The latest view of Jewish genealogy is that it was based on generational groups under the name of the patriarch of that generation. This means that we are not only viewing it according to the lifetime of the patriarch himself, but the lifetimes of all his descendants within that generational group. Because lifetimes were so much longer, up to 1000 years in one case, it is quite believable that the time frame could be many thousands of years more than Ussher and others calculated.
For example, many have wondered how Cain could have gone to the land of Nod and married a wife, when he had just killed Abel. The fact is, that there is no time frame mentioned between the killing of Abel and Cain going to the land of Nod. It could have been a couple of hundred years later, when there was a sizable population in the land of Nod.
Also, after the Flood, people had very lengthy lifetimes, and we don't know how long it was before the Tower of Babel event, nor how long before Abraham departed from Ur of the Chaldees to Canaan. We could be talking about thousands of years here.
Therefore, the Bible has many time-frame gaps, which could very well account for humans to be inhabiting the Americas 20,000 years ago. It is an established fact that they were, as well as the discovery of sea shells and fossilised sea creatures high up in the Andes mountains which shows that the sea covered them at one stage, and that could only have happened in a world-wide flood.
So, if we decide to believe the Bible to be true, we can say what took place as being true, but how these things happened cannot be reliably determined because we just don't have enough information to make that determination.