system
Given the following post, respond as if you're commenting on Reddit:
human
If the Earth rotated on its axis where one side always faced the sun, and the other side was always dark, how might that have affected evolution?

Just for one, I'm assuming sleep cycles as they exist today would never have come about.. But what would that mean? Is it possible these organisms would require no sleep? Or is it likely they would still form some type of resting? And what would that likely be influenced by?  
  
What would be the likely differences between the light and dark side of Earth? Would any of what we have on the world today even be possible without constant 24-hour rotation?
gpt
We don't even know what climate and weather are like on tidally locked (same side always facing the star) worlds, so any discussion of biological evolution on such a planet is beyond speculative.

This is an interesting question, because many exoplanets that we've discovered are expected to be tidally locked to their stars.  Those that might be habitable but still tidally locked are generally in tight orbits around dim, low mass stars.  We have no idea if only the terminator (a ring on the planet where it's always "sunset" or the whole planet might be habitable--or at least harbor liquid water.