North America was entirely colonised by Denisovians 12,000 years ago.
White caucasians from Europe with super-hardened bones (Neanderthals) and the Greek Foot (Neanderthals) and strong forearms with impressive finely manufactured tools (Neanderthals) colonised the entire continent long before Indians arrived. The single most likely reason that Indians have such a generous allotment of Neanderthal genes is the rape of Neanderthal women - the same reason than European people all over the world have Neanderthal genes.
Neanderthals had control over their own birth rates. Other races of people didn't. When the climate warmed, these other races were able to swarm over geographic land masses that formerly barred their entrance and survival because of cold temperatures. Neanderthals never stood a chance in the face of these numbers and were rapidly overwhelmed despite being vastly superior hand-to-hand warriors, having superior weapons and being far superior in strength and agility. None of those things matter when you are fighting a thousand-to-one.
The biggest cull of the Clovis people was during the event that shows as a black line of diamond-laced dust in the fossil record around 12,000 years ago, when water supplies worldwide were rendered toxic by blue-green algae that thrived in the enormous amounts of iron dust that fell all over the planet. How smart were the Clovis people? Do you think they didn't know something was wrong with the water? A heap of their bones were found beside a well they had dug to filter lake water through deep clay. The water was still poisoned that emerged. Their bones are tinged green because of it. Only osmosis or distilling could have purified this water, a good thing to keep in mind in case it ever happens again. Another good reason to store water in tanks underground where it can't be contaminated.
Last of all and strangely significant, the Clovis people were a damned good-looking race of people, with far superior facial symmetry and a general harmony of features not seen in moderns at all. Their women would have probably looked angelic to the savages they fought against.