SAD is an STD (and we caught it from the Neandertals!)
There are two fascinating reports in the last two weeks in Science:
Members of our species had sex with Neandertals much earlier—and more often—than previously believed, according to a new study of ancient DNA. As some of the first bands of modern humans moved out of Africa, they met and mated with Neandertals about 100,000 years ago—perhaps in the fertile Nile Valley, along the coastal hills of the Middle East, or in the once-verdant Arabian Peninsula. This pushes back the earliest encounter between the two groups by tens of thousands of years and suggests that our ancestors were shaped in significant ways by swapping genes with other types of humans.
These early modern humans’ own lineages died out, and they are not among the ancestors of living people. But a small bit of their DNA survived in the toe bone of a Neandertal woman who lived more than 50,000 years ago in Denisova Cave in the Altai Mountains of Siberia, Russia.
Far more fascinating is the other report about the Neandertals:
A breakthrough came when…[researchers] realized that they could fish for Neandertal gene variants in a medical database, the Electronic Medical Records and Genomics (eMERGE) Network. This consortium in nine U.S. cities links patients’ genetic data with their medical data, in the form of specific billing codes that record diagnoses for illnesses and other conditions. Thus eMERGE allows researchers to track correlations between genes and symptoms in tens of thousands of people.
Here is a graphic of what they found.
The most interesting finding for me was a link to depression:
The significant replicated association of Neandertal SNPs with mood disorders, in particular depression, is intriguing because Neandertal alleles are enriched near genes associated with long-term depression, and human-Neandertal DNA and methylation differences have been hypothesized to influence neurological and psychiatric phenotypes. Depression risk in modern human populations is influenced by sunlight exposure, which differs between high and low latitudes, and we found enrichment of circadian clock genes near the Neandertal alleles that contribute most to this association. The replicated nominal association of Neandertal SNPs with actinic keratosis (precancerous scaly skin lesions) further links introgressed alleles in AMHs to a phenotype directly related to sun exposure.
These findings are particularly exciting to me because here is a direct link back to Neandertal’s minds and how they felt. Neandertals must have gotten depressed just like we do—to a point. Of course they were evolved to live in northern glaciated Europe, where it was very dark for half the year. So-called SAD, or seasonal affective disorder, has long been hypothesized to have been an adaptive “hibernation” state, because this form of depression manifests with symptoms of extreme fatigue and lack of initiative in dark winter days, and it is successfully treated with bright lights and trips to Florida. All that makes a lot of sense: we “caught” SAD from the Neandertals. BUT..
Before we think that we are going to find many more answers to mental illness from sequencing our ancestors, I will throw in this caveat:
SAD is the rare exception of a modern human mental illness that could be conceived as being adaptive and thus holding some hidden benefits for sufferers. Very early in my psychiatric career, when I was first becoming interested in the relevance of the major illnesses to human evolution, I came to this firm conclusion and have never altered:
Severe conditions such as the major depressions, panic disorder, schizophrenia, and bipolar disorder are simply TOO DEBILITATING TO HAVE ANY ADAPTIVE FUNCTION WHATSOEVER. Rather, I concluded that these major mental illnesses are an epiphenomenon—or “side effect”—of the major adaptation that bestowed upon us the miraculous capacities of reflective self-consciousness and the deep complexity of syntactical language that has enabled our rich symbolic culture. Therefore we should regard the mentally ill for paying the price for our extraordinary capacities.
