Rather than motivating environmental conservation out of some species-level nostalgia for "the biodiversity of our youth", there's another, forward-looking motivation for it: the species that evolved at the same time we did — our "evolutionary cohort" — are the species of our own Environment of Evolutionary Adaptedness. While they may not share our ecological niche, they all share a dependency on the same air mix we do, the same temperature ranges, the same water osmolarity and pH, etc.

This implies two things:

- Insofar as we want to keep the Earth a good place for us to live, we can measure our success by the number of our evolutionary cohort of species that stick around;

- Insofar as we plan to live in space, terraform other worlds, etc., it's only animals of our evolutionary cohort that will be able to comfortably survive in the places we optimize for our own well-being. If we increase global CO2 levels here on Earth to such a point that species on Earth that like our current air mix begin dying off, and we see an evolution of replacement species that prefer higher CO2 levels (and we ourselves retreat to dome cities or something to avoid that fate); but then we later terraform Mars to have the environment of Earth ca 5000BCE; then the species that inhabit Earth by that point, won't be able to be transplanted to Mars. Only the species we had conserved — keeping them in the dome-cities with us — would be able to be transplanted.

(Another way to say that second one, is that if we screw up the Earth's environment, but later fix it, we'll lose biodiversity twice — we'll have killed off our evolutionary cohort with the screw-up, and we'll kill off the newer-evolved species with our "fix.")