Recent archaeological evidence suggests that humans became increasingly docile and less reactively aggressive around the time of becoming Homo sapiens, a process that started about 300,000 years ago.
There are certain changes that appear in human fossils that also appear in domesticated animals.
There’re four bone characteristics of domesticated animals. They tend to have smaller bodies than their wild ancestors. Their faces tend to be shorter. Differences between males and females are less. And they tend to have smaller brain cavities (and brains). Our brain size increased steadily over the last two million years, until about 30,000 years ago, when our brains started becoming smaller.
As language became sufficiently sophisticated, our ancestors’ increased communication ability led not only to a more peaceful species but also to new kinds of hierarchy. The physical force of an individual was no longer the prerequisite for ruling human groups. Once capital punishment came along, then anyone aspiring to control the group by force couldn’t get away with just being a killer. He had to be a politician, too.
The result of such selective pressure over generations means that maybe human beings are an animal species that’s been domesticated — like dogs, horses or chickens.