Final answer:
The theories of Thor Heyerdahl and Erich von Daniken both lack evidence and involve external influences on human accomplishments. Mainstream research attributes achievements to human ingenuity instead. Scientific explanations for human migration to the Americas are widely accepted and evidence-based.
Step-by-step explanation:
The theories proposed by Thor Heyerdahl and Erich von Daniken both suggest that ancient civilizations or beings apart from the indigenous peoples of the Americas may have influenced the development of those civilizations. These theories, while lacking substantial evidence, share a common thread: the belief in external or extraordinary influences on human accomplishments. Despite their popularity in some circles, these ideas are not supported by mainstream historical or archaeological research, which credits the ingenuity and adaptability of human beings themselves for major historical achievements, like the construction of the Great Pyramid at Giza, or the establishment of complex civilizations in the Americas. Scientific explanations, such as the Bering Land Bridge and the coastal migration theories, are based on a variety of data, including archaeological and genetic evidence, and remain the most widely accepted models for human migration to the Americas.