South American Genetic History

I remain skeptical that the Australasian ancestry is as ancient as claimed. It is much less than 2%, maybe a hundred times less, and the regional variation in its frequency is far too great for it to be ancient. I suspect an origin in Polynesian sea farers that may be obscured by natural selection against some signature Polynesian genes. 

I have not yet seen any solid evidence that it has been present for 10,000+ years. South American ancient DNA samples are few and far between at that time depth.

I'll examine the two source articles at greater length if time allows (which it might very well not).

[A] study published today in Nature reveals these migrations were anything but simple. Examining ancient and modern genomes collected from across South America and beyond, the team found that genetically diverse groups populated the continent in at least three separate pulses. And some people or communities carried with them possibly advantageous genes acquired from long-ago Australasian ancestors. . . .

His team published a complementary study today in Current Biology, finding evidence of unexpected genetic diversity and otherwise invisible migrations in 52 ancient genomes from Argentina and Uruguay.

In the Nature study, Tábita Hünemeier, a geneticist at the University of São Paulo and the Institute of Evolutionary Biology, collaborated with researchers and Indigenous communities across Latin America to sequence 128 whole genomes from living people from north Mexico to southern Argentina. The team then analyzed them alongside existing databases and previously published ancient genomes.

Previous work had identified the first two waves of settlement in South America, the earliest of which included people related to the Anzik child, who was buried in Montana 12,700 years ago. A second dispersal followed about 9000 years ago and ultimately contributed more to the genomes of most ancient and modern South Americas, including those Posth studied.

Hünemeier and her team found evidence of a third dispersal, whose genetic signature first appears in their data about 1300 years ago and then spreads widely across the continent and even into the Caribbean. The newcomers show hints of being related to Mesoamericans from Mexico and Central America, but so far, researchers don’t know exactly where they came from or who were their closest relatives. “Without the source population and more direct evidence [of a third pulse] from ancient DNA, it’s hard to really wrap our heads around” how and when a third migration might have happened, Posth says.

The study also digs deeper into a mystery that has bedeviled the genetic history of the Americas for over a decade: How did traces of Australasian ancestry end up in some ancient and modern South American genomes? Genetic variants from this lineage make up only about 2% of ancestry in the people who carry it, but that proportion has stayed remarkably consistent over the past 10,000 years. “This signal is found again and again and again,” Posth says. “It must mean something.”

Hünemeier suspects people carrying this ancestry were among several distinct populations that lived for thousands of years in Beringia, the now-drowned landmass that connected eastern Siberia to Alaska, and that it eventually spread southward into the Americas from there. (This Australasian ancestry, sometimes known as Population Y or the Ypykuéra signal after the Tupi word for “ancestor,” is different from the genetic sequences some Polynesian populations share with South American ones. Scientists continue to debate how that more recent gene flow happened—for example, whether Polynesian voyagers may have reached western South America about 800 years ago—but the findings from the Nature paper have no bearing on that mystery.)

From Science.

Astronomy Meets Climate

A 100,000 year climate cycle on Earth can be explained by Earth's orbit.

The 100,000-year problem concerns the dominant period of glacial-interglacial cycles over the past 800,000 years and their correlation with Earth's orbital eccentricity, despite eccentricity's weak influence on solar radiation. 

Two theories compete: the astronomical theory, in which orbital forcing drives the cycles with amplification from Earth system feedbacks, and the geochemical theory, in which internal dynamics dominate with orbital forcing synchronising oscillations. We investigate these theories using conceptual models. 

Augmentations to the Budyko energy balance model fail to reproduce the 100,000-year period, revealing formulation limitations. Linearised versions of existing non-linear ice volume models perform comparably to their full counterparts, indicating the data does not necessitate non-linear dynamics. We develop two simple linear models: a feedforward model aligned with the astronomical theory and a feedback model aligned with the geochemical theory. 

The feedforward model reproduces the ice volume record well and offers a novel explanation for the absence of eccentricity's 400,000-year period, arising from oceanic heat storage and tropospheric energy responding with differing phase lags. Conservative estimates show bulk ocean temperature variation can be explained by eccentricity alone, challenging the geochemical theory's core assumption. 

We also show that widespread use of Q65 may bias models towards geochemical explanations by underrepresenting eccentricity. The feedback model's improvement is concentrated around Marine Isotope Stage 11, suggesting this anomalous interglacial reflects Earth-based events rather than a general requirement for feedback mechanisms. We conclude that 800,000 years of glacial cycles can be largely reproduced by a linear astronomical model, emphasising the importance of parsimony when interpreting palaeoclimate data.

Liam Wheen, "A First Principles Approach to the 100,000-year Problem" arXiv:2604.12143 (April 14, 2026).