Why Don't We Know the Age of the New Ancient Human?

Because dating fossils is hard, and it’s not always possible.

Last Thursday, the world said hello to Homo naledi, a new species of ancient human discovered in South Africa’s Rising Star cave. As I reported at the time, scientists extracted 1,550 fossil fragments from the cave, which were then assembled into at least 15 individual skeletons—one of the richest hauls of hominid fossils ever uncovered.

But one significant problem clouded the excitement over the discovery: The team doesn’t know how old the fossils are. And without that age, it’s hard to know how Homo naledi fits into the story of human evolution, or how to interpret its apparent habit of deliberately burying its own kind. Everyone from professional paleontologists to interested members of the public raised the same question: Why hadn’t the team dated the fossils yet?

The simple answer is: Because dating fossils is really difficult. Scientific papers and news reports about new fossils so regularly come with estimates of age that it’s easy forget how hard-won such data can be. I asked John Hawks, a biologist at the University of Wisconsin and one of the heads of the Rising Star expedition, to talk me through the various available methods—and why they have been difficult to apply to the latest finds.

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.

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 layers—which 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.

But in southern Africa, hominid fossils are almost always found in caves like Rising Star. Here, there are no convenient volcanic layers. Instead, the bones are typically embedded within breccia—a concrete-like mixture of gravel, sand, and other junk that accumulated in the floor of the cave.

These blocks might still be surrounded by informative layers called flowstones—sheets of calcite formed when water drips down the walls and floors of a cave. The water carries soluble uranium, which remains in the flowstone and decays over time into thorium and lead. Again, the levels of these elements reveal the age of the layer. “If you've got a fossil, and it's in breccia, and there's flowstone over and under it, you've got a bracket for age,” explains Hawks.

That was certainly the case for Australopithecus sediba, the hominid that Lee Berger discovered in another South African cave in 2008. It was the fossil filling in a delightfully thin flowstone sandwich, bracketed by slices just a thousand years apart in age. “It spoiled us,” says Hawks. “You had this instant that was captured in time. It's the best-dated site in South Africa.” Homo naledi isn't being quite as cooperative.

Good news: There are certainly flowstones over the Homo naledi fossils, which should hint at their minimum age. Bad news: The bones aren’t preserved in breccia but in much softer sediment, so they may have shifted from their original locations with respect to the flowstones. Worse news: The team doesn't know if there are flowstones under the H.naledi  fossils because they haven't dug down that far yet. Without such layers, they can’t estimate the maximum age of the fossils.

Their plan is to use radar to visualize the floor of the chamber and work out where they can take samples from its very bottom. “It could be that nature will cooperate and the maximum and minimum ages will be close to each other,” says Hawks. “Or they could be millions of years apart.” He’s bracing himself for the latter.

Flowstones aside, the team can also look for layers of magnetic minerals, like particles of iron. The alignment of these minerals depends on the Earth's magnetic field, which has repeatedly flipped direction over the eons, so that north becomes south and vice versa. We know a lot about the history of these reversals, which we can use to date layers of magnetic sediment. The Rising Star team is going to try this approach, but it’s laborious work that will take a lot of time.

That’s arguably true for all of these techniques. They’re all challenging and they all depend on luck, on specimens being preserved in just the right way, or in just the right rocks, or in just the right timeframe.

“People want to know why this is a problem?” says Hawks. “Little Foot is the best example.” He’s talking about an Australopithecus skeleton that was found in South Africa’s Sterkfontein cave in the 1990s. Over the last decade, many people have tried to date the specimen using animal remains, paleomagnetism, flowstones, and more, and their estimates differ by around 2 million years depending on the technique they use. “The different lines of evidence just don't agree,” says Hawks. “We're really hoping to avoid that scenario. We’re committed, as a team, to not publish a date estimate until we have multiple estimates that arrive at the same result.”