It appears to me you are both wrong in the recent debate re: objective morality.
But here are a couple points I think you may find useful:
W. D. Ross
C. S. Lewis
If it were valid, this would work: “If human beings disappeared, KNOWLEDGE would disappear with them; THEREFORE all knowledge is subjective.”
You appear to be saying that human nature is sufficiently constant enough to ground morality. This is half-right.
Human nature has to be NORMATIVE, that is, something we can read off from intelligibly how we OUGHT to act.
An evolutionary account can’t ultimately supply the normativity.