Neanderthals once dominated Europe in numbers rivalling contemporary modern humans. Despite their abundance, genetics shows they lived in isolated groups, often inbreeding.

Researchers have long been curious about how this impacted the Neanderthals. Could it be the reason they went extinct? Or was it actually not a big deal, despite making my skin crawl.

So researchers Ríos et al. have been studying a family of Neanderthals from El Sidrón (Spain). With more than 1,500 fossils from individuals living together, this site is a great cross-section of a Neanderthal group. As such, it could reveal how common inbreeding health problems were1.

It turns out these health problems were abundant at El Sidrón. Ríos et al. found many conditions that are rare (<2%) in modern humans were way more common (>15%) in the El Sidrón fossils1.

Keep it in the family

El Sidrón is a fascinating cave in Northern Spain. It was discovered during the Spanish civil war, when soldiers hid inside (and were almost buried alive by dynamite detonated by the other side)2.

Since then, excavations have proceeded a bit more carefully. In fact, archaeologists frequently wore full lab gear whilst digging up some of the >1,500 Neanderthal bones (representing at least 13 individuals). This prevented contamination from modern human DNA, allowing a wealth of Neanderthal genetic data to be extracted2.

Someone digging up a Neanderthal in full kit 2.

This genetic data confirmed that all of the Neanderthal fossils were part of the same family. This is a big deal, as it means they were all alive (and thus killed) at around the same time2. As such,
El Sidrón provides a snapshot into Neanderthal life that is extremely rare in Palaeolithic archaeology, where sites (and burials) normally build up over millennia.

In fact, that genetic data tells us they were extremely close relatives. It seems they were getting a little more friendly with each other than most modern human families.

Searching for signs of inbreeding

The snapshot nature of El Sidrón makes it an important Neanderthal site. For instance, archaeologists have found all 13 of them show signs of cannibalism, suggesting they might have died (and been butchered) in a violent attack2.

However, Ríos et al. were more interested in their lives. Specifically, whether their frequent inbreeding had any negative health effects. Importantly, since these Neanderthals represented one population Ríos et al. could get an idea of how common any discovered health effects were1.

As I spoiled in the introduction, it turns out they were very common. Pretty much everyone in the cave had a congenital anomaly caused by inbreeding. In fact, 4 even shared the same condition; giving it a 23% occurrence rate at El Sidrón. For context, it’s only seen in <3% of modern humans1.

That’s the same pattern Ríos et al. saw with most of the other congenital anomalies, with 16 conditions seen in 1-3% of humans occurring in 7-23% of these Neanderthals. Many of them are in the foot, leg, or spine; raising the possibility they may have made walking harder for them1.

Congenital anomalies caused by inbreeding in the El Sidron Neanderthals, and their frequency in the population (blue circle)

And of course, these are just the ones we can find in the skeleton. Inbreeding at this high a level is associated with increased disease risk in modern humans1.

Implications of inbreeding

As with any new discovery about the Neanderthals, the first question everyone asks is “could this be why they went extinct”.

To figure this out, we have to establish if inbreeding amongst Neanderthals was harmful and widespread. The fossils from El Sidrón confirm the former; whilst the fact fossils from across Europe also feature evidence inbreeding confirms the latter. This includes Neanderthal fossils from places as far away as Croatia and Siberia3.

It seems Neanderthals were getting cosy with their family across the continent.

Possible family trees of another Neanderthal with signs of inbreeding, this time from Siberia

These facts make it likely inbreeding was bad for Neanderthals. However, it doesn’t necassarily have to be the ultimate cause of their extinction. Rather, it could just be the symptom of it.

After all, if they were already going extinct then the resulting population crash (or group isolation) could very well cause the inbreeding we see here. This, in turn, may have tipped them over the edge of extinction; but not be what started them on this road1.

So if you have a pet theory about why Neanderthals died out, don’t panic. It could still be true!

Or it might not be. After all, it could the case that inbreeding and isolation was just part of their culture, and their extinction was all but inevitable. Which is somewhat troubling, as xenophobia and isolationism also seems to be becoming more popular positions in Western politics.

We should be anthropologically alarmed, as it turns out those ideas may have already driven one human species to extinction.

References

  1. Ríos, L., Kivell, T.L., Lalueza-Fox, C., Estalrrich, A., García-Tabernero, A., Huguet, R., Quintino, Y., De La Rasilla, M. and Rosas, A., 2019. skeletal Anomalies in the Neandertal Family of el sidrón (spain) support A Role of Inbreeding in Neandertal extinction. Scientific Reports9(1), p.1697.
  2. Lalueza-Fox, C., Rosas, A. and de la Rasilla, M., 2012. Palaeogenetic research at the El Sidrón Neanderthal site. Annals of Anatomy-Anatomischer Anzeiger194(1), pp.133-137.
  3. Prüfer, K. et al. A high-coverage Neandertal genome from Vindija Cave in Croatia. Science358, 655–658 (2017).

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