  Consciousness is presented not as a unified and uniquely human
characteristic, but rather as an emergent property of several building blocks,
most of which are demonstrably present in other species. Each block has its own
rationale under natural selection and could have arisen independently, and the
jumps between blocks -- which culminate in consciousness -- are small enough to
be evolutionarily plausible. One underappreciated block involves unconscious
engram playback and discrimination, and plays a major role in brain storage
optimization. This function is present in birds and nearly all mammals and is
recognized by its side-effect: dreams.
