Jacquelyn Gill analyzes fungal spores in her lab.
If 13,600 years ago Eva and her people turn up in Mexico, when did they enter the Americas?
With no written texts from this time period or stone tool "breadcrumbs" aged older than Eva, how is it possible to know?
The answer is dung.
Fossil evidence tells us that during the Ice Age the Americas were home to more than 135 species of megafauna, including: mammoths, mastodons, camels, horses, giant ground sloths, and beavers the size of bears.
For thousands of years the dung they produced was being swept away by meltwater from the ice sheets into Indiana's Appleman Lake.
In 2010 Jacquelyn Gill from the University of Wisconsin, Madison, extracted samples of that lake sediment. It's because the dung of herbivores is naturally rich in fungal spores that can become preserved like tiny fossils that this dung is precious.
By placing her samples under a microscope, Jacquelyn and colleagues counted the numbers of fungal spores from different layers of lake sediment. What they saw was dramatic: 16,000 years ago the numbers dropped off.
Her conclusion was simple: the spore numbers dropped because North America megafauna were being hunted at this time. In other words, Eva's ancestors had arrived.
Their earlier presence in North America suggests the entry point into the Americas was in the North, and that they were moving southward as they colonized the double continent.