To me that suggests that the impulse towards population growth was always there but was limited by the scarcity of food and that once agriculture created a (comparative) surplus of food that lid was lifted off population growth, not than that people generally held off shagging until they started farming.
it is impossible, but we still must try not to project our sexuality as conditioned by our environment onto those whose lives are organized in DRASTICALLY different ways.
Abstinence after a newborn for periods of up to 1 year is common, and so is infanticide, as ways to control population. Scarcity of food may also have been a factor, but i'm reluctant to believe to a great extent, because, again, of a much more plentiful earth.
And we must not use our moral codes to judge their practices either, for one thing, we are always completely biased to down play our own systematic cruelty when looking at the other: if infanticide is a part of a sustainable way of life in which the well being of every one is accounted for, it makes sense for that system.