What are the reasons for the similarity between the nature of northern Eurasia and North America?

The reason for the similarities existing in modern times plants and animals of the two continents, as well as many fossil species, is that these territories have been connected for a long time. In the geographical past, they were first united into one continent under the name of Laurasia, and after the split of this supercontinent, where the Bering Strait is now, there was a natural bridge, and animals could freely migrate along it between the continents. The indigenous peoples of America also crossed this bridge.