The individuals being selected are only the "best" if you value killing over everything else. If you, for example, value preservation of genetic material then war selects some of the worst people.
I think there's a lot of jobs that war would select for that don't involve killing. Early man interested in treating wounds would study the human body and try to repair it, as one example. These valuable and intelligent humans would be promoted just like today's .mil and be more likely to procreate.