Ruminating on life, evolution, sentience, and intelligence can be interesting as well as mind boggling. Mankind has defined the conditions required for life to exist, and is continually updating that definition as we discover how persistent life actually is. Sentience, however, is a whole different issue.
Sentience is defined as the ability to feel, perceive, or experience subjectively. If anyone wants to get really confused, look up the word subjectively. Anyway, most of us simplify the meaning of sentience to be a condition of being self-aware. Isn't humans trying to define sentience, kind of like an artist painting a self-portrait without ever seeing a reflection of their face? If the presence of ego is needed for sentience, then humans certainly qualify.
How will we determine sentience when we meet life-forms that have evolved on other worlds? And, how does sentience evolve? Is attaining sentience the condition that biological organizations, like humanity, strive for, or is it simply a rest stop on the road of evolution?
Let's change direction. Does anyone question that there is life on other planets? I don't know of anyone that does. The only real question is, how has life evolved on other planets? The big question is, has any of that life evolved into sentience? We can guess at the evolution of other life, but sentience isn't really related to how life evolves.
In the thirteen billion plus years since the big bang, how many different kinds of life forms have evolved in our galaxy, or in the universe? Billions of worlds, with every kind of environment imaginable, could have spawned life in unimaginable ways, and in unimaginable places. If just a tiny fraction of that life evolved to sentience, then how many sentient species have evolved? Your guess is as good as mine, but the only limit is your imagination.
Change direction again. Hang on ... String theory is a theoretical mathematical framework in the field of physics. The set of theories hints at other dimensions that exist alongside our own reality. Assuming string theory is valid, could life also exist in those other dimensions? There is no reason to believe that life wouldn't evolve there, but what kind of life would it be? Once again, our imagination is the only limit to imagining what that life may be.
Give your imagination free rein, and think about life, and sentience, and evolution. We are surrounded by a vast universe of possibilities. Adding the hints of other dimensions only increases the scope and range of those possibilities. All of these questions are relevant to the story that follows, and may even have a glimmer of some answers.
Enjoy the story.