[Agrippa]

Dienekes dealt with the subject too now and mentions interesting aspects in his comment:

Dienekes\' Anthropology Blog: Hawks et al. paper on accelerated recent human evolution

Originally Posted by Dienekes

The paper has a simple but powerful idea: that really useful mutations are more likely to occur in large populations than in small ones: mutations are random accidents that happen in bodies; the more bodies you have, the more likely it is you'll get a really neat one.
All populations have the capacity to evolve by shifting around the frequencies of the different alleles in their gene pools. But, the kinds of alleles that evolution can work with are not the same. Large populations have a greater repertoire of alleles to work with.
But, some species with really huge numbers don't really evolve that fast. This is because they have already reached an adaptive plateau; they are already well-suited to their environments and don't face large selection pressures.
New challenging environments + growing population = accelerated human evolution.

Abstract from the paper:

Recent acceleration of human adaptive evolution
John Hawks, Eric T. Wang, Gregory Cochran, Henry C. Harpending, and Robert K. Moyzis
Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9-million HapMap SNP dataset, we found that selection has accelerated greatly during the last 40,000 years. We tested the null hypothesis that the observed age distribution of recent positively selected linkage blocks is consistent with a constant rate of adaptive substitution during human evolution. We show that a constant rate high enough to explain the number of recently selected variants would predict (i) site heterozygosity at least 10-fold lower than is observed in humans, (ii) a strong relationship of heterozygosity and local recombination rate, which is not observed in humans, (iii) an implausibly high number of adaptive substitutions between humans and chimpanzees, and (iv) nearly 100 times the observed number of high-frequency linkage disequilibrium blocks. Larger populations generate more new selected mutations, and we show the consistency of the observed data with the historical pattern of human population growth. We consider human demographic growth to be linked with past changes in human cultures and ecologies. Both processes have contributed to the extraordinarily rapid recent genetic evolution of our species.

Dienekes\' Anthropology Blog: Hawks et al. paper on accelerated recent human evolution