Ex-ante, at the time of the first ("lower")species, it would have been logical to predict that "higher" life forms would probably arise at some time, if life on earth was to survive.
Since Carrol posits time travel, we must be allowed to take the thinking of Hegel,Darwin,Mendel Watson and Crick with us back in time.
So, ex-ante, we would have known 1)everything changes ;2)new species emerge from old species
based on making
better adaptions than old species make to changes in their environment; 3)New species arise from random mutations in the gonadal genes of the existing species.
So, we could have predicted (ex-ante)that 1) at many times in the future, there would occur changes in the earth's environment ; 2)at many times in the future there would occur random mutations in the existing species gonadal genes ;3) the now existing "lower" species would be less able than some of its mutant forms to adapt to some of the changes in the environment; 4) some better adapting, mutant forms' difference from the existing "lower" species would be the characteristic of being "higher" 5) Those "higher" characteristics would be a better adaption to one or many of the changes in the environment.
Ergo, there would come a time when relatively "higher" species would persist... for a while...
...but nothing lasts for ever, my fellow "higher" beings. (smile)