What came before what came before what came before? Read 1491 by Charles Mann available in a second reprint in paperback. Among the many revelations in this book is the environmentalist myth that Indians were passive and living lightly on the land before the incursion of the white man.
Central America was probably one of the most populated areas in the world, prior to the arrival of the Spanish and by 1500, the Inca Empire was larger than that of any other on Earth, including China.
What caused the population of the early arrivers in the North America to shrink so rapidly? Not guns but disease, especially smallpox. The Europeans had built up immunities, the last of the early arrivals like the Indians, Incas, had not and thus died in large endemics. What happened to the pre-Inca and pre-Indians can only be speculated on but diisease, cold, (glaciers) volcanoes, drought and famine, and warfare, probably played major roles.
An interesting read.