Dating a Neanderthal
Clash of cultures or ships in the night?
Radiocarbon dating relies on the fact that the radioactive isotope carbon 14 (14C) has essentially identical chemical properties to common carbon 12. Whilst organisms are alive they constantly exchange carbon with the environment, some of which is 14C. However when the organism dies that exchange ceases so the amount of 14C is fixed and the radioactive decay clock begins. If you measure the proportion of 14C to 12C in something like a bone and compare it to that in the environment, you can work how old that bone was when it died. At least that’s the theory.
Unfortunately as with so many things in science the reality can at times be rather more complex than the theory and this can stir enormous debates in every field from art history to archeology.
One such debate rages around the coexistence of modern humans (Homo sapiens) and Neanderthals in Iberia (modern day Spain and Portugal). In Northern Iberia there’s evidence to suggest that Neanderthals used tools and decoration similar to the Homo Sapiens which some people say may reflect an exchange of ideas between the two groups. However such interaction did not last long because the Neanderthals did out very soon after the arrival of modern humans around 42,000 years ago.
In the south, the story appears to be quite different. Radiocarbon dating of bones there suggests Neanderthals survived 5,000 years after modern humans were in the north. Why two adjoining regions should have such different histories is one of the great puzzles of archeology.
There are many possible explanations but Dr Rachel Wood from the ANU Research School of Earth Sciences is gathering evidence that suggests the answer may simply be that we have the dates wrong. She believes that the samples used to determine the age of Neanderthal bones from southern Iberia were contaminated. Not by bad scientific practice, but by inevitable natural interactions of bone with the warm environment of the region.
“Iberia has a warm climate which means that organic material does not survive well. This means that contamination is a massive problem because the molecules we are looking for are very scarce, and the younger contaminating molecules very abundant. The type of contaminant depends on what material we want to date, but normally they are things like carbonates and humic acids (the group of molecules that make soil brown). It’s contaminants just like these that bones are exposed to when buried for thousands of years” Dr Wood explains.
Because the half-life of 14C is quite short, it doesn’t take much contamination to throw the date off by a long way. “If you have a 60,000 year old bone and you swap out just one in a hundred carbon atoms for modern carbon the dating will return an age of only 37,000 years,” Dr Wood says, “And that’s a really important difference when we’re trying understand what was happening at this critical point in human history.”
As if the contamination in the ground wasn’t enough of a problem, the scarcity of Neanderthal bone and measures taken to preserve it have added to the difficulty. “Often Neanderthal remains from the period were protected as they were removed from the ground with acrylic sealants but unfortunately that can also contaminate the samples.” Dr Wood says, “So what we’ve been looking at is animal bones which are far more common and easily accessible.”
If archeologists find animal bones in the same location as Neanderthal remains and those bones bear the cut marks of stone tools then it’s fairly clear that those animals were killed and eaten by the Neanderthals. “Because animal bones are far more common and ironically because less effort has been made to preserve them, they sometimes provide a better window on what’s happening than Neanderthal bones.” Dr Wood says.
Bone is made of mineral, hydroxyapatite, and protein, collagen. Carbon in the mineral is found as a carbonate ion, very similar to the carbonate in water, and the two will exchange places easily. So instead of looking at the mineral which is highly prone to contamination, the scientists instead look at residual proteins in the bone. Carbon atoms tied in the amino acids that build proteins like collagen are relatively immune to substitution by groundwater carbon so provide a more reliable dating method. Of course the tricky bit is getting at and refining that protein.
“Decay is a huge factor in samples from Iberia,” Dr Wood explains, “out of 200 bones we found 6 that had sufficient protein to work with!”
The protein was extracted from the bone in solution, and