Shells are durable. How many examples of art have been unable to last so long? The way our consciousness develops progressively from egg to adulthood and beyond means there isn't a clear line we can draw between modern and primitive thinking. I'm not sure there was an event, a change in lifestyle, or a physical adaptation where we could draw a line, to mark the development of self awareness, no matter how accurate our definition.It's not as though the development of consciousness, or self awareness has stopped. It is still going on, but we generally manage to remain ignorant. Unwilling to believe there is any reason to improve.