Severe brain injuries are all too common in modern times and were probably more common in prehistoric times. X-rays of (mostly) men with foreign objects through their brain are all too common. I suppose being flexible about which part of the brain does what would make the difference between a survivable and recoverable injury, and a debilitating injury, probably leading to death.
Edit: Steven Pinker, in "The Better Angels of our Nature: Why Violence Has Declined", makes it out to be 15% average death by violence, so, pretty common. Explaining his graphs, he says, “The topmost cluster shows the rate of violent death for skeletons dug out of archaeological sites.” “The death rates range from 0 to 60 percent, with an average of 15 percent.”
Edit: Steven Pinker, in "The Better Angels of our Nature: Why Violence Has Declined", makes it out to be 15% average death by violence, so, pretty common. Explaining his graphs, he says, “The topmost cluster shows the rate of violent death for skeletons dug out of archaeological sites.” “The death rates range from 0 to 60 percent, with an average of 15 percent.”