Trust

SM Stirling, "Shikari in Galveston" IN Worlds That Weren't (New York, 2003), pp. 63-148.

I think that the secular bases of morality are as follows:

we were naturally selected to help others either because they bear the same genes or because they might help us in return and we experience this motivation as moral obligation, not as calculating self-interest, which is what it sounds like when expressed in biological terms;

as members of a social species, we have collective interests, like speaking a common language, that transcend a simplistic selfishness-altruism dichotomy.

It follows that lists of commandments and precepts are formulations of an evolved morality, not divine instructions. However, could humanity lose its moral basis?

"When men hunted each other to eat, there could be no trust, and trust was what let even the wildest men work together. Usually man-eaters had no groupings larger than an extended family, and often they barely retained the use of speech and fire. Human beings were not meant to live like that..." (pp. 118-119)

Loss of speech would be loss of humanity. In Olaf Stapledon's Last And First Men, Venerian human beings artificially adapted in haste to colonize Neptune spread across the Neptunian surface but degenerated into animality, one species retaining the custom of gathering in a circle to hear a single individual howl. Later, quadrupeds migrating into an area dense with tall plants began to walk upright, thus freeing forelimbs for manipulation and growing new fingers above the vestigial digits of their ancestors.

Stirling's cannibals have not degenerated that far yet but, if they lose speech entirely, then they will no longer be human.