Thanks for your thoughts.

"We are all from Earth" is not a bad place to start, but I think we do ultimately need to go further than that.

After all, I think most conservatives and liberals would already agree with that statement - and yet they will still bicker for decades about abortion, climate change, and LGBT rights.

And the reason is because they don't share the same fundamental values, or the same strategies for establishing facts. To a conservative Christian, good and evil are defined solely by God's will (as told by the Bible, and interpreted by their community). Whereas to a secular liberal, good and evil will have more of a basis in human well-being - which is to say, human well-being is a noble end in itself. But to a conservative Christian, human happiness is not considered an end in itself, and may or may not coincide with what they interpret to be good in the eyes of God.

These two strategies for assigning value are simply not compatible within one society. And this is why we need to explicitly define a value system. The value system I've proposed here is human happiness, or, more broadly, the happiness of all sentient beings - the latter of which would, I think, speak to your concern about including all species.