TL;DR: No such thing as moral objectivism? Evolution (which, mind, is precisely what you just cited) provides a lot of principles to work off of (like fear of death! And pain! And desire to reproduce!), and I'd argue that they are pretty objective. Even if morality grows from the barrel of a gun, fear of guns is a pretty objective response, and a gun is a pretty darn convincing argument too.
Woo, meta-ethics discussion!
Fear of guns is not an objective response. Matter of fact, evolution-based meta-ethics IS morality growing from the barrel of a gun, it's just that the gun is abstract and its bullets may travel very, very slowly. This argument presupposes that an individual
wants to continue existing. This is true for most people - but not all, so the objectivity of this system falls apart. You're, unconsciously, I think, arguing for enlightened self-interest meta-ethics in the guise of evolution.
Furthermore, evolution itself doesn't really have an
agenda, it's a description of emergent behavior. Things that propagate become common. If you accept purely evolutionary meta-ethics, aggresively breeding and neglectfully abandoning your children to their own devices (r-selective strategy), for example, is absolutely ethically consistent. As is taking good care of your two-and-a-half kids. Both are things that humans can, physically, do, but one of these is nowadays seen as highly irresponsible.
There's also situations such as asexuality or congenital analgesia, in which cases those drives are more or less completely irrelevant to the affected individual, so they don't really *care* what evolution has to say about their need to breed or avoid walking into a ADS crowd control devices.