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.).
I've underlined some thoughts that I don't agree with fully. It looks like the Younger Dryas was a much bigger factor in mass extinctions relative to the overkill hypothesis than we previously expected.
And then there is the genetic evidence. Unlike Cochran, I am quite convinced that it is basically mathematically impossible for this pre-Founding population to have been the source of "Paleo-asian" autosomal DNA in South America, which is much more likely to have arrived ca. 1200 CE via Oceanians (although other possibilities in that time frame could work as well). The frequency of the Paleo-Asian component is just too variable. And, any population that stayed isolated population genetically for more than 14,000 years to finally burst out into South America would be incredibly genetically distinctive (much like the Kalash people who are incredibly genetically distinctive after their small population was isolated for much less long a time period than that).
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.
I think the that Precursors were probably derived from one or more small expeditions from a population close to the ultimate Founding population of the Americas rooted in Northeast Asia that survived where many who left no trace at all died, as something of a fluke.
The Precursors probably had no dogs or other domesticated animals on their boat(s). But dogs were, in my opinion, probably a major fitness enhancing technology for the post-Papuan/Australian aborigine wave of modern humans in mainland Asia (the Papuan/Australian wave didn't have dogs), and for the Founding population of the Americas.
They probably had a small founding population that suffered technological degradation similar to what Tasmania experienced when it separated from Australia for 8,000 years, including the loss of maritime travel technology needed to reunite with kin left behind in Asia or Beringia. See Joseph Henrich, "Demography and Cultural Evolution", 69(2) American Antiquity 197-214 (April 2004) but see Krist Vaesen, "Population size does not explain past changes in technological complexity" PNAS (April 4, 2016) (disputing this conclusion, unconvincingly IMHO).
For example, as Cochran notes, they may have known how to fish and hunt from boats, but not how to make boats, or how to hunt terrestrially, at first.
The effective population size of the Founding population of the Americas was ca. 200-300 people; Henrich's hypothesis sees major degradations in technology as the population size falls significantly below 100, which would have been a typical size for a one-off exploratory expedition that was stranded and unable to return. Once technology is lost, it can be recovered or rebuilt over time, but it takes much longer to innovate than to preserve culture transmitted from previous generations or to imitate neighboring civilizations.
The loss of technology may not simply have been a matter of raw numbers either. Vaesen's counter-examples are very small, stable, complete hunter-gather communities. But an exploratory expedition may have consisted of bold young people not fully trained in reproducing their culture's technologies, even if they had enough raw numbers of people to do so, rather than the tribe or band's skill craftspeople.
They probably suffered inbreeding depression greater than the main founding population of the Americas due to a smaller founding population size. A scientific report in Nature (March 5, 2019) notes that:
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.
The possibility that the Precursors derived from one or more expeditions with an effective population size less than 50 in the short term, regardless of its long term size, seems plausible. Even if there were several dozen men on the expedition, it is very plausible that there could have been fewer than twenty-five reproductive age women on the expedition at the outset needed to avoid short run inbreeding depression, and many of them were probably cousin and/or siblings.
These inbreeding depression effects could have lasted many, many generations without input from an outside population source, leaving a much less smart, much less fit group in the next few generations than in their first generation.
They probably faced challenges to thriving at hunting and gathering due to the ice age that caused the Last Glacial Maximum that their degraded technology didn't help them to overcome. They may have landed in North American fairly near to the glacial area that was particularly impaired.
They probably did go extinct in all, or almost all, of their range, after not too many generations. They may not have reached South America until after the Last Glacial Maximum at all, and if they did, may not have penetrated very far into it, sticking to the Gulf Coast.
To the extent that they didn't go extinct, they probably lost many of their uniparental genetic markers during sustained periods of stable low populations or population busts, as opposed to the preservation of the markers usually found in expanding populations.
They also probably weren't all that genetically distinct from the Founding population of the Americas from which they were only separated for a few thousand years. A small effective population takes many generations to generate distinct mutations and both the Founding population of the Americas and the Precursors would have had small effective populations for most of the Last Glacial Maximum ice age.
The Founding population, due to a larger founding population, better technology retention, less inbreeding, dogs, and better climate conditions, expanded rapidly. When the much more advanced Founding population arrived, the remnants of the Precursors that remained, if any, would have been diluted almost invisibly into the very genetically similar Founding population and may have died out from competition, or at least, shed any of its distinctive uniparental markers, with it as well.
"Failure to thrive" is a phrase most commonly used in medicine to describe phenomena without a specific and well-determined cause of a child's lack of development at the pace of normal children in health environments. Usually, it is attributed to poor nutrition either in quality or quantity.