For the last 15 years or so, molecular anthropologists have been comparing the DNA of living humans of diverse origins to build evolutionary trees. Mutations occur in our DNA at a regular rate and will often be passed along to our children. It is these differences (polymorphisms) that, on a genotypic level, make us all unique and analysis of these differences will show how closely we are related. However, different approaches used by molecular and physical anthropologists have led to opposing views on how modern humans evolved from our archaic ancestors.
The two main hypotheses agree that Homo erectus evolved in Africa and spread to the rest of the world around 1 - 2 million years ago; it is regarding our more recent history where they disagree.
Evidence from DNA studies generally supports a recent African origin but these conclusions have been criticised for a lack of statistical support. One possible reason for this is because these studies have focussed mainly on the polymorphisms in a small section of the mitochondrial genome called the D-loop, which comprises around 7% of the mitochondrial genome. The reason for this section's popularity lies in its particularly high mutation rate, meaning that scientists can analyse this relatively short sequence and still resolve differences between closely related sequences. Unfortunately, it is now becoming increasingly clear that this very high mutation rate is actually obscuring the informative information.
Three main problems with data from the D-loop section have been identified:
- back mutation - sites that have already undergone substitution are returned to their original state
- parallel substitution - mutations occur at the same site in independent lineages
- rate heterogeneity - there is a large difference in the rate at which some sites undergo mutation when compared to other sites in the same region; data shows evidence of 'hot spots' for mutation
Now that the entire genome can be sequenced, we should get a clearer picture of the origins of modern humans.
Although the mitochondrial genome is one of the first genomes to be sequenced in its entirety, it was not until recently that the progression of technology allowed sequences of that length to be obtained with relative ease and a study of any appreciable size using whole genomes was undertaken. This study became an important landmark in the field of population genetics and perhaps will be a precedent for a new field, already coined "population genomics." These researchers (Ingman et al., see references) found that although sequencing the whole genome was considerably more work, it provided some important advantages.
Although the D-loop was evolving at a much higher rate, the greater length of the complete genome allowed for the analysis of twice as many informative polymorphic sites (sites that show the same polymorphism in at least two sequences).
The numbers of back- and parallel mutations found outside of the D-loop were practically zero.
The rate of evolution of the rest of the genome was surprisingly even between different sites, different genes and also between the different gene complexes.
The robust phylogenetic tree reconstructed with this dataset of complete mitochondrial genomes gives strong support to the 'recent African origin' theory. By determining the substitution rate of the genomic sequences, it is possible to derive dates for points on the tree and build a chronology of events in the evolution and migration of our species.
The most important date, in relation to the competing evolutionary theories, is the time when all the sequences coalesce into one unique source, the "mitochondrial Eve". See a simplified (just for explanation) evolutionary tree for 16 individuals as proposed in Professor Bryan Sykes book "The seven daughters of Eve", 2001.
From this study, a date of 150000-200000 years was obtained which fits remarkably well with that proposed in the "out of Africa view".
For us to accept multi-regionality, we would expect a much older date, as it would represent the common ancestor of Homo erectus rather than of Homo sapiens.
The evolutionary history of aboriginal populations still remains a mystery. This study is only the first and population genomics is in its infancy. The future will provide more studies with ever-increasing numbers of sequences from yet unanalysed populations and perhaps an interface between genetic data from different loci. For example, a recent study of ancient human remains in Australia integrated genetic data with the information collected by physical anthropology. There are many important questions that remain to be resolved such as how and when the Aborigines arrived in Australia and the evolutionary history and relationships of North and South American Indians.
