In the words of de Acosta himself:
We may easily infer by these arguments [speaking of the similarities of the faunal communities] that the first Indians went to inhabit the Indies more by land then by sea; or if there were any navigation, it was neither great nor difficult, being an indubitable thing, that the one world is continued and joined with the other, or at least they approach on nearer unto another in parts.
(Dixon quoting de Acostas,p.3)
It was the deductions of a 16th century priest that was the first to put forth the hypothesis that: 1)man was not indigenous to the Americas; 2)man arrived in the Americas via a land bridge following the migrations of the animals that were the main source of their food. De Acostas further assumed that the connection was in the northern regions of the Americas that had not yet been explored.
However, there remained the questions that de Acostas had not answered to the satisfaction of modern scientists; namely when did they arrive, and where did they come from. Slowly answers are being arrived at using tools and discoveries made recently.
It must be understood that new theories develop because of new finds that may contradict