The land bridge to Asia was available from ~30,000 YA to ~10,000 YA. There have been no boats yet found of that antiquity so we presume the migration from Asia couldn’t have happened without that land bridge.
Also, there was no blood group B among the first Americans. The original South and Meso American Indians were all type O. The original North American Indians were either O or A. These facts have fueled the hypothesis that the migrations came in two small but separate and closely-related groups.
There have been human remains found in the nether reaches of South America that still bore Asiatic features and were carbon dated to too soon after the presumed +/-15,000 YA Bering Strait crossing hypothesis for them to have migrated the necessary 8,000+ miles from the crossing point. In other words, there is archaeology to support the hypothesis that humans were in the Americas well before 15,000 YA.
As for the slow rate of development of primitive man, no great civilization can exist without a cereal grain and domesticated meat-producing livestock. Cereal grains because it is the technologically simplest form of food that can be stored for years and still be consumable, and meat livestock because they convert cellulose into a concentrated source of calories that humans can digest. But there were no cereal grains until primitive man discovered natural mutations of seeded grasses.
Somewhere in Southwest Asia, a wheat grass naturally crossed with a goat grass, resulting in a fertile hybrid with a large head full of seeds known today as emmer wheat. Humans began domesticating emmer in about 8000 BC. The domesticated emmer crossed naturally again with another variety of wild goat grass, producing another fertile hybrid, bread wheat. Which gave man the first cereal grain crop, which was the beginning of agriculture. It also gave them a vital building block necessary to the creation of cities and civilization.
And that’s why human development was so slow for most of 200,000 years (NOT 3 MILLION) since H. Sapiens arose. Because nomadic tribesmen, people whose life needs are met day-to-day, don’t see much need for innovation. Farmers who cultivate the same plot of land year in and year out do.
Real intellectual progress was only possible once a there was means for thousands of people to live together in a common community where people could specialize in different roles. And all of that specialization starts with farmers raising cereal grains and livestock, producing far in excess of their own needs, so that not everyone has to spend their days seeing to it that their belly gets filled.
Very interesting and persuasive post. My hypothesis was that it was the desertification of the Sahara which forced the people living there to migrate to the Nile, thereby creating the “critical mass” of population which you allude to. But that didn’t explain the Mesopotamia civilization, the Indus civilization, or the civilizations of the Far East and central/southern America, and other interests and consequent lack of time resulted in me not investigating that.