Genetics reconstructs evolutionary events by inferring the past from the present. It is very powerful at building phylogenetic relationships within and between species and to a lesser extent at locating them in time and space. However, as currently used, genetics can say little or nothing about populations that have not persisted through to the present day and nothing about context. Furthermore, although the situation is bound to change, most evolutionary genetic studies are concerned with noncoding parts of the genome, and thus, little can be said about the functional or structural nature of present or past human biology.
The complementarity of the two disciplines is thus built on the fact that paleontology can reach the world of extinct populations that provided the context in which humans evolved and locate it in time and space, whereas genetics can provide the details of recent history. Thus far, the sole focus for genetic research has been phylogeny and timing; the mechanisms and population-level processes involved have not been addressed, largely because of the priority given to solving the "two theory problem." With that solved, greater emphasis can now be placed on process and hence on the integration of genetic and paleontological evidence.
Multiple Events, Multiple Dispersals: Reconstructing the Context of Human Genetic Diversity
What do the fossil and archeological records reveal? Box 1 shows the climatic curve for the last half million years, on which have been highlighted a number of key events that may be relevant to the evolution of modern humans. "Single-origin" models of human evolution have tended to imply that the origin is a single event. Perhaps the first point to note about the diagram is that the fossil and archeological evidence has for some time indicated that this is not the case, and, increasingly, genetic evidence is bearing this out. Any valid model of modern human evolution must incorporate multiple events and multiple dispersals. In particular, it is clear that although anatomically modern humans may have evolved ~150,000 years ago, modern dispersals did not occur until much later, and then there were several. Future models of human evolution based on genetics should recognize the full complexity of the events of the last quarter of a million years (Foley and Lahr 1992
; Lahr and Foley 1994).
The second point is that the Neanderthals are just a small part of the story and, perhaps, distort the way we look at the past. There has been considerable discussion about whether the Neanderthals are a separate species. However, although there is more and more evidence that they did not interbreed with early European modern humans, the species concept is in practice largely irrelevant to the problems of the later Pleistocene. What the fossil record shows is a number of populations, modern and archaic, sparsely scattered across time and space (Fig. 2). Many of these, modern or archaic, may well not have made it through to the present day and thus are not reflected in current genetic diversity. Given this, and the complexity of the events, caution should be maintained when looking for links between the living and the fossil populations. Essentially, we need population- not species-based models. Recent human biological history is at heart a question of microevolution.
Finally, there has been a tendency to collapse a number of separate problems together: coalescence times of genes, divergence and dispersal of populations, and the evolution of anatomical and behavioral modernity. Increasingly, the archeological and fossil records are showing that these may be separate events, and yet to many they are all the "origins of modern humans." The complexity of both genetic and fossil evidence should remind us that there may be many such "origins" and that they could be scattered temporally and geographically. A move away from simple phylogenetic questions and approaches to human evolution might help us to untangle these various events and thus develop a more integrated evolutionary model of biological and behavioral history. Such a move will require knowing far more about the context of human genetic evolution, and that is what the paleobiological record can best provide.
Answers to all your Biology Questions
