Many would ask, without the promise of heaven or the threat of hell, where does morality come from?
I believe that morality in its most basic sense, empathy, is not just a social construct but also a product of evolution. In extended families or tribal clusters as our ancestors must have lived co-operation would have been paramount to survival. Feeling a desire for your tribal members' survival and well-being was a survival trait of itself.
Millions of years ago our ancestors started walking upright at first moving a bit more slowly than other primates until the Achilles tendon came into the picture. Imagine you are a slow-moving, meter high person with very little defensive equipment in the way of sharp teeth, strong jaws or razor sharp claws. You aren't even strong enough to kneecap a lion even if you were large enough to pick up a big enough club. Your children are born quite helpless, unable to cling to their mothers' upright backs. While standing tall allows you to see predators from great distances away you really aren't as great at climbing trees as your distant ancestors may have been. You could probably yank loose a prickly branch from a thorn bush and wave it in a big cat's face but she or her family could easily circle behind you and your mama's contribution to the gene pool would end up as a light meal.
So how did something this frail and dare I say paw lickin' good survive or even evolve in the first place? Team work. The little fellows learned to look out for each other both from a budding advancement in empathy and blatant self-interest. A lone pre-human (even a sturdy and healthy male massing perhaps as much as a young German shepherd dog) would not do so well on the African plains amidst large predators. Every man for himself just doesn't work when every man is three feet tall and delicious.
Those little mothers also had to be very delicate with their large-headed, weak infants. Big brains made early hominids feeble, floppy fetuses even after birth. Those proto-human women had to coddle and cuddle their immature infants or they would have lost them soon after giving birth. Everybody knows dead babies don't pass on their genes. Even early hominids likely had a long childhood requiring extra protection and help acquiring food long after most animals would have been self-sufficient. Thus empathy and even love were survival traits. Survival, enlightened self interest, and love of family - even distant family - these are the roots of morality, conscience and civilization. They are part of our evolutionary makeup both from a social and biological sense. I believe evolutionary psychology explains the origin of morality quite well.
Love really is all you need.