M_0, basic moral approximation

Reason arises from reason as I think anyone could see it to be the more reasonable conclusion. Morals are rooted in person-hood.

Morals are rooted in reason, with special emphasis on conclusions regarding person-hood. Reason arises from awareness of reasons. Reasons arise from predictive control. Control arises from things with dynamic feedback whose continued existence is contingent on the nature of the feedback. Such things arise from any system or pattern which self-replicates. Self-replicating systems arise in all sorts of situations, including purposeless ones with no reason involved.

But you don't need to think about things that clearly (sketchy as it was) to get a basic rule of thumb: When investigating the morality of X, begin with the question "If X had been a normal, acceptable practice that most people in the last ten thousand years engaged in, where would we be now?"

Loving your neighbor? We'd be a lot better off than we are now. Love FTW!
Cannibalize your neighbor? We'd have died out. Double plus unbetter!
Eat pie? Not so much different if we did or didn't, so not a moral issue.

Now, you can certainly object that people aren't infallible in their abilities to figure out what would have happened if something had been some other way, but that doesn't mean there's no objective truth of how it would have been. It simply means that, like people aren't infallible in their abilities to speak on behalf of gods, getting a grasp on the objective stuff is hard, despite it being there.

Remember, though, this is only one rule of thumb. A first step, an approximation, not the entirety and final ending point. For some things it's entirely sufficient, but clearly not for everything. When you're dealing with simple stuff like "you and your ancestors failing to even have been able to exist", no problem. Things like "stealing and bearing false witness in order to prevent adultery" need later approximations, since teasing out whether the resulting state of things would be worthwhile or not is a lot harder. You might think it'd be good, someone else might disagree. Better tools are needed, I don't disagree.

But, looking back to reply number 7, even this simple rule of thumb is sufficient for anyone (who's mildly sentient and not psychopathic) to condemn cannibalism far, far more than they can condemn eating pumpkin pie. And it works for anyone, not just atheists. No supernatural references required.