I think your depiction of existentialism is a little simplistic. The existential condition which we're supposed to recognize to live "authentically" can include our relationship to the environment, or our status in the universe. But existentialism isn't a single creed. It's more like a level of discourse, I think.

The evolution of life certainly seems like a natural process, so all living things are at some level part of the universe. But whether that process makes life meaningful depends, I think, on how that process tends to end. If living things have no metaphysically satisfying role to play, but are left accidently to observe the universe's mindless approach towards increasing disorder and lifelessness, as the stars will eventually devour their planets and wink out, that's not necessarily the kind of meaning we'd prefer. The meaning or purpose we'd prefer isn't so naturalistic or cosmic; on the contrary, we'd find meaning in rebelling against that larger pointlessness. Existentialism would enter the picture to show why we act personally and socially to create artificial meanings that oppose nature's absurdities.

The transhumanist speculation is interesting. I've pondered that question, too, since it raises a quasi-theological possibility that our cosmic role may not be so grim, after all. It's a sci-fi speculation, though, one that calls as much for religious faith as for science.