There’s a special place in South Africa called Border Cave where our ancestors already in modern human form lived continuously for nearly a quarter million years.

Excavating in the cave is like reading backward in a history book, finding evidence of each earlier generation in each deeper layer.

The cave has yielded 69,000 artifacts, a treasure trove of clues to human prehistory, and digs continue to this day, with surprising new findings.

For instance, scientists recently discovered the remains of sleeping beds from 200,000 years ago. These consist of fossilized fragments of grass and, surprisingly, ash.

It appears these early modern humans built beds using bundles of grass—the same species that still grows at the cave mouth—on a base of ash. Fine ash is an effective insecticide and repellent as it can clog the breathing organs of crawling insects.

Based on the evidence, scientists believe the cave dwellers would periodically burn the grass bundles in place to kill infestations of fleas, bed bugs or ticks, then leave the ashes and build new grass beds on top of them.

It’s a surprisingly sophisticated cultural practice for such ancient people, which makes us wonder what other knowledge of their environment they had and passed down.

I’m sure researchers will find more clues in Border Cave, and you may hear about them on EarthDate.