Lead appears to be less neurotoxic for the human brain than it was for some ancient hominins. Has this greater tolerance helped us?
Exposure to lead is harmful to humans, but it must have been even more so for some ancient hominins now extinct. Many prehistoric species lived in habitats naturally contaminated by lead, so much so that they absorbed it in significant quantities in their teeth.
THE sapiens modern humans would have evolved the ability to tolerate lead poisoning better than Neanderthals, Australopithecines, Paranthropus and ancient representatives of their own species. This increased tolerance may have given us a competitive advantage, allowing us to outlast everyone else and populate the Earth. This is the bold and yet to be verified hypothesis contained in a study published on Science Advances.
Research in the teeth
A team of scientists from Southern Cross University in Lismore, Australia, looked for signs of lead exposure in 51 fossil teeth of ancient great primates, apes and hominins from various parts of the world. Traces of lead contamination were found in 73% of the teeth analyzed, including those of Australopithecus africanus, Paranthropus robustus (an extinct hominin who lived between 2.3 and 1.2 million years ago), Neanderthal, Gigantopithecus blacki (a huge extinct ape that lived in southern China 2 million years ago), of ancient orangutans and baboons.
The analysis shows that lead poisoning, traditionally associated with industrial pollution, mining and – until the 1980s – red petrol containing the metal, was also a reality for our ancestors 2 million years ago. Since lead can be found naturally in rocks, as a product of volcanic eruptions, in water, in soils and in plants that have roots therein, ancient hominins and other primates may have acquired it from the environment.
The damage caused by lead to the brain – especially the developing brain – would, according to the study, constitute a “ceiling” to the ability of those species to develop complex social skills.
Lead: do sapiens “bear” it better?
In fact, the study suggests that modern man tolerates the neurotoxic effects of lead a little better than his predecessors did, thanks to some evolutionary adaptations. To understand what effect sometimes “industrial” quantities of lead had on ancient hominins and other primates, scientists worked on brain organoids (i.e. agglomerations of cells whose structure resemble small replicas of the cerebral cortex) in which they studied the archaic and modern versions of a gene – the NOVA1 – which helps organize brain development.
By exposing the organoids to very small, lifelike amounts of lead, the archaic ones expressed mutations in another gene, called FOXP2, that are linked to problems with communication and language development.
In contrast, mini-brains expressing the modern form of NOVA1 appeared protected from this type of damage.
The idea is that in modern humans, natural and continuous exposure to lead may have selected some genetic variants, including the modern version of NOVA1, useful in protecting the brain from metal toxicity. This may have helped make our species more communicative and socially cohesive.
Many points to clarify
The study has some limitations. For example, the small number and few geographical locations from which the fossil teeth come (China, Southeast Asia, Australia, South Africa and France) make the hypothesis that all ancient hominids were continuously exposed to lead for 2 million years arbitrary.
Furthermore, it is not possible to demonstrate when this exposure occurred, and whether it really began in the most delicate moment for neurodevelopment, that of childhood. It is also unclear whether exposing brain organoids to lead reflects what happens to the human brain in the event of natural contamination. In short, the conclusions are courageous, given the type of evidence on which they are based, and will need to be supported by other evidence.