Living in a dangerous world
This study is not the first evidence that ancient hominins were exposed to lead in the environment. Two Neanderthals who lived 250,000 years ago in France were exposed to lead as children. according to a 2018 study. At the time, these were the oldest known examples of lead exposure (and are included in a recent study by Joannes-Boillot and colleagues).
Just a few thousand years ago, no one smelted silver, built baths, or emitted lead fumes from car exhaust fumes. So how did our hominid ancestors become exposed to the toxic element? Another study published in 2015, showed that Spanish caves inhabited by other Neanderthal groups contained enough heavy metals, including lead, to “meet modern 'contaminated soil' standards.”
Today, we mostly think of lead as a man-made pollution, so it's easy to forget that it also occurs naturally in bedrock and soil. If this were not the case, archaeologists could not use lead isotope ratios to determine where particular artifacts were made. And some places—and some rock types—have higher lead concentrations than others. Some common minerals contain lead compounds, including galena or lead sulfide. And the lead exposure documented in Joannes-Boillot and colleagues' study would have occurred at an age when small hominins were very likely to put rocks, cave dirt, and other random objects into their mouths.
Some of the fossils from the Keke cave system in China, including an extinct 1.8-million-year-old gorilla-like ape called Gigantopithecus blackhad lead levels above 50 ppm, which Joannes-Boiau and colleagues describe as “significant levels of lead that could cause some developmental, health, and possibly social impairments.”
Even for ancient hominids that did not live in caves full of lead-rich minerals, forest fires or volcanic eruptions can also release lead particles into the air, and erosion or floods can wash buried lead-rich rocks or sediments into water sources. If you're an Australopithecus, for example, living upstream from an outcrop of lead-rich mica, erosion could introduce poison into your drinking water – or into the drinking water of the gazelle you eat, or into the root system of the bush from which you get those delicious berries… .
