Out of Africa
There is an interesting paper that will soon be published in the journal Paleo^3 (Palaeogeography, palaeoclimatology and palaeoclimatology) regarding the exodus of hominins from Africa. In the paper, "Using Pliocene palaeoclimatic data to postulate dispersal pathways of early hominins" (doi:10.1016/j.palaeo.2006.11.012 K. Holmes), the argument is made that, from a paleoclimatic perspective, the exodus out of Africa would have been most favorable 1.8 million years ago (Pliocene-Pleistocene boundary). The analysis was made using a GIS (Geographic Information System) model coupled with climatic information from climate models for that interval of time.
There is considerable debate in the paleoanthropology community regarding the timing of the exodus from Africa. Some groups favor an early exit from Africa based on poorly dated finds of fossils from places like Atapuerca (Spain). The Atapuerca find for example, contains hominid bones thought to date to about 800,000 years. In Java (Indonesia) there is controversial evidence for hominins outside Africa by 1 million years ago. Older estimates for fossils dating the exodus back to ~ 2 million years are also controversial. What is clear is that ancestors to homo sapiens left Africa sometime post 2 million years ago.
This paper approaches the problem from a paleoclimatic standpoint. The argument is that the exodus was driven by environmental issues and, more specifically, climatic issues. The paper looks at potential pathways for the exodus using a combination of GIS and paleoclimatic models. Holmes looks at the pathways of animals that also dispersed from Africa at about the same time and argues that physical barriers for the exodus were not an important consideration (i.e. it was easy to leave Africa). Her model considered the following pieces of information most critical for the analysis:
1. Pliocene topography
2. Pliocene vegetation
3. Distribution of carnivore faunal remains
4. Distribution of carnivore/scavenger faunal remains
5. Distribution of herbivore faunal remains
6. Distribution of herbivore/root/omnivore faunal remains.
The model predicts a number of favored pathways out of Africa. Some are confirmed by fossil finds while a few known hominid finds confound the predictions of the model. What is most interesting is that the model predicts several potential locations for future discoveries. Holmes predicts, for example:
The results suggest that a pathway through the more northerly countries of central Asia, following the Asian grasslands into southern China, has the highest probability of having been used for hominin dispersal in the Plio/Pleistocene; thus archaeological/palaeontological work in this area would provide a critical test of the results of this model.
Image Copyright Elsevier Science 2007
This is a useful illustration of how scientific predictions are made using independent methods. The model reported in this paper suggests potential new sites for hominid discoveries and also highlights preferred paths for the exodus based on physical and environmental barriers.