1) Which specific tribes are you referring to? I don't think that you can generalize the experience of a few isolated, backward tribes and assume this is how all human societies functioned before the advent of civilization. For example, there are tribes that practice cannibalism. But it would be a stretch to suggest that cannibalism was, at any time, common human behaviour. By the same token, just because some group of weirdos on an isolated island like to share their women does not mean that this behaviour was previously widespread. There are reasons why those tribes are still struck in the stone age and this may be one of them...
2) Soldiers do what they do due to extreme mental conditioning. The main purpose of military training is break you down as an individual and turn you into a machine that would obey orders, and, if necessary, sacrifice itself without question. This type of self-sacrifice is most definitely not natural.
3) If cooperation was a natural human state, you would not be seeing breakdown of social order in the wake on natural disasters, wars, and other calamities. When civilizations fail, cooperation goes out the window and it's everyone for himself. Thus, cooperation is a product of civilization.