How does the existence of a site like cactus hill demonstrate the challenges that archeologist face making theories ?

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Cactus Hill site has human remains that have been dated to around 20,000 years ago. Given its location in Virginia shows that human beings had reached America earlier than this. This contradicts the theory that postulates humans came to America from Siberia through the modern day Bring Strait. The theory asserts that at the end of the Ice age humans were able to cross to America through the Beringia land bridge and other means such as canoes and rafts around 10,000 years ago.  

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