Comment by Symmetry

So, one the one hand all Earth life depends on proton gradients over membranes in some form or other for energy, as in the mitochondria powering us. But because prokaryotes and archea seem to share a common ancestor but use radically different cell membranes it looks like their common ancestor might not have generated its own membranes but used naturally occurring lipid membranes. And we'd probably want to call that common ancestor alive.