It cannot be accounted for by the drive of individual selfish genes to perpetuate themselves. Quite the contrary: it may lead humans to make sacrifices that lead to great personal suffering, injury, or death, without any evidence of benefit. And yet, if we carefully examine that inner voice we sometimes call conscience, the motivation to practice this kind of love exists within all of us, despite our frequent efforts to ignore it.
Sociobiologists such as E. O. Wilson have attempted to explain this behavior in terms of some indirect reproductive benefits to the practitioner of altruism, but the arguments quickly run into trouble. One proposal is that repeated altruistic behavior of the individual is recognized as a positive attribute in mate selection. But this hypothesis is in direct conflict with observations in nonhuman primates that often reveal just the opposite -- such as the practice of infanticide by a newly dominant male monkey, in order to clear the way for his own future offspring.
Another argument is that there are indirect reciprocal benefits from altruism that have provided advantages to the practitioner over evolutionary time; but this explanation cannot account for human motivation to practice small acts of conscience that no one else knows about. A third argument is that altruistic behavior by members of a group provides benefits to the whole group. Examples are offered of ant colonies, where sterile workers toil incessantly to create an environment where their mothers can have more children. But this kind of "ant altruism" is readily explained in evolutionary terms by the fact that the genes motivating the sterile worker ants are exactly the same ones that will be passed on by their mother to the siblings they are helping to create. That unusually direct DNA connection does not apply to more complex populations, where evolutionists now agree almost universally that selection operates on the individual, not on the population. The hardwired behavior of the worker ant is thus fundamentally different from the inner voice that causes me to feel compelled to jump into the river to try to save a drowning stranger, even if I'm not a good swimmer and may myself die in the effort. Furthermore, for the evolutionary argument about group benefits of altruism to hold, it would seem to require an opposite response, namely, hostility to individuals outside the group. Oskar Schindler's and Mother Teresa's agape belies this kind of thinking. Shockingly, the Moral Law will ask me to save the drowning man even if he is an enemy.
If the Law of Human Nature cannot be explained away as cultural artifact or evolutionary by-product, then how can we account for its presence? There is truly something unusual going on here. To quote Lewis, "If there was a controlling power outside the universe, it could not show itself to us as one of the facts inside the universe -- no more than the architect of a house could actually be a wall or staircase or fireplace in that house. The only way in which we could expect it to show itself would be inside ourselves as an influence or a command trying to get us to behave in a certain way. And that is just what we do find inside ourselves. Surely this ought to arouse our suspicions?"