The Incredible Human Journey (38 page)

Given that we now know so much about Neanderthals, there are still many questions about their disappearance. Although there
is no evidence of Neanderthals and modern humans actually living in precisely the same places at the same time, there was
certainly a period when both species were present in Europe. It used to be thought that this period of overlap lasted around
10,000 years, but new calibrated radiocarbon dates suggest that the overlap was shorter: about 6000 years in north and central
Europe, and perhaps only one or two thousand years in western France.
¹³
But why did the Neanderthals disappear? Did we kill them off or out-compete them? Or perhaps they are actually still around
– could Neanderthals have been assimilated into the expanding modern human population as it flowed westwards across Europe?

There are certainly some researchers who think so. They put forward specimens – mostly skulls – like Oase and Cioclovina from
Romania, the Mladeč fossils from the Czech Republic and the Lagar Velho skeleton from Portugal1 – as physical evidence for
interbreeding between Neanderthals and modern humans. Palaeanthropologists João Zilhao and Erik Trinkaus suggest that archaic
traits in these fossils are not just ‘throwbacks’: they may be evidence of Neanderthal genes in the early modern human populations
of Europe.

Neanderthal Skulls and Genes: Leipzig, Germany

So I made my way to Germany, not to the Neander Valley, but to the Max Planck Institute for Evolutionary Anthropology in Leipzig,
where I had arranged to meet Dr Katerina Harvati, who had recently analysed the Cioclovina skull. Katerina met me on the other side of the revolving door at Max Planck, and we walked into an enormous space,
some three floors of atrium with light streaming in through glass walls on two sides. Stairs and ramps to the upper floors
seemed to float in the air. Katerina led me up to the labs on the second floor, where she was going to show me CT scans of
the Cioclovina skull.

But my attention was first drawn to a composite skeleton, put together from casts of fossil bones from different sites, standing
in the corner of the lab. It was the first time I had laid eyes on a complete, assembled Neanderthal skeleton, and it was
interesting to see just how stocky he looked. The ribcage flared out at the bottom, quite different from the modern human
chest shape. Individual bones were generally quite similar to modern human bones, but nonetheless very rugged.

‘We can tell from their body form and proportions that Neanderthals were showing some level of cold adaptation: they were
stocky, with short limbs,’ said Katerina.

But how much of an advantage would this have given them, compared with modern humans?

‘It has been calculated that the advantage would be – perhaps not as great as we originally thought – maybe the equivalent
of one business suit.’

It didn’t sound that impressive. Cultural adaptations, like clothing and use of fire, must have been more important to the
Neanderthals’ survival in Ice Age Europe.

But the most ‘different’ part of all the Neanderthal skeleton was the skull. Neanderthals have very long, low skulls, whereas
modern human human crania are much rounder. Neanderthal faces are big: they have massive browridges, large, goggly orbits
(eye sockets), large nasal openings and projecting, prognathic jaws.

So what about this Cioclovina skull that had been suggested to be a Neanderthal/modern human hybrid? The skull itself had
been discovered in the cave that gives it its name in southern Romania, in 1941 – during phosphate mining – and had recently
been radiocarbon dated to about 29,000 years old. The skull was really just a braincase: most of the face was missing. Although
its general shape was definitely modern, some researchers had suggested that the shape of the browridge and the back of the
skull were Neanderthal-like.
¹

Obviously, before you can confirm or reject a claim that a skull represents a hybrid, you have to have an idea of what a hybrid
might look like. Is it likely to have an even mix of features from each parent? Or might it be mostly like one parent with just a few features from the other? Katerina had looked into the features of hybrids
in other primate groups and she found that a common feature of hybrids seemed to be a size change – either bigger or smaller
– than would be expected from the parent populations. Some hybrids – like a gibbon–siamang cross in Atlanta Zoo, and hybrids
from different macaque and baboon species – looked anatomically like a mixture of the two species they came from. It also seemed that hybrid populations tended to be more variable than the parent species, and also had rare anomalies popping
up more often than usual.
¹

So Katerina had analysed the Cioclovina skull to see if it showed any of these signs of being a hybrid: an appreciable size
difference, a mixture of features, a high level of variability, or any strange anomalies. But she had also measured the skull
so that she could compare its size and shape with those of other modern human and Neanderthal skulls. Describing features
in skulls, even measuring them, is fraught with problems, as I had seen so vividly in China, but Katerina had also used a
technically sophisticated and perhaps more objective approach to the problem of comparing skull shape and size.

