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New Ideas about the Neanderthal Extinction

A modern human cranium (left) and a Neanderthal cranium (right). Modern humans have a globe-shaped braincase with steep sides, our foreheads lack a prominent bony ridge about the eye sockets, and our faces are shorter and flatter with scalloped cheeks. Credit: BirdImages/iStockphoto

A modern human cranium (left) and a Neanderthal cranium (right). Modern humans have a globe-shaped braincase with steep sides, our foreheads lack a prominent bony ridge about the eye sockets, and our faces are shorter and flatter with scalloped cheeks. Credit: BirdImages/iStockphoto

By Darren Curnoe

Were modern humans so superior that they drove Neanderthals to extinction, or did their lonely existence leave them genetically vulnerable?

The Neanderthals (Homo neanderthalensis) are a species of extinct human relatives that occupied Europe, West Asia and Central Asia from around 400,000 to a little less than 40,000 years ago. Science has known about them for more than 180 years, and they were the first extinct hominin to be discovered, pushing our understanding of human origins beyond a Biblical timeline for the Earth.

Scientists studying human evolution are fascinated by them for a multitude of reasons, with more written about them than any other extinct species. This year has already seen dozens of scientific articles published about them, and we’re only halfway through the year!

The Neanderthals are also deeply embedded in Western culture as the archetypal “caveman”, and have historically been given a bad wrap. In the English language we even use the term “Neanderthal” in a derogatory way to describe an ignorant or unenlightened person. So, why the fascination with them?

A Little Bit of History

The first Neanderthal fossil was found at Engis Cave in Belgium by Philippe-Charles Schmerling in 1829. Schmerling found fragments from the skull of a Neanderthal child. Slightly earlier, in 1823, a fossil had been found at Paviland Caves in Wales, but we now know it to be a member of Homo sapiens.

Quite remarkably, these two European caves provided the earliest evidence for humans living beyond the timeline inferred from the Bible. Paviland is now thought to be around 30,000 years old, while Engis is still of uncertain age but older than Paviland Cave.

The first skull of an adult Neanderthal was found at Forbes‘ Quarry in Gibraltar in 1848, but it wasn’t announced to the scientific community until 1865.

However, the most important prehistoric human remains from this time were those recovered from the Neander Valley in Germany in 1856 at a site called Feldhofer. The Feldhofer Cave skeleton came to light during quarrying, with much of it destroyed before it was rescued by a local teacher. The top part of the braincase survived and was examined by the anthropologist Hermann Schaafhausen, who drew attention to its “savage” and “brutal appearance” – implying great antiquity.

When Schaafhausen’s work was eventually translated into English, the amateur archaeologist George Busk emphasised several ape-like characteristics, such as its pronounced eyebrow ridges.

Thomas Henry Huxley, the famous anatomist and founder of human evolutionary science who became known as “Darwin’s bulldog”, published the first detailed study of the Feldhofer fossil in 1863. While acknowledging it to be the most ape-like of fossils found so far, he felt that, on account of its large brain, it would have fallen within the wide range of variation for living humans.

At the time, the human brain was considered the cardinal feature that distinguishes us from all other organisms, including our Stone Age ancestors.

At least one worker felt the Feldhofer Neanderthal was distinct enough from humankind to belong to a different species, and in 1863 William King dubbed it Homo neanderthalensis.

Soon after, more Neanderthal remains were recovered, yet their place in human evolutionary history was far from settled, with scepticism from several scientific quarters. Only with the discovery of several further Neanderthal remains at Spy in Belgium in 1886 were the sceptics beginning to be silenced.

Another important figure from the early 20th century was the French anthropologist Marcellin Boule. He believed that the Neanderthals had played no role in the evolution of contemporary humans, but were instead a very primitive evolutionary side-branch or dead end.

Boule did all he could to portray the Neanderthals as brutish, stupid and very different to humans. It is fundamentally because of his work that we see them in popular culture as an unsophisticated cave-dweller – the archetypal dumb “caveman”.

For the remaining 20th century up until now, the precise place of the Neanderthals in evolutionary history has been hotly debated. Importantly, anthropologists have argued about whether they played a role in our evolution – as direct ancestors – or were simply close (kissing) cousins.

When and Why Did They Disappear?

Current evidence suggests that the Neanderthals disappeared around 40,000 years ago, and perhaps slightly more recently. This is about the same time that H. sapiens settled Europe after our ancestors migrated out of Africa maybe 60,000 years ago.

We don’t yet know why they disappeared, just that it is broadly coincident with modern humans settling Europe. Yet they seem to have overlapped for several thousand years or more in West Asia (Middle East) and probably even interbred with H. sapiens in this region.

The sequencing of DNA from several Neanderthals and comparisons with the DNA of living people from Asia and Europe has provided a pretty convincing case that we interbred. Mating between different species is surprisingly common in nature.

Even if we interbred with them, which seems reasonable, they were not our ancestors as such. The ancestors of Europeans seemed to have hung around Asia for 10,000 years or more before they migrated into Europe. Why? Was it because of the climate? Or because of competition with Neanderthals in Europe? Were the Neanderthals already in decline before modern humans settled Europe, providing impetus for the colonisation of the region by our species?

