This is a retread. It originally appeared in the January 2001 edition of my AH newsletter. Turtledove did something similar with Homo erectus in a series of short stories published quite a while before this was. Neanderthals have some different impacts because they were considerably more technically advanced than Turtledoves H. erectus.
Let’s say that somehow—and to be honest I don’t have the slightest idea how—a breeding population of Neanderthals reach North America. Time: sometime between 100,000 and 300,000 years ago. I initially visualized a hop from Europe to the Eastern US, but crossing Asia actually makes more sense. Some ecological barrier apparently kept Neanderthals from spreading across northern Asia to Siberia and from there to North America in our time-line, but if they somehow got past that barrier they could easily enough spread to the New World.
Neanderthals were a cold-adapted subspecies (or possibly species) of humanity. Think Eskimos but with less technology and more extreme physical adaptations to make up for the missing technology and you won’t be too far off.
Where do we go from there? The Neanderthals initially spread through the coldest parts of North America, areas that they are already adapted to. They find the hunting easy, because North American animals haven’t had a chance to adapt to human predators. In our time-line, the first Indians may have killed off 70% of the large animals of North and South America, either directly through hunting or indirectly by modifying the habitat or competing for scarce resources like water or sheltered areas for the winter. (That’s very controversial, with paleontologists splitting fairly evenly between a camp that blames climate change and one that blames the Indians for the die-off. I lean toward the Indians as the cause.)
In any case, Neanderthals are less technologically advanced than our Indians and somewhat less destructive. As they spread, they still change ecological balances in hundreds of subtle ways, just as the Indians did when they arrived in North America in our time-line. Large species quickly become rare as Neanderthals move into an area, and a few species that are already on their way out get pushed over the edge by the new elements in the environment. For the most part though, the Neanderthals have more subtle, long-term impacts on their new environment. The balance between slow but powerful animals like Ground Sloths and fast animals like horses and antelopes shifts in favor of fast animals.
The Neanderthal occupation of the Americas is much slower and less complete than the Indian occupation was in out time-line. The Neanderthals simply don’t have as flexible or advanced a tool-chest as the Indians did, so it takes them longer to adapt to new climates and new prey species. Ecological frontiers stop or reroute their expansion for generations, especially as they reach warmer climates. It takes them thousands of years to reach the southern tip of South America, and large parts of both North and South America remain uninhabited for even longer.