The problem with the idea of an early, pre-Amerindian settlement of the Americas is that ( by hypothesis, and some evidence ) it succeeded, but ( from known evidence) it just barely succeeded, at best. Think like an epidemiologist ( they’re not all stupid ) – once humans managed to get past the ice, they must have had a growth factor greater than 1.0 per generation – but it seems that it can’t have been a lot larger than that, because if they had averaged, say, 3 surviving kids per generation ( r = 1.5) , their population would have exploded, filling up all the habitable territories south of the glaciers in less than 2000 years.
(1.5)^40 multiplies the original population by a factor greater than ten million !
A saturated hunter-gatherer population inhabiting millions of square miles leaves a fair number of artifacts and skeletons per millennium – but we haven’t found much. We have, so far, found no skeletons that old. I don’t think we have a lot of totally convincing artifacts, although I’m no expert at distinguishing artifacts from geofacts. ( But these were modern humans – how crude do we expect their artifacts to be?)
For-sure footprints we’ve got, and intriguing genetic data.
A priori, I would expect hunter-gatherers entering uninhabited America to have done pretty well, and have high population growth rates, especially after they become more familiar with the local ecology. There is good reason to think that early Amerindians did: Bayesian skyline analysis of their mtDNA indicates fast population growth. They were expert hunters before they ever arrived, and once they got rolling, they seem to have wiped out the megafauna quite rapidly.
But the Precursors do not seem to have become numerous, and did not cause a wave of extinctions ( as far as I know. check giant turtles.). What might have limited their biological success?
Maybe they didn’t have atlatls. The Amerindians certainly did.
Maybe they arrived as fishermen and didn’t have many hunting skills. Those could have been developed, but not instantaneously. An analogy: early Amerindians visited some West Coast islands and must have had boats. But after they crossed the continent and reached the Gulf of Mexico, they had lost that technology and took several thousand years to re-develop it and settle the Caribbean. Along this line, coastal fishing settlements back near the Glacial Maximum would all be under water today.
Maybe they fought among themselves to an unusual degree. I don’t really believe in this, am just throwing out notions.
Maybe their technology and skills set only worked in a limited set of situations, so that they could only successfully colonize certain niches. Neanderthals, for example, don’t seem to have flourished in plains, but instead in hilly country. On the other hand, we don’t tend to think of modern human having such limitations.
One can imagine some kind of infectious disease that made large areas uninhabitable. With the low human population density, most likely a zoonosis, perhaps carried by some component of the megafauna – which would also explain why it disappeared.