Numbers suggest mating with humans might have led to Neandertals’ demise

Scholars are turning the disappearance of humans’ closest cousins into a numbers game.

More than 150 years ago, German schoolteacher Johann Carl Fuhlrott realized that fossils from a local limestone quarry were almost, but not quite, human. More recently, scholars have expanded their research beyond bones and stones to figure out what became of the Neandertals. And a Danish physicist is taking an actuarial approach to the puzzle: Bent Sørensen of Roskilde University thinks numbers may explain why humans sit around today puzzling over the Neandertals’ fate, and not the other way around.

There have been two leading theories to date. One posits that increasingly wintry conditions gradually made life in Europe and Asia unbearable for the Neandertals until, about 30,000 years ago, they literally couldn’t take it anymore. The other points to the expansion of modern humans into the heart of Neandertals’ range beginning about 45,000 years ago, arguing that Eurasia just wasn’t big enough for both groups.

Both explanations have been vexed by a lack of evidence. Fossils reveal no obvious physiological reason why Neandertals would have been less able than humans to adapt to harsh ice age conditions. Records of ancient climate show that Neandertals repeatedly bounced back after cold snaps just as harsh as the one that came on at the beginning of their end. And there is no profusion of cracked Neandertal skulls to suggest widespread warfare. For that matter, last year geneticists reported DNA evidence indicating that Neandertals and Homo sapiens had actually interbred.

So what gives?

Sørensen starts with a relatively simple formula that calculates a population’s size by adding up each year’s births and then subtracting deaths. Then, he appeals to archaeological evidence suggesting that the Neandertal population in Europe never got much higher than 10,000. He also points out that the species survived a pretty nasty cold snap about 135,000 years ago. Using those simple observations, he derives likely ranges of fertility and death rates that depend on two factors: population size (populations above about 12,000 increase the death rate due to food shortages) and temperature (deviations in either direction from the average annual mean of 10° Celsius increase death rate).

Based on that model, Sørensen reports in the March Journal of Anthropological Archaeology, there’s no way the Neandertals could have survived nearly 200,000 years of ice age conditions only to be frozen out of existence by gradually worsening climate 30,000 years ago. Something about the arrival of modern humans, who entered Europe from the Near East about 45,000 years ago, must have led to the Neandertals’ demise.

But that something wasn’t competition for food. By building a similar model to calculate the population size of modern humans in Europe — one that even gives Homo sapiens a leg  up in terms of birthrate and life expectancy —  Sørensen shows that the newcomers couldn’t have grown so numerous in just 15,000 years to eat the Neandertals out of house and home.

So what could have wiped out the Neandertals? Recall that this is a numbers game: Whatever it was had to have been introduced by humans, and it had to have either increased Neandertals’ death rate, reduced their birthrate, or both.

Sørensen dismisses warfare as unlikely, since there’s no sign of it in the archaeological record. That leaves disease. Perhaps, like the European settlers who brought smallpox to America a thousand generations later, modern humans brought with them a disease that the locals just couldn’t handle. Sørensen suggests searching DNA from Neandertal fossils for signs of viruses.

So here’s a theory: Given the new evidence that the two groups interbred, maybe humans killed off the Neandertals by giving them some kind of sexually transmitted disease.

Medical historians believe syphilis was originally a New World disease, introduced to Europe by veterans of Columbus’ voyages. The timing is right: The first major European syphilis epidemic began in Naples, Italy, in 1495.

Is it likely that the Neanderthals were wiped out by syphilis? Well, no, especially considering that if the first humans to arrive in Europe had brought the disease with them, it would have  had to disappear from the continent and then be reintroduced 45,000 years later by Columbus and his men.

But there is a real possibility that something about their amorous relationship — maybe  another STD, maybe something else — put the Neandertals at a demographic disadvantage. While interbreeding might have been of minor consequence for modern humans, who came away from the rendezvous with nothing more than a trace of exotic genetic material, the other side of the bargain could have sealed Neandertals’ fate like a black widow’s kiss.

SN Prime | June 17, 2011 | Vol. 1, No. 2