The most famous of our extinct hominin relatives –  there were once 20 other such species – are the Neanderthals, who evolved in Eurasia and disappeared around 39,000 years ago. 

Neanderthals had been established for 100,000 years before modern humans evolved. They were strong, they had large brains; there were many reasons why they could have prevailed as the dominant hominins.

From fossil and molecular evidence, we now know the Neanderthal lineage evolved for at least 400,000 years and occupied a wide range of habitats across Europe and Asia, from the warm shores of southern Spain to the steppes of Siberia. 

Around 100,000 years ago, several hominin species, including our own species in Africa, shared the ‘Old World’. 

The coincidence of modern humans moving into Europe around 45,000 years ago and the last appearance of Neanderthals dated to not long thereafter led to the view that direct competition between modern humans and Neanderthals ultimately caused the extinction of the latter.

This idea was often related to a stereotypical view of Neanderthals: oafish, without ‘modern’ culture and lacking the physiological sophistication of modern humans. But in reality, Neanderthals sported absolutely larger brains and complex tool technologies

Some scientists have argued that the Neanderthals’ close-range hunting techniques, which had been so effective in mixed environments, were probably less effective in open spaces and so they may have been forced to follow these retreating woodlands into more southern latitudes. 

Modern humans on the other hand were supposedly rather better suited to hunting in open terrain – they were well adapted for long-distance running and had mastered the use of projectile weapons. These traits were likely helpful for Homo sapiens, but the new and unstable climatic conditions would have been difficult environments for both groups, and in places where the species overlapped competition for fast-changing resources was probably fierce.
From what we know of Neanderthal genetics, population sizes across Europe were quite small, even before ecological conditions began to change. 

With low genetic diversity it is likely that worldwide populations of late Neanderthals mostly numbered only in the tens of thousands, and this may well have been the case for modern humans too – a far cry from the 7 billion of today. However, it has been posited that modern humans generally lived in larger social groups than Neanderthals did, and that as the habitats of the latter became fragmented, so too did their networks of relatively small social groups. 

The fragmentation of Neanderthal social groups would have prevented the benefits of gene flow, population movement, and cultural diffusion, and the effect of these in buffering extreme changes in climate would therefore have been reduced. However, due to a geographic bias in excavations, our picture of Neanderthals is extremely Eurocentric: the factors that contributed to Neanderthal extinction in Asia, for example, may be entirely unrelated to the factors in Europe.

It has been suggested that interbreeding between the two species may have endowed modern humans with some of the Neanderthals’ evolved immunities to Eurasian pathogens. 

In fact, the evidence for interbreeding between modern humans and Neanderthals across Europe and Asia has led some to suggest that Neanderthal populations were not eradicated, but merely subsumed into the larger modern human groups. The frequency and implications of these interbreeding events are, however, still poorly understood. 

The extinction of the Neanderthals and their interactions with modern humans during this period remain enigmatic, and the emerging view is clearly complex. Compared to that of other hominin species, our data for Neanderthals are quite comprehensive, and this gives us some idea of how much more there is to understand about the factors leading to the extinction of the numerous other hominins that preceded us. 

Modern humans evolved relatively recently, and in that time the technological leaps have been extraordinary, allowing us to colonise habitats as diverse as Melanesia and the Arctic Circle. Our technologies increasingly shield us from many of the risks that seemingly resulted in the extinction of our relatives, such as infectious disease, climate change and predation, and it can be tempting to imagine ourselves, as the last remaining hominin species,  immune to selective and environmental pressures. 

However, after over 100 years of study of Neanderthal extinction, our current picture of events should perhaps warn against this hubris. As incidence of non-infectious disease increases, climate change accelerates and biodiversity plummets, the threat of humans becoming yet another biological failure is more acute than ever.
Julia Galway-Witham and Professor Chris Stringer are palaeoanthropologists (scientists of human evolution) at the Natural History Museum in London. Julia enjoys scientific communication via print and radio, and her research interests include exploring the intersection of classification and morphology through the study of fossils and the philosophy of biology.

Chris is one of the leading experts on human origins and has spent the last 40 years investigating human evolution, and is the author of the bestselling books The Origin of Our Species and Homo britannicus. Julia and Chris can be found on twitter at @JGalwayWitham and @ChrisStringer65