How could myths of indigenous religions be so similar with no contact among them, and why are the similarities significant?

First, it should be noted that it is far from clear that remote indigenous cultures had no contact at all. For example, Egyptian mummies from 2000 BCE and earlier frequently test positive for cocaine, which derives from a plant that only grows in the Americas. While later Mesoamerican cultures were not synchronous with ancient Egypt, oral traditions can live on for centuries and may explain some similarities. Additionally, there are hints that prehistoric cultures may have been capable of ocean travel, with the Chinese possibly visiting western North America, and so on. So we really don’t know whether any indigenous culture was truly isolated. Modern bias tends to attribute little capability to ancient peoples.

With that said, however, some patterns do seem to repeat across cultures that have had no apparent contact. One possible explanation is that there’s something “genetic” about these patterns, for example, that building massive stone monuments in pyramidal shape is somehow “obvious” or “inevitable.”  Similarly, traditions from across the world have recollections of a great flood that devastated humanity, leaving only a single family or a small number of families as survivors. More likely, however, is that such stories and traditions have some kernel of truth in them that represents a trace of actual historical experience. It seems likely, for example, that at the close of the last Ice Age, the rise in sea level as glaciers melted could have triggered any number of dramatic floods, and since human beings like to live along coastlines, lakes, and rivers, it should not be surprising to note a lot of flood myths. Also, Jung believed that there were universal archetypes that all people could tap into at an unconscious level. If true, that would mean that on a fundamental level, all humanity really is connected, and that too would explain the universality of patterns in indigenous myths. No matter what the link is among people, however, the key significance of such myths is that it demonstrates the ultimate brotherhood of all of humanity. We are in some way connected—whether via unknown cultural interactions or shared genetics or a common unconscious. And that means that human beings are all one family in truth.