The first step was to convert a real skull into a mathematical model, a cloud of points in 3D space that described the shape
and size of the skull, using features or ‘landmarks’ that could be recognised on any skull. Rather than measuring a skull with calipers to get distances and angles, Katerina showed me how she had captured the 3D shape
and size of skulls in two ways: using an electronic digitiser and CT scans. The digitiser was an elegant piece of equipment
– an articulated arm ending in a stylus that could be placed on the surface of a skull – and points could be captured in 3D
space, with x, y, z coordinates. It was a piece of apparatus that was widely used in design and engineering – and was now
beginning to be applied to the study of old bones. 3D coordinates could also be taken from detailed CT scans of skulls, which
would allow points on the inside as well as the outside of the skull to be recorded.

Having captured and quantified all that information, Katerina could then compare different skulls, and she did this in the
context of variation among different primates.

‘The difference between Neanderthals and modern humans is not similar at all to the differences that you would find between
subspecies of primates living today,’ she said.

‘It is much more similar to the distances you’d find between closely related species.’

‘So you can be absolutely sure that Neanderthals are a separate species?’ I asked.

‘Yes, that is what I’d say. They are too different to be another population or even a subspecies of modern human. They were
our sister species. Closely related – but a different species.’

This seemed to refute the multiregionalist idea that all species since
Homo erectus
have essentially been one.

‘So what about Cioclovina?’ I asked. Katerina showed me a 3D computer model of the Cioclovina skull, based on CT scans that
had been done at a local hospital. She spun the model round on the screen, and pointed out the relevant features. The browridge
was big, but it was broken in the middle, unlike the uninterrupted ‘monobrow’ of Neanderthals. The occipital bone at the
back of the skull did bulge out, and the nuchal line where neck muscles attached was well marked, but not really Neanderthal-looking.
There didn’t seem to be anything unusual about its size, nor were there any odd anomalies in the skull.

So what about the results of the shape analysis? Katerina had compared the 3D ‘landmark configuration’ of the Cioclovina skull
to Neanderthal and modern human (including Upper Palaeolithic) skulls. Using different sets of statistical analyses to make the comparisons, Cioclovina always came out closer to modern humans.
¹

‘From my analysis, I wasn’t able to see any resemblance to Neanderthals,’ Katerina told me. ‘There is no evidence to support
the claim that this is hybrid. It actually turns out to be very typically modern human in its anatomy.’

It was clear that Katerina couldn’t wait to look at the other proposed hybrid specimens, like Oase. She was open-minded about
what her results meant, and what she still might find.

‘Of course this doesn’t mean that hybridisation didn’t happen. It could have happened and we just haven’t found the hybrids
yet. Or, some of the other proposed hybrids that I haven’t examined yet might fit the criteria. Or it could be that it was
so rare that it hasn’t left a trace in the fossil record. And the genetic evidence to date suggests that
if
admixture happened, it was so low that it was really not significant in an evolutionary sense.’

Indeed, it wasn’t just the shape and size of Neanderthal bones that was being studied in Leipzig, it was genes as well. In
1997, a team of scientists led by Svante Pääbo of the Max Planck Institute published the first analysis of DNA from an extinct
human. They had managed to extract mtDNA from one of the original fossils from the Neander Valley. Pääbo chose to look for a non-coding, fast-mutating section of mitochondrial DNA that had already proved useful in
studies into evolutionary relationships between living species.

