How did humans come to be monogamous?

According with Frederick Engels in Origins of the Family, Private Property, and the State
     "The Athenian family is the origin of monogamy as far as we can trace it back among the most civilized and highly developed people of antiquity. It was not in any way the fruit of individual sex-love, with which it had nothing whatever to do; marriages remained as before marriages of convenience. It was the first form of the family to be based, not on natural, but on economic conditions – on the victory of private property over primitive, natural communal property. The Greeks themselves put the matter quite frankly: the sole exclusive aims of monogamous marriage were to make the man supreme in the family, and to propagate, as the future heirs to his wealth, children indisputably his own. Otherwise, marriage was a burden, a duty which had to be performed, whether one liked it or not, to gods, state, and one's ancestors. In Athens the law exacted from the man not only marriage but also the performance of a minimum of so-called conjugal duties."
        So, the monogamy arise because it made sens economically that time. In ancient time almost all economical duties were made by men and it was easier for him  to take care of a single wife.
        Polygamy is still accepted in Islam and somewhat in Hindu and yes it predate Christianity.
How to relieve muscle pain in my upper back

When looking at muscle pain, it is always important to start by understanding where the pain is located. Upper muscle pain is often due to the muscle trapezius (see below). The main task for the trapezius is to hold stabilize and move the scapula, thus your shoulders (this is important information for the avoidance analysis below).

What are the effects of doing sit ups?

Sit-ups cause your spine to roll against the floor, overtime displacing plates in your spine. Some believe that stretching and flexing a muscle is the only way to stimulate hypertrophy (muscle growth). This isn't true though for some muscle groups. The abdominals being apart of the,