I felt his book was more arguing against racist notions of developmental inequality by painting a picture of how systemic and environmental factors influence societal evolution. We've learned more but the central thesis holds..

That said, do you know of a more current layman-accessible book on the topic?

One possible way, but the one best supported by evidence. The west coast of North America continues to yield evidence of human settlement and migration, sometimes just offshore. Genetics also supports the out of Siberia hypothesis.