Monkey is a genetically engineered man, an enhanced clone created by PyramidHe may be forged by many years of surviving alone, but Monkey's strenght, resilience and agility look more often than not superhuman. He may have been completely created in a laboratory, or be subject to genetic modifications when he was very little, and at least part of his memories be not real. Pyramid wanted to create sort of a Super Soldier for his cause, so what better than a bigger and stronger version of himself? However, Pyramid ended up putting a small part of his conscience that considered his cause wrong into the clone, which escaped his control. The clone spent some time confused, his mind completely blank, surviving, until he became the Monkey we know. That would explain why Monkey, after fighting so fiercely until then, was falling prey to Pyramid's illusion quite easily near the end of the game, and why both characters are played by Andy Serkis.