Elton:

If you can point to reliable evidence of human occupation of the Americas at 250 KYA, you'll overturn the entire field of archaeology.  Even the most aggressive interpretation of sites like Pedra Furada only goes back perhaps 48 KYA -- and those interpretations are both doubtful and hotly contested.  The earliest uncontested occupation of the Americas is about 14,800 BP at Monte Verde.

Frankly, I find assertions of high-tech in layers where people are chipping stone points to be utter bunk.  All the "mysteries" of how the pyramids or Stonehenge were built turn out to be lots and lots of people rolling things around on logs and understanding the principle of leverage.  My mind isn't closed -- but there's a very large burden of proof to meet.

JS

Kenneth:

That's a specific case of the general problem, which is that humans are changing our environment much more quickly than we can adapt to it.

JS