Francesca Tomasi received her B.A. from the University of Chicago and is now a microbiologist.
Homo sapiens (which translates to “wise person”) are the only extant human species on earth – us. The genus homo includes other hominids such as Neanderthals, which died out as H. sapiens infiltrated the planet and changed life on earth forever. Neanderthals were widely distributed up until about 35,000 years ago and are characterized in the fossil record by a receding forehead and prominent brow ridges (below). But why did H. sapiens come out on top and not another species of homo?
Last week, a study was published by researchers at Cambridge and Oxford Brookes that seeks to answer this very question. The fact that Infective Perspective has an article about it should give you a pretty good idea about the budding theory: infectious diseases. It turns out that pathogens carried by humans migrating from Africa to Europe may very well have contributed to the demise of the Neanderthals. The reason is simple: individuals coming out of Africa could easily have acted as reservoirs for tropical diseases endemic to their home continent (and that they were therefore immune to). On the other hand, Eurasian homos were naïve to these pathogens. What’s the result of exposure to pathogens without medical care or a previous immune response? A catastrophe.
Fossil evidence dating back to 60,000 years suggests that humans and Neanderthals interacted – and even mated – with each other. In fact, modern-day humans (you and I) carry 2 to 5 percent of Neanderthal DNA. If bodily fluids were being shared, it is safe to assume that communicable diseases were shared too. Culprit microbes include some nuisance we still face today: tapeworms, stomach ulcers (H. pylori), genital herpes, and tuberculosis.
Historically, the epitome of the prolonged interaction between different species of homo has been described during the agricultural revolution, which took place about 8,000 years ago and led to an eruption of infectious diseases we often read about in historic accounts. But the new study suggests that pathogens preceded this “big bang” and lent their destructive hand well before endemic disease settled into society. In fact, as you saw earlier, Neanderthals went extinct tens of thousands of years before the agricultural revolution. So how did pathogens eclipse Neanderthals’ time on earth?
By now, you’re probably forming a comparison to the H. sapiens-Neanderthal interaction with the early European explorers who wiped out Native American populations with plagues like small pox. However, the infections that may have killed off the Neanderthals were different. Rather than acting on a full population level, little flares of outbreaks likely weakened individual communities and eventually tipped the equilibrium against overall species survival. In fact, Neanderthals used to live in groups of 15 to 30 individuals and took to a hunter-gatherer lifestyle. Such small communities allowed for this transient spread of pathogens: the introduction of a virus, bacterium, or parasite could easily have caused individuals to succumb to illness and thus rendered them unfit to continue hunting and gathering. This subsequently weakened groups as a whole. Agricultural communities thousands of years later simply provided networks of reservoirs for infectious diseases to spread on a larger scale and therefore earn the adjective “endemic.”
So how does science today tell us about disease yesterday? The researchers from Cambridge and Oxford Brookes used pathogen genome sequences and DNA from ancient bones to date some infectious diseases to well before we had previously placed them on a timeline of life on earth. For instance, the scientists propose that tuberculosis, which has long been called a zoonosis (transferred from animals – in this case cows – to people), actually went the other way around: humans may have transferred m. Tuberculosis to cows, creating an animal reservoir for the disease. The geographical and temporal overlaps of Neanderthal-H. sapiens interactions and the former’s extinction point to these conclusions. Evidence of protective genes against certain diseases such as bacterial sepsis (blood poisoning from wound infections) and tick-caused encephalitis further suggests some benefits of interbreeding between the homo species. But for the most part, mixing two populations from geographically diverse locations (Eurasia and Africa) could easily have led to the swapping of resident microbes.
It shouldn’t come as a surprise that infectious diseases have been co-evolving with other forms of life for millions of years. After all, we are all descended from microbes, and our prokaryotic friends and foes have had billions of years of evolutionary advantage over us. Genomic analysis of herpes simplex 2 virus, which causes genital herpes, from ancient samples suggests the virus was transmitted to humans over 1 and a half million years ago in Africa, likely from another species of humans that acquired the disease from nonhuman primates. The same thing could easily have happened between H. sapiens and Neanderthals, leading to this emerging theory of Neanderthal extinction as a result of H. sapiens emigration. Of course, it is unlikely that infectious diseases alone contributed to Neanderthal extinction. But the introduction of a pathogen to a naïve population is a very conceivable catalyst.
Why did Neanderthals draw the short end of the stick, when they, too, likely carried geographically-specific microbes that H. sapiens had never seen before? What tipped the balance in one direction over the other? Part of it comes to the fact that when it comes to infectious diseases, biodiversity wins. The dense tropical landmass of now-Africa was (and still is) home to incredible species diversity unparalleled by now-Europe. As a result, anomalous inter-species interactions occurred more readily, allowing for increased likelihood of the emergence of human pathogens - this, too, is a consequence of closer contact between wild animals and humans.