Caveman, thru circumstances of time and geography, lived short, brutish lives in chronic hunger, cold, etc. And the poverty of their lives was matched by the poverty of their spiritual existence - by a dearth of Revelation, knowing not the consolations of grace, the Spirit, Jesus and not even having been given the Law, which, imperfect as it was, was an improvement over the pagan notion of religions which imagined the deities cruel and heartless. The idea of "God is love" was still foreign. The progressive nature of revelation is comparable to the evolution of a seed developing into a young plant developing into an oak. The tiny plant has the worst time of it – it is subject to degrees of cold and is vulnerable to an extent the mature oak is not. That is nature. So why should God not show us, through the physical laws, his plan for the spiritual? Is it because we think we are better than the oak, that human life is more precious and that humans should be coddled? The problem therein is that we are told that we were coddled and that we, via Adam, spoiled it. We were born to a greater dignity. But we chose the harder way - the progressive revelation path.