There's only one primate species currently in the continent. Us. Other mammalian groups -- carnivores, rodents, ungulates, insectivores, bats, and so on -- have done fine here, flourishing and diversifying and lasting for tens of millions of years.
Primates haven't been so successful.
The first primates -- well, proto-primates -- in North America were the plesiadapiformes, which first appear in the fossil record in the early Paleocene Epoch, right after the Chicxulub Impact pretty well wiped out all the big animal species (most notably, the non-avian dinosaurs). To modern eyes, they would have looked a bit like squirrels:
[Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Plesiadapis NT, CC BY-SA 3.0]
Despite the superficially squirrelly appearance, their skulls, and especially their teeth, show clear affinities with primates, not with rodents.
These guys were widespread, living throughout North America, Europe, and Asia. All of those continents were still connected at this point -- what had been Pangaea had broken into a northern continent (Laurasia) and a southern continent (Gondwanaland, made up of what are now South America, Australia, and Antarctica). But things were changing, as they are wont to do. The Central Atlantic Magmatic Province had kicked into high gear, rifting Laurasia and splitting what would become North America from the rest of the continent, opening up the North Atlantic Ocean. At that point, the primate species (and everyone else) in North America were pretty well stuck there.
And they lasted a while. But at the end of the Eocene Epoch, around 34 million years ago, the North American continent got significantly cooler and drier. This drove all the warmth-loving native primates to extinction.
[Nota bene: South American monkeys come from a different lineage. Recall that at this point, North and South America were pretty far apart, and there was a lot of ocean in between. South America was a great deal closer to Africa, though -- and was colonized by primates from Africa, probably by monkeys and other species clinging to rafts of plant roots and brush torn loose during storms. They seem to have made this amazing journey in several pulses, starting about thirty million years ago. In any case -- the genetic and structural evidence is clear that South American monkeys are related to primates from Africa, not the extinct groups in North America.]
In any case, North America was primateless for about four million years. Then, suddenly, a primate appeared in what is now Nebraska. This species, named Ekgmowechashala (the name is Sioux for "little cat man"), weighed about three kilograms, and looked a bit like a lemur. But where the hell did it come from?
The whole topic came up in the first place because of new research into this odd creature, which appeared in the Journal of Human Evolution last week. A thorough analysis of Ekgmowechashala fossils dating from around thirty million years ago found that they most closely resemble primate species in China and eastern Siberia. Apparently, the ancestors of Ekgmowechashala did what the ancestors of the Native Americans would do, millions of years later. They took advantage of the fact that the cooler conditions locked up more sea water in the form of ice, lowering sea levels. Among other things, this turned what is now the Bering Sea into a broad valley with rolling hills (nicknamed Beringia), allowing them to cross into North America.
"The 'Lazarus effect' in paleontology is when we find evidence in the fossil record of animals apparently going extinct -- only to reappear after a long hiatus, seemingly out of nowhere," said Chris Beard, of the University of Kansas, who was senior author of the paper. "This is the grand pattern of evolution that we see in the fossil record of North American primates. The first primates came to North America about 56 million years ago at the beginning of the Eocene, and they flourished on this continent for more than 20 million years. But they went extinct when climate became cooler and drier near the Eocene-Oligocene boundary, about 34 million years ago. Several million years later Ekgmowechashala shows up like a drifting gunslinger in a Western movie, only to be a flash in the pan as far as the long trajectory of evolution is concerned. After Ekgmowechashala is gone for more than 25 million years, Clovis people come to North America, marking the third chapter of primates on this continent. Like Ekgmowechashala, humans in North America are a prime example of the Lazarus effect."
So the "little cat man" didn't last very long -- the continual cooling of the climate, peaking with the repeated continental glaciations of the "Ice Ages," was more than primates could cope with. But as Beard points out, that didn't stop our own species from doing the very same thing, eventually colonizing all of North America, and more inhospitable places yet.
But it's odd to think that thirty million years ago, there was something very like a lemur living near what is now Omaha.