I framed my question a few different ways so I'm not sure which one you're referring to.

I'm certainly not sold on the biological fitness = morality hypothesis I mention above. You pointed out some holes such as altruism and selfishness within a species; although I think they can both play roles in reproductive fitness. I also suspect that my statement of reproductive fitness being an evolutionary objective good could be a gross assumption. I'd like to give it more thought and hope people will poke more holes in it.

I recall a conversation from years back, where I ventured that there was an objective moral order, which I was then told was an absurd thing to believe. — Wayfarer

What are your thoughts on an objective moral order? Is this still your stance?

Although at this point I'm beginning to think the OP is a drive-by. — Wayfarer

No drive-by, just distracted :)

I believe a lot of our social behavior, including morality, is built in, human nature. — T Clark

The problem I have with this (and mine is an unpopular view), is that it invariably reduces morality to survival. After all, the only criterion for success in biological theory is reproductive fitness. — Wayfarer

I find myself leaning towards morality derived from evolutionary biology as my example of pro-existence morality may suggest. As T Clark said, morality, or in my opinion at least the first principal of morality, could be built into us as a species. Could it not be plausible that human morality is framed around the biological objective of reproductive fitness? For instance we notice throughout the animal kingdom that certain species will join together in order to increase reproductive success. A fish may swim in a school, an ant may work within it's colony, and a higher primate may operate in a social group. All of these behaviors came about through natural selection, and appear to benefit the organisms carrying them out.

In a sense one could consider the morality of the fish to stay near it's fellow fish. Now the fish likely does not consciously consider this morality, and we humans would likely prefer to call this an instinct; but can we be sure that our human morality is not a biological instinct as well? Other higher primates have shown altruistic behaviors while interacting with their social groups. It seems form an evolutionary standpoint, these behaviors would increase the reproductive fitness of the group and thus be a desirable or "fit" behavior. Could it be possible that we primates evolved certain moral "instincts" through means of natural selection? Could my urge to help a dying child I see on the side of the road, be instinctual in nature, similar to my fear of heights? I find a genetic morality plausible, though maybe not empirically provable for the moment.

So - it seems to me that what is needed to anchor morality is an objective good, a summum bonum. That is generally translated as the 'highest good' which naturally sounds religious — Wayfarer

As for a "summum bonum," since the overall objective in evolutionary biology seems to be fairly straightforward, could the objective good simply consist of reproductive fitness? It seems to me that most human moralities help humans coexist together in their respective social groups. Social groups which, when compared to the rest of nature, have proven themselves to be one of the most powerful reproductive fitness tools. In this sense I could see how reproductive fitness could be a basis for morality.

Objective morality as defined by you, that is, "equal consideration of equal interests of sentient beings per life time," does indeed appear to be impossible in reality as it is now.

You asked "how can separateness of persons and equal consideration of equal interests be both required for objective morality and impossible to achieve?" I can think of a few (unrealistic) ways this could be done. If a society were to be created where everyone had interests which either aligned, or did not conflict with the interests of others. Such a society would essentially need to control the interests of its individuals or make conflicting interests harmless in a sense.

Examples:
1) a society with the ability to psychologically manipulate it's population's interests from birth so that interests never conflict (extreme brainwashing)
2) A society made of people who naturally don't have conflicting interests (e.g. a society of clones or perhaps a genetic variant of sentient being that can cannot have conflicting interests)
3) Some combination of the two.
4) Random chance. I suppose I can imagine an Earth that miraculously contained nobody with conflicting interests during a certain time period out of pure chance (very very small odds).

I realize some of these examples (like clones) may not qualify for your "separateness of person" criteria in objective reality. That aside, I don't think such equal consideration of interests can ever be attained outside of these examples. As you mentioned, the terrorists interests are not conducive to a layman's interests. It is this conflict of interests in today's reality that makes objective morality impossible from a practical standpoint.

A few other issues with your objective morality include the "equal life" criteria. In nature lifespans are never equal. To have truly objective reality, we would need equal lifespans so that our equal interests may span equal times. Again, solving this problem brings me back to the homogeneous society where everyone's interests and lifespans are the same; however this directly conflicts with the separateness of personas clause. It seems to me that objective morality may be a logical paradox. Perhaps you can reconsider your initial assumption that objective morality, as you define it, is true?