Getting DNA out of an ancient bone was always going to be a huge challenge – DNA starts to fall apart after death – but Pääbo
and his team had hoped that some tiny fragments might still be there. The extraction was done in a sterile room, to try to
reduce the possibility of contamination with modern DNA. The bone sample was ground into powder and then the sample was treated
to amplify up any DNA – by getting any fragments to make copies of themselves. Then the sequencing could start, and the results
were quite stunning: when they compared the Neanderthal sequence with the equivalent mitochondrial DNA sequence from nearly
1000 modern humans, they found that it was distinctly different. The modern human mtDNA sequences differed from each other
by an average of eight different base pairs out of almost four hundred. But the Neanderthal sequence had an average of twenty-six
base pair differences compared with the modern human samples. This difference suggested that Neanderthal and modern human
mtDNA had been evolving along separate pathways for about 600,000 years. Although this seems a very long time ago, compared
with the dates of the earliest known Neanderthal (about 300,000 years ago) and the earliest known modern human (about 200,000
years ago) it still makes sense, as the lineages would have started to diverge within an ancestral population of
Homo heidelbergensis
.
²

This result seems to support the theory of a recent African origin of modern humans, and a replacement of any earlier human
populations. In contrast, the multiregional hypothesis suggests that archaic populations in Africa, Europe and Asia developed
into modern human populations. A halfway house theory has modern humans originating in Africa, then spreading into Europe and Asia and interbreeding with
existing archaic humans.

Pääbo’s findings suggested that the mitochondrial DNA lineages, at least, had separated (and stayed separate) hundreds of
thousands of years before modern humans appeared in Europe. Even if you ignore the timings, then the multiregional model with
hybridisation suggests that Neanderthals should be genetically closest to modern Europeans, but there was no evidence of this
in the mitochondrial DNA: the Neanderthal sequence was equally different from all modern humans across the globe. Another study compared ancient DNA extracted from two 25,000-year-old European modern human fossils, and found that the Cro-Magnon
mtDNA fell in the modern human range of variation, and was very different from the Neanderthal sequences.
³

Looking at mtDNA variation as well as modelling the population expansion of modern humans in Europe, researchers in Switzerland
came up with a
maximum
interbreeding rate between the two populations of less than 0.1 per cent. Statistically, this is so low as to be practically
non-existent, and the Swiss scientists go as far as to say that this suggests the two species were biologically separate –
and could not produce fertile offspring even if they had seized upon the chance to have sex with each other.
⁴

So do these mtDNA results represent definitive evidence that the Neanderthals could not be counted as among the ancestors
of modern Europeans? Well, they certainly seem to point in that direction, but, actually, it’s impossible to completely rule
out
any
hybridisation between modern and archaic populations. Neanderthal genes could have entered the human gene pool, but those
lineages might have died out, leaving no trace of them today. And what if only Neanderthal men, not women, had interbred with
the incoming modern humans? That wouldn’t show up in the mitochondrial DNA – which is inherited only from the mother. So although
these Neanderthal mtDNA studies are amazing, and suggest that hybridisation didn’t happen, they can’t rule it out. So would
it be possible to probe further, to go after more Neanderthal DNA – perhaps nuclear DNA?

When Svante Pääbo was interviewed for
Science
magazine after the publication of the Neanderthal mtDNA paper in 1997, he was very pessimistic about the chances of anyone
ever managing to recover and sequence nuclear DNA from Neanderthal bones.
⁵
But just over a decade later, I was visiting his lab at the Max Planck Institute – and they were doing just that.

The genetics labs were just along the (very beautiful, sky-lit, gently curving) corridor from the bone lab. The Institute
felt like a modern monastery, with an all-pervading calm and scholarly atmosphere. But instead of monks painstakingly copying out biblical passages, scientists were locked away in their high-tech scriptoria,
sequencing the Neanderthal genome.