Russian (Soviet?) scientist Dmitri K. Belyaev began a series of experiments in 1959 in which he crossbred generations of wild animals selecting only for increased tolerance for human beings. He considered this characteristic to be the hallmark of tameness or domestication. Most famously, Belyaev eventually bred foxes that in only a few generations became as tame as dogs. The New York Times has a fascinating article today looking at how Belyaev and his successors have also bred tame rats, minks, and so forth. Tamed animals look more like younger versions of their forbears.
Researchers have noted that domesticated animals generally have smaller brains than their wild ancestors did. In fact, modern humans also have smaller brains than our ancestors did. This has led some researchers to suggest that human beings are self-domesticated. As the Times noted: "Richard Wrangham, a primatologist at Harvard, has proposed that people are a domesticated form of ape, the domestication having been self-administered as human societies penalized or ostracized individuals who were too aggressive."
Researchers reported last year that one variant of the microcephalin gene affecting human brains was strongly selected for and swept through human populations about 37,000 years ago. Another study found that a variant of the ASPM gene, which is also involved with the development of human brains, arose merely about 5,800 years ago and has since swept to high frequency under strong positive selection. Could these genes be involved in our ongoing self-domestication? Is self-domestication (and the increased tolerance of others that goes with it) responsible for our ability to build large-scale civilizations?