“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.

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.

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.

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.

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. 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. 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.

That’s impossible here, because was the only occupant of its particular chamber, save for a bird and some assorted rodents.