There is pretty strong evidence (especially, well dated footprints from 22,000 to 30,000 years ago) of modern humans in North America, at least prior to and during the Last Glacial Maximum ca. 20,000 years ago, long before the Founding population of the Americas should have arrived around 15,000 years ago.
West Hunter considers conditions that would fit their "failure to thrive" in the New World, as evidenced by a complete lack of human remains, few and marginal possibly human made tools, and a lack of mass extinctions, in "virgin territory" without competition from other hominins. He notes:
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 . . . .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.).
Statistical artifacts of the methods used and/or selective pressure against some Oceanian distinctive genes that make the remaining component look Onge rather than Papuan-like, could explain the closer f-test statistical matches to these populations. A relict non-Oceanian Paleo-asian population from Northeast Asia arriving in that time frame could work also but seems less plausible. Loss of distinctive Oceanian autosomal DNA during periods of stable population with low effective population size or during periods of shrinking populations during times of adverse conditions could also purge so statistically important genetic identifiers in what is already a small share of the total genome in the people who have it.
Further, some of the more convincing earlier pre-14,000 year ago finds of arguable stone tool making and fire use in the Americas were in North America (including Mesoamerica) or northeastern South America, not in the greater Amazon jungle basin where Paleo-asian ancestry has been found.
But the points he makes about a "Precursor" (I like Cochran's choice of words here) population's lack of success are still valid. There shouldn't have been Malthusian limits to their population in the New World that allowed them to sustain their population, but population growth would have had to be exceedingly slight in the long run to be consistent with the available evidence. He considers some possibilities and I underline the ones that I think are too implausible to take seriously:
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.
What do I think?
In a nutshell, the Precursor population was probably just slightly below the tipping point that they needed to thrive, in terms of population size, knowledge, and the resources they brought with them, but just large enough to establish a community that was marginally sustainable in the long run with inbreeding depression and degraded technology.
Franklin has proposed the famous 50/500 rule for minimum effective population size, which has become the threshold to prevent inbreeding depression. This rule specifies that the genetic effective population size (N(e)) should not be less than 50 in a short term and 500 in a long term.