Religion has been associated with some terrible atrocities, but these outrageous acts were committed by men [sic] in the name of man-invented-religion. The problem, as I see it, has always been human nature, and that, in its turn, results, if anything, from the blind-failure of evolution. That is, biological evolution does not move in any idealized direction. Humans are successful, but not innately morally superior.
Biological evolution generated us from a common ancestor that also gave rise to chimpanzees and bonobos. (This hominid timeline implies a linear evolution for hominids, whereas a pruned bush would be more accurate.) We share more than 99% of our DNA sequences with chimpanzees, indicating that our evolutionary lines diverged 7 to 5 million years ago. In general, the later the analysis, the more recent the esimated split. The ancestor of chimpanzees and bonobos are estimated to have split again between 0.89 and 0.86 million years ago, and the two common chimpanzee subspecies are estimated to have diverged about 0.46 million years ago.
Even though our advantages over our cousins result from a regulatory-gene-mutation that permitted development of a greater brain/body size ratio, biological mutation/reproductive selection did not care whether or not we were otherwise inherently vastly superior in rationality/morality.
We seem to have inherited from this common ancestor a nature that lies somewhere between aggressive chimpanzies and sex-obsessed, peaceful bonobos. Possibly chimpanzees and bonobos diverged from a midway-natured common ancestor. The environments in which chimps and bonobos now live are comparatively more challenging in the case of chimps, no doubt necessitating – and so selecting for – their aggressive behaviour.
Humans live in environments that are even more challenging than the jungle habitats of chimps, but we can cope in such environments because our greater brain/body size permits us a degree of inventiveness unsurpassed in the animal kingdom. However, we have needed to retain bonobo-like cooperation in order to survive, particularly in hostile environments.
Further, our greater brain/body ratio has imposed an inordinately long, parent-dependent childhood, and this has necessitated prolonged male-female pair-bonding such as is innate in many bird species, but which is not characteristic of chimp or bonobo societies.
Having fathered a child, a human male can only ensure the perpetuation of his genes (reproductive success) by staying close enough to ensure survival of his offspring and/or by fathering children with many women. Men might, by their emotional/sexual nature, opt for both strategies, but society has long imposed a cost on fathering. So, official polygamy has traditionally been the exclusive reserve of the wealthy.
Human females could similarly ensure reproductive success (survival of their children) by nuturing each child and by mating with several fathers (ensuring a variety of genetic possibilities for her offspring). However, human males, like the males of many species, are typically less invested in the survival of the offspring of other males, so human females can best secure the survival of their children by maintaining the support of one male.