The fossil footprints around an ancient lake in White Sands have been known for some time, but now we have what look to be perfectly respectable C-14 dates. They’re about 22 thousand years old, close to the Last Glacial maximum (LGM) and, as such clearly predate all existing evidence of human settlement of the New World (south of the glaciers, anyhow).
There were already hints: Amerindian populations in South America, mainly in Amazonia, carry a trace of a different genetic heritage. The existing population closest to that trace are the inhabitants of the Andaman Islands, between India and Burma. Other populations such as Australian Aborigines and the inhabitants of New Guinea are also close. There is reason to believe that, until a few thousand years ago, all of Southeast Asia (including the islands) was occupied by related populations, known as Australo-Melanesians.
Here’s the key insight: the fact that the Andaman-like genetic trace is found in Amazonian Amerindians, but not in North America, suggests that it was picked up from a pre-existing population as the Amerindians expanded into South America. There are other possible scenarios, but it is hard to fit them with certain known facts. For example, we have ancient DNA from a Clovis kid, which is genetically close to modern Amerindians in South America, but does not contain this Andamanese-like trace. Hard to make this scenario work.
This implies that there was an earlier, less-successful colonization of the New World, one that preceded the Amerindians. One could have predicted this, and I did.
North America looked something like this in those days:
Note that crossing by boat would have been difficult at this time, assuming that settlers closely followed the coast. There is a long stretch of icebound coast before you reach somewhere habitable. The LGM was perhaps the most difficult time for a population to reach North America from Asia, so you have to suspect that they had crossed even earlier.
I say “less successful” because this population left very little sign, compared to the later Clovis culture. We find many artifacts and some skeletons from the early Amerindians, but very little from this (hypothetical) earlier population: in part this might be because their artifacts are harder to recognize ( because primitive) , but you have to think that their population density was considerably lower.
They may have done better in Brazil, because its climate was more favorable for hunter-gatherers than most places were back in the dreadful LGM.
One of the interesting differences between this somewhat hypothetical early population and Amerindians is that they seem to have been far less competent hunters. By the time of Clovis, Amerindians had atlatls and could apparently kill any animal, no matter how large. Ecologically dominant. In fact, they seem to have driven almost all of the megafauna in North and South America into extinction over a fairly short period of time.
This [hypothetical] earlier population may not have had atlatls simply because they hadn’t been invented yet: the earliest known example is about 18,000 years old. Considering that they may have entered North America some time well _before_ the LGM, this seems likely.
It may be that the alternative to being ‘ecologically dominant’ in the Americas was not very pleasant. The Amerindians could make a living hunting megafauna, and probably could deal effectively with the big predators sustained by those megafauna – at minimum, they were tough enough to make those predators think twice. After the extinction of the megafauna, most of those predators disappeared. This means that every other strategy of making a living – hunting lesser game, fishing, gathering plant foods – could be pursued without much risk. The whole landscape was theirs – the only thing they had to fear was other Amerindians.
For our hypothetical Precursors, this may not have been the case. Predators may have excluded them from much of the landscape, reducing their access to resources, which may not have been very abundant anyhow in the most severe part of the Ice Age.
It is safe to say that they didn’t leave a lot of skeletal fossils, since so far we haven’t found a single one. At the same time, any skeletal sample with useable DNA would be very valuable: as with the key Denisovan sample, where we have learned much about a whole separate branch of humanity from part of a little finger!
We can hope for luck finding skeletal fossils, but there may be another, more fruitful approach: looking aDNA in sediments in ancient sites. People have successfully retrieved ancient DNA from Neanderthal sites, and we may hope to do the same for pre-Clovis sites in the Americas. People ( other than James Bond) only die once, but they take a crap many times before their deaths. This approach might let us acquire genetic evidence from very early cultures, cultures with low population size, perhaps essentially failed colonizations or short-term explorations.