www.edge.org: My ANSWER
I have read yesterday some chapters of your book, The New Hunanists and I have been delighted. I have almost totally agreed with the authors of the first chapter. Even before having gone through, I have tried to give an answer to the question "Why did human development proceed at such different rates on different continents for the last 13,000 years." I have seen that it has been due to the evolution of culture and to its relationship with the environment rather than to any species or racial differences since biological evolution cannot proceed at such a rapid rate. I have also advanced that the chance factor has come also into play; more precisely, I have pointed to the probability of the Bayesian statistics. I have also underlined the factor of the density of population and I have realized that the civilsations at or close to the Mediterranean Bassin were prime incentives; this is close to your point of view of the geographical axis of travel and of spreading civilization that I have accepted then. One point that I have raised my attention is that you have advanced that if the rhino and the hippopotamus could have been domesticated, a cavalry mounted on them "would have made mincemeat of an Eurasian cavalry mounted on horses." However, it is obvious that could not have happened even if they have been domesticated, since the hippo needs to be close to water ponds and the rhino cannot travel so far and so fast as the horse (a bad comparison!) Hannibal of Carthage has almost conquered the Roman Empire with a cavalry mounted on elephants and a chance factor could have led to the conlusion and to its final defeat by the romans.
As for the second chapter which regards the importance of the genes in our development, I have come with objections. The author(s) has(ve) advanced that half the difference in human behavior is due to the genes, but also to other interelated developmental factors that show up when they raise twins. However, what if the culture or the education of your prototypes is very different and not only a little bit. With the exception of developmental abnormalities (whatever small they may be) due to genes or to other causes, the influence of culture will have precedence and will render negligeable any genetic influence. In fact, most of the differences attributed to genes might only be due to a chance factor, to probability.