Why Are Humans' Predominately Right Handed?

Or even left handed, have a dominant handed usually? Wouldn't it Be Advantageous to Be ambidextrous?
A professor came up with a compelling theory. He studied ancient stone tools, and would explore how they were made by getting students to make new ones. He noticed that he could tell whether they were made by lefties or righties.

Next step was a stroke of brilliance, he started to look back at the stone tools in museum collections, in order to trace when humans stopped being ambidextrous (like most ape species, so he knew there was a point when this occurred), and there was a clear sign at about 500,000 years ago.

This is the same time that human skull differentiation showed clear signs of growth in the speech centres, such that language was becoming more important. The theory at the moment stands that handedness is an accident of this development of the side of the brain that controls speech.

So if we follow speech to right-handedness theory to be true, then all humans use the left side of the brain to process speech and grammar, because of evolutionary history (no good reason, those parts of the brain were just ready to become speech centres). These well developed pathways somehow led to a small favouring of the hand that it controls, the right hand in most people. But the right hemisphere also has all the correct pathways to control the left hand well. You know this if you think that righties can still do some amazing things with their left hand AND can train themselves to be ambidextrous. Most of us don't bother, and the small differences in favouring hands when we are young, result in big differences in handedness when we are older. It just takes practice.
The reason that left-handed people still exist, is probably likely due to an evolutionary stable strategy.

Despite the pressure to be a right handed person, in a world where 90 % of people are right-handed, there is just enough of an incentive to be left. We can only hypothesise what that incentive is (but we DO know there is one, because there are still left-handed people). But one working hypothesis is the sword hand. In a world where 90%+ people are using their right hand for sword-fighting, everyone practices against right hands (including the lefties) most of the time. In combat, lefties defeat righties, because they have more practice. Cool. But if the number start to shift towards the lefties, the incentive starts to go away (in our example, because there are more lefties that everyone gets some practice against them) and the pressure from the speech center effect pushes humans back towards being more right handed. The see-saw effect wavers around 90% right handed. 

Source 1,2
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