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.
In a commentary, Chris Stringer from the Natural History Museum of London said that the Rising Star team could have tried carbon dating “even if only to test whether the material lies beyond the effective range of that method.” Hawks says they plan to but says the technique “involves destroying material, and we didn't want to do that until we had published a description of the species.”
They’re also going to try to extract DNA from the fossils themselves. The study of ancient DNA has repeatedly revolutionized our understanding of human evolution, revealing the presence of Neanderthal DNA in all modern humans outside Africa, and the existence of an entirely new hominin species—the Denisovans. Homo naledi’s DNA would reveal its evolutionary relationships to ourselves and other ancient humans.
“We're investigating it, but it’s not a hopeful scenario,” says Hawks. The chances of finding intact DNA are higher when “it’s dry and super-cold, and we’re wet and warm,” says Hawks. “But the bones are exceptionally preserved, so if there's a chance of finding ancient DNA anywhere in southern Africa, it’s here.”
An alternative technique, known as electron spin resonance or ESR, requires no destruction and is great for dating teeth—which 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 “excited”—that 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.
But knowing the natural radiation levels is “sort of nightmarish,” says Hawks. It involves, for example, installing actual radiation dosimeters and taking out vertical cores of sediment. And even then, the results from ESR typically need to be cross-checked against other sources of data.
Paleontologists can sometimes date a new fossil by looking at its companions in death, by finding nearby bones of other extinct animals that died within a known timeframe. That’s impossible here, because Homo naledi was the only occupant of its particular chamber, save for a bird and some assorted rodents.