Your comment about the Cane Toads may have been made in a flippant manner, but in fact they are an example of how invasive species can spread quickly. And humans in the Americas were an invasive species.
I think that the reason nobody but modern humans made it to the Americas is a matter of physical adaptation. We and all of our extinct relatives evolved from African progenitors. Up until fairly late in the evolutionary process most of the species of Homo were basically warm adapted, and no matter how you cut it, getting to the Americas involved travel through a very cold climate in NE Siberia. Until the first Homo species invented clothing and good control of fire they couldn't have survived the climate.
The Neanderthals might have been able to, but they were for some reason limited primarily to Europe, so they didn't get close enough to find a land bridge when it was available.
We don't know enough about the Denisovans to really be able to tell what their abilities were. The few fossils we have (a finger bone and some teeth) don't tell us much about their culture. Most of what we know is due to a fantastic bit of luck. They were able to get DNA from the finger bone and sequence the genome. Comparing the DNA they recovered with modern human DNA they found some matches between the Denisovans and Humans. But only for people outside of Africa. And the strongest intermixture is for people in SE Asia. So it appears that the Denisovans didn't get very far North.
Bill Gill
