Anthropology

crossing from Siberia to North America does not say that they were a culturally and bioligically homogenous group. I'm not sure why you think that that's part of the theory.

DNA studies show 5 different mtDNA haplogroups, associated with varying locations in Asia. That is probably due to migrations that took place in Asia before people began to enter North America.

There was a period of time when people in the now submerged bridge of Beringia intermixed while glaciers blocked further eastward movement into North America. But Beringia covered a large area and even with some intermixing culturally and biologically, some variations would have remained. Once in the vast continent of North America, with its variety of climates and resources, the need to adapt accounts for how even more variations developed.

Theories of entry into the Americas are not limited to crossing the land bridge of Beringia. The presence of human footprints in New Mexico that date back 22,000 years ago indicate that there were people on North America before the ones who arrived across Beringia. Many anthropologists are accepting the idea of people arriving by boat, before the Beringian arrivals.

I respect the right of the indigenous people of the Americas to have their own beliefs about their origins. Those beliefs are part of a cultural identity that has meaning for them. But, from a scientific perspective, there could not have been human beings in the Americas from the time that the continents were formed. Humans originated in Africa and spread around the world from there. Human beings around the world look different today due to evolutionary adaptations to their environments as they migrated to different places, to localized mutations that spread within some groups (like the mutations that resulted in variations of eye and hair color), and to some groups being isolated from others.

For Native Americans to have "always" been in the Americas suggests that human evolutionary ancestors like Homo Erectus were here before modern humans arrived and evolved separately from them. But biology and archaeology do not support that possibility.

As I said earlier, I respect the right of people to have their own cultural beliefs about their origins that form their identity as a people. But I also respect the evidence of science.