I am finding tracking through the publication references to be pretty tough going.
I thought it might be helpful to post some general information on the topic of dating ancient DNA. In the context of this article ancient means on the ranging from tens of thousands of years in the past up to 100,000 or so, beyond which DNA is generally destroyed.
Why Don't We Know the Age of the New Ancient Human, Homo Naledi? - The Atlantic
The technique people are most likely to have heard of is carbon dating. It hinges upon the presence of carbon-14, a radioactive isotope of carbon that accumulates in the bodies of animals throughout our lives, and gradually decays after we die. By measuring the amounts left in a specimen, scientists can calculate when its owner died. The problem is that carbon-14 decays relatively quickly, as radioactive isotopes go, so this method only works well for samples this side of 50,000 years old. Homo naledi is likely far older than that.
Electron spin resonance dating
An alternative technique, known as electron spin resonance or ESR, requires no destruction and is great for dating teethwhich the team found plenty of. When the crystals in tooth enamel are hit by natural sources of radiation, like underground uranium deposits, the electrons inside them become excitedthat is, they move to a higher-energy state. Some become trapped like that. So, a tooth acts like a dosimeter for radiation, in a way that depends on two things: the levels of natural radiation in its environment, and how long it was buried for. If you know the former, you can deduce the latter.
Layer dating techniques
If adjacent bones provide no clues, the surrounding landscape might. In East Africa, hominid fossils are often preserved within layers of rock, like an opera gateau that took millions of years to bake. These layers include slices of ash deposited by erupting volcanoes. Scientists can date them by measuring radioactive isotopes of potassium within those layerswhich is the same principle as carbon-dating, but applicable to much older samples. And once they know the age of the layers, they can tell the age of the fossils sandwiched within them.
At least the first two techniques might reasonably apply to human dna in the range of dates that are of interest here. I have not found references to the DNA examined in the Nature article.
Edited by NoNukes, : change date range.
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