To Christians who aren't creationists

[The Grand Nudger]

You're describing cladogenesis at the very least, but more likely anagenesis. That's what isn't open to debate. That's why our evolution doesn't allow for 900 year old human beings. The mutations required to push us there by any means we are capable of proposing would leave us decidedly different than what we currently are. The mutations that we could not envision would be even more radical (what with the ones we could envision already having such a profound effect on our biology). In either case, we'd be renaming ourselves.

The mammalian metabolism is a massive stumbling block on the road to longevity, our endothermic/homeothermic biology depends upon it and it supports one of our most important organs -between your ears-. Live fast, die young. Ectotherms, reptilian metabolisms, regenerative survival strategies, "biological immortality"- these are not the roads that mammalian evolution went down, though we do see them expressed here on earth. In many cases, these things are mutually exclusive. Give us a few billion years and who knows, but whatever it is, it won't be human (and in all likelihood, still won't be living 900 years by any natural means).

What would be open for debate would be the likelihood of this sort of mutation (or more aptly, the likelihood of all of the mutations required and what that would mean for us). I don't think we'd be willing to wait quite that long personally, (nor do I think it's a very likely path for us to take-but what I think is likely and what life may do aren't equivalent are they). We could always just tinker with ourselves, little bit of this, little bit of that.

But by all means, continue.

Here's a fun question, why would a "trait" for longevity be selected for? The longevity we see in thigs like tortoises appears to be a side effect, not a "goal". Just how long is it going to take you to have kids anyway? 899years?