If there is no basis for moral reasoning, then nobody has ever done wrong. Consider the usual examples about how evil the past has been: if there is no moral right or wrong, if there is no good or evil, then it would follow that the Nazis were not wrong and the slave trade was not wrong. It would follow that there is no moral reason why a serial killer or sex predator should be brought to "justice"; what is justice? If morality is an artificial construct, society may come to agreements about rules, but does society have a right to pass judgment on dissenters or conscientious objectors?
"Is it natural or artificial?" is a complicated question, even if we look for a simpler topic than morality. If we get milk from the store, is it natural or artificial? Was the cow given hormones? How about antibiotics? How much selective breeding was involved to produce the herd? Was the milk pasteurized? An artificial process may be applied to a natural thing, and there may be difficulty in attaining an absolutely natural state; that does not imply that there is no natural state. When all the arguing is done, mammals produce milk whether anyone has given them hormones or antibiotics.
So what about morality? Is there a natural state? In tracing its roots, I have not found more basic than this: Life is good. To explain that more fully: Life naturally comes with the beauty of the natural world and the enjoyment of that, with an intrinsic bond to those who gave us life and those who share it. Our original sense of good seems to be our innate sense of the worthiness of life itself. And if anything is good, then opposing or attacking or sabotaging it is not good.
From that, we can derive all the laws that protect life, protect freedom, and promote quality of life. Even traffic laws are, in the end, about not deliberately endangering a life.
If life is good, if it is intrinsically valuable to the one living it, then there is an objective basis for morality.
Next I hope to explore whether there are other intrinsic and natural bases for moral reasoning.