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Genes inherited from our ancestors crossbreeding with an extinct species of humans may have left a significant and lasting impact on our mental health, according to researchers.
A new study, published in the journal PLoS Genetics, concludes that these genes are one of the most widespread traces of our ancient ties with the Denisovans, who are believed to have mated with modern humans leaving Africa around 60,000 years ago.
As a result of that interbreeding, us homo sapiens appear to have inherited a genetic adaptation involved in zinc regulation that may have helped them weather the colder climates of the day — but at the same time may have been responsible for predisposing us to depression and all kinds of mental disorders.
And it could be affecting massive swathes of the global population — which, especially when compared to other mutations we got from Denisovans, is incredibly ubiquitous.
"For example, a variant in the EPAS1 gene inherited from the Denisovans allows adapting to life at altitude, but is found only in Tibetans," explained study co-lead author Elena Bosch at the Institute of Evolutionary Biology in a statement about the work. "However, in our case, the impact extends to all populations outside Africa."
Unlike with Neanderthals, the more famous species of extinct human, little physical evidence remains of the Denisovans. Most of what we understand of them comes from their genetic legacy — of which this latest work expands our understanding greatly.
During the researchers' genomic analysis, they found that a genetic adaptation in modern human populations was remarkably similar to part of the genome of the Denisovans, strongly suggesting that it came from our ancestors interbreeding with them. Because this adaptation does not appear in Neanderthals, the researchers have ruled them out as its source.
"We discovered that this mutation surely had implications for the transport of zinc within the cell," Bosch said in the statement.
Zinc is an essential micronutrient for humans and animals, serving, among many other crucial roles, as a messenger for cells. Hormones, enzymes, and proteins all depend on the trace metal, and various disorders, including growth and neurological, have been linked to zinc deficiency.
The fact that the zinc mutation survived suggests it must have provided some sort of advantage. Lab experiments conducted by the researchers attempted to prove what it was.
They found that the gene, SLC30A9, can influence a cell's metabolism by regulating its zinc balance — providing a "possible adaptation to the cold," said co-author Rubén Vicente, an associate professor at Pompeu Fabra University in Barcelona, in the statement.
But what proved a boon to cellular zinc transport came with some pretty heavy consequences down the line. According to the researchers, this same mutation has been associated with a greater risk of mental health problems, including depression, bipolar disorder, obsessive compulsive disorder, schizophrenia, and autism.
Still, the role of genetics in depression and other mental disorders is not well understood, and inheriting certain genes doesn't necessarily mean inheriting depression, though it may mean you will be more susceptible to it. The nature of this susceptibility is something that the researchers hope to iron out in follow-up studies.
"In the future, expanding this study to animal models could shed light on this predisposition to suffering from mental illnesses," Vicente said.
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