Single-celled organisms are almost infinitely more complex than, say a self-replicating RNA molecule. That again is vastly more complex than a protein or an ami

Single-celled organisms are almost infinitely more complex than, say a self-replicating RNA molecule. That again is vastly more complex than a protein or an amino acid. Similarly, a human is nearly infinitely more complex than a single-celled organism. Evolution causes organisms to fill an ecological niche. Simple niches for simple organisms will always exist, and simple life will always exist, even as the upper bounds of organic complexity trend unerringly upward. Life tends toward complexity, but that doesn't obviate the need for simple organisms.