An academic writing in Nature had noted how a paleontologist, asked by a colleague whether he thought an old skull was varnished or not,
had licked its top and announced that it was.
"In the process," noted the Nature article, "large amounts of modern human DNA would have been transferred to the skull," rendering it useless for future study.
I asked Harding about this. "Oh, it would almost certainly have been contaminated already," she said.
Just handling a bone will contaminate it. Breathing on it will contaminate it.
Most of the water in our labs will contaminate it.
We are all swimming in foreign DNA.
In order to get a reliably clean specimen you have to excavate it in sterile conditions and do the tests on it at the site.
It is the trickiest thing in the world not to contaminate a specimen.
So should such claims be treated dubiously? I asked. Harding nodded solemnly. "Very," she said.
If you wish to understand at once why we know as little as we do about human origins, I have the place for you.
It is to be found a little beyond the edge of the blue Ngong Hills in Kenya, to the south and west of Nairobi.
Drive out of the city on the main highway to Uganda, and there comes a moment of startling glory when the ground falls away
and you are presented with a hang glider's view of boundless, pale green African plain.
This is the Great Rift Valley, which arcs across three thousand miles of east Africa, marking the tectonic rupture that is setting Africa adrift from Asia.
Here, perhaps forty miles out of Nairobi, along the baking valley floor, is an ancient site called Olorgesailie,
which once stood beside a large and pleasant lake.