentiation continues there on a path unconnected to outside of Africa. Clearly this model contains a possible means of differentiation of the human population not found in the base Out of Africa hypothesis. Furthermore, this model in no way hinders other theories such as the "Weak Garden of Eden" theory (Harpending) that attempt to explain more specifically human origins than does the Out of Africa hypothesis. This model fits all of the facts that support the Out of Africa hypothesis as well as providing mechanisms for differentiation and expansion that agree with observed results.
A bigger picture
What exactly, then, are the observed results? What final compilation of human expansions and movements can be compiled from the available data discussed in this paper? What information is from what evidence? The results are outlined below.
Approximately 200 000 years ago anatomically modern humans originated in a single event in East Africa. Over the next 80 000 years they migrated slowly, eventually reaching South Africa and some of the Middle East, being at these positions approximately 120 000 years ago (fossil evidence). Around 80 000 years ago a population expansion occurs (genetic evidence), and by 60 000 years ago mankind has reached Australia (archaeological evidence), having gone through Southern and Southeastern Asia to get there.
50 000 years ago mankind crossed the Sahara and populated West Africa (fossil), and 40 000 years ago a population expansion (archaeological, source unknown) established the Australian aborigines (morphological). A similar population expansion occurred in the rest of the world approximately 30 000 years ago (genetic), which set of a rash of colonization. This triggered the final death of the Neanderthals in Europe and fueled the expansion of South Asian peoples (genetic demonstrated) into North Asia and Siberia between 35 000 and 20 000 years ago (archaeological). This expansion also pushed the people of Southeast Asia (genetically demonstrated) north, where they eventually crossed into North America just over 20 000 years ago. This is evidenced by the rapid extinction of more than 200 species of game animals (cultural ecology). By 15 000 years ago mankind was in Meadowcroft, Pennsylvania and Monte Verde, Peru (archaeological). These populations featured several minimally differentiated Asian traits (fossil). A second wave populated much of the rest of the New World between 10 000 and 15 000 years ago as evidenced by the more differentiated Asian dental pattern (fossil).
The first domesticated crop appeared in Turkey, also just under 15 000 years ago (cultural ecology). This was followed shortly thereafter by the first civilizations in the Mesopotamian area of what is now Iraq approximately 10 000 years ago (archaeological). And from there the rest is literally history.
The final picture (in this paper, at the least).
Generally then, it can be stated with fair certainty that mankind originated in Africa. Several different forms of data collection and multiple disciplines combine to create a single unified picture as to the extent, direction, and chronological order of human expansion out of Africa. The origin of modern man in Africa is supported by data in several forms, and in general the support for both the Out of Africa hypothesis and the Multiple Dispersals Model is very robust.
If you would like to see the rough bibliography of this paper, there is a link for that below.
The final page(s)