Some interesting research about Neanderthals has been published this year. I’ll outline the findings of two studies that received quite a bit of media attention to provide a taste of some of the current research on them and how our ideas about them are changing.

DNA Diversity

In research published in Proceedings of the National Academy of Sciences by Sergi Castellano and a large team of geneticists (doi: 10.1073/pnas.1405138111), the DNA of two Neanderthals – one from Spain dated ~49,000 years old and one from Croatia aged about 44,000 years old – was studied to try to understand how much diversity there was in the species. The team studied more than 17,000 genes that produce proteins or play a role in the physiology of the body, and compared them to the DNA of living people.

We know that genetic diversity in our species is quite low, especially compared with our living cousins, the chimpanzees, even though there are seven billion of us. This is because we all descend from a very small Stone Age population that was living in Africa around 200,000 years ago.

Genetic diversity can be an indicator of recent evolutionary history and, when it is especially low, a species is vulnerable to extinction. This could have been an issue for the Neanderthals.

It seems that the Neanderthals had much less genetic variation than living humans and probably lived in very small and isolated groups.

Other important findings of this research relate to the particular genes that have changed between Neanderthals and us after we shared a common ancestor 500,000 years ago or more. Castellano’s group found that after this split, Neanderthal DNA shows an abundance of genetic changes related to metabolism, the cardiovascular system, hair distribution and physical features affecting the genitals, palate, face, limb extremities, joints, fingers and toes, thorax, and other parts of the skull. There is also evidence for even later changes to genes involved in the curvature of the spine.

In modern humans they found a large number of changes in genes associated with skin and hair pigmentation, and also some behavioural traits. They identified genes that are clinically associated with disorders like psychomotor retardation, autism and Tourette syndrome, including some involved in “hyper­activity” and “aggressive behaviour”.

It is difficult to know precisely how these genes would have affected the behaviour of our Ice Age ancestors. Human behaviour is controlled by much more than just our DNA, and the authors concluded that even if these genes changes affected behaviour, “the way in which they occurred is unknown. For example, if they affected activity or aggression levels, it is unclear whether they increased or decreased such traits.”

The Neanderthal Demise

In the second research article, published in PLOS One (doi: 10.1371/journal.pone.0096424), Paola Villa and Wil Roebroeks set out to test whether the archaeological evidence for Neanderthals indicates that they were behaviourally less sophisticated than modern humans in the Ice Age – yet another study in a long history of such investigations beginning with Boule.

Villa and Roebroeks acknowledge the common view that “the disappearance of the archaic populations, including Neandertals, is routinely explained in terms of the ‘superiority’ of modern humans, who had developed in Africa the ability to evolve complex cultural traditions and had become equipped with cognitive capacities which allowed them to expand globally and replace all other hominins”.

They undertook a detailed review of the archaeological evidence for Neanderthals, comparing it with similar evidence for modern humans who lived at about the same time. In particular they examined issues such as:

  • language and symbolism;
  • hunting methods and diet;
  • organised use of space;
  • capacity for innovation;
  • the size of social networks; and
  • hafting procedures, heat treatment and cognition (i.e. tool-making as evidence for complex thinking).

They end their review by concluding that “the Neandertal archaeological record was not different enough to explain their demise in terms of inferiority in archaeologically visible domains. Thus, if Neandertals were not technologically and cognitively ‘disadvantaged’, how can we explain that they did not survive?”

Unfortunately they don’t have an answer. Instead they conclude that there was unlikely to have been a single cause, and that it certainly wasn’t the result of more sophisticated behaviour by migrating modern humans.

As I have already noted, the DNA evidence suggests that they could have been vulnerable to extinction on account of their low genetic diversity and living in small and isolated groups.

To be frank, I’m pretty sceptical that we can really get at the cognitive capacities of any hominin by looking just at archaeological evidence. There is so much missing, like wooden tools, and we don’t really understand the reasons why certain types of tools were made or how they were used or why they took a particular form over another.

Neuropsychologists experimenting with living people have difficulty enough understanding how the brain functions and why people do the things they do, even when they have the luxury of being able to study their brains with sophisticated techniques like magnetic resonance imaging and asking people questions in surveys. We don’t have fossilised brains and have no chance of really understanding why particular choices were made or cultural traditions developed.

Moreover, we know that many other animals are capable of very complex behaviours as well, but this doesn’t mean that their brains function the same way that our does. Different species have brains that function differently, and clearly also have distinct behavioural repertoires. How can it be any other way?

I find stone tools to offer a crude window into past behaviour and motivations, and think that we need to be a bit more careful when deriving thinking capacities from what is ultimately very limited evidence.

Why the Fascination?

In the end, our fascination with the Neanderthals stems from a long historical knowledge of them and their place in history in overturning creationist ideas. We shared the landscape and seem to have interbred with them; we feel that they are almost but not quite us; they provide insights into a lost world from which we evolved; and they perhaps reveal an even deeper, more natural or “animal” side to ourselves and our psyche.

Darren Curnoe is a specialist in human evolution at the University of NSW. He blogs at http://www.walkingontwofeet.com