Agreed. We are all descended from the perpetrators of genocide.
we need well lubricated armpits to work weapons in battle for hours at a time.
Disagree. Tribal combat never consists of hours of stand up toe to toe fighting. It is raids and ambushes, remarkably like how you describe animal fighting.
At the very earliest, ranked infantry fighting didn't come in till agriculture forced people to defend particular territory. And in the sense you are using the term, probably not till the Greeks invented hoplite shock infantry warfare.
The trait of well lubricated armpits under stress from other humans is likely related to combat somehow, directly or indirectly. Agriculture is too new an invention to account for such a well developed trait different than other animals. Tribal warfare has been with us much longer than agriculture. A primitive surprise attack massacre of a competing tribe would take many hours preparation and execution. If not in actual fighting, the tribes with superior weapon manipulation endurance were selected somehow.