It's complicated: How people first inhabited the Americas

In the largest examination to date of genetic material from ancient Americans, researchers found some genetic similarity among remains in Montana, Nevada and Brazil — showing there was likely a rapid migration of people from the tip of North America.

Why it matters: Researchers want to better understand how people first migrated, dispersed and settled into the different areas of the Americas. These genetic samplings — including some more than 10,000 years old — bolster some theories of what may have happened, but also bring forth new questions, particularly about new lineages that were discovered.

Background: Existing theories range from people arriving via what was then the Bering Land Bridge between Russia and Alaska or coming along the Pacific Coast — and at different times.

• Relatively new information has shown there were likely people in North America before the so-called Clovis occupants around 13,000 years ago.
• Previous genomic studies have suggested the first American populations diverged from their Siberian and East Asian ancestors nearly 25,000 years ago, and then split into distinct North and South American populations about 10,000 years later.

What's new: Three new studies published Thursday present comprehensive genetic evidence that reveal new insights into how people migrated to the Americas.

1. A study in Science analyzed 15 diverse bone samples — 6 of which are more than 10,000 years old — from sites across North and South America.

• The findings suggest the population expanded "very rapidly" thousands of kilometers south, but in uneven spurts, and then diversified into multiple population types.
• For instance, as co-author Eske Willerslev of the University of Copenhagen pointed out in a press briefing, genomes from the Spirit Cave Mummy found in Nevada are related to those of the Anzick child found in Montana as well as remains found in Lagoa Santa, Brazil.
• "The implication of that is that, if you’re moving that far that fast across space... there wasn’t anything to obstruct you or your movement, or mobility. And yet we know that we have people in the Americas prior to this time ..." says study co-author David Meltzer of Southern Methodist University.

"So that raises the really interesting question: Was no one else home? Yet we suspect archeologically there was and we’re actually getting some hints in the genetic record that there were other populations present as well."

• The researchers also discovered a population about 11,700 years ago that had Australasian ancestry evident only in South America (it hasn't been found in North America yet).

2. Another study, in Cell Press, examines 49 individuals spanning about 10,000 years in Belize, Brazil, the Central Andes and southern South America.

• The findings show a North American Clovis-associated genome (called the Anzick-1) from around 12,800 years ago "shares distinctive ancestry" with the oldest Chilean, Brazilian and Belizean individuals.

• The authors say this supports the hypothesis that the Clovis people also migrated to Central and South America. However, they add, because the Anzick-1 gene itself is not found throughout South America, this means there must have been at least one other founding group.
• They also say there was evidence that ancient humans from California's Channel Islands have significant genetic similarities with those living in the Central Andes at least 4,200 years ago.

3. A third study, in Science Advances, looks at when people may have begun living in the harsh extremes of the South American Andes.

• They compared ancient genomes of Andean highlanders to various other prehistoric and modern populations, and suggest permanent highland populations were established between 9,200 and 8,200 years ago, which is younger than earlier reports.
• Environmental stresses may have prompted genetic modifications in both the inhabitants' cardiovascular and digestive systems to help with high altitude and starch digestion.

The bottom line: The way in which people dispersed through the Americas is far more complex than current models predict, says Dennis O'Rourke from the University of Kansas. O'Rourke, who was not involved in the new research, tells Axios: "These are really important in helping us refine the simple models we have used in the past."

Go deeper:

• Native Americans may have arrived in the Americas via several routes
• To find the first Americans, search below the sea