Why do you think diverse and distinctly separate cultural groups grew, to be densely over populated, by approximately 9000 -3000 years ago (long before European contact), only along the western coast?

The number of completely different and unique languages, many hundreds, are found along the western coast. Do a bit of research online (be careful online, it’s a minefield of myths). Then tackle these two topics. Why do you think diverse and distinctly separate cultural groups grew, to be densely over populated, by approximately 9000 -3000 years ago (long before European contact), only along the western coast? What kinds of problems and issues might you find in a culturally diverse and densely populated area? 100-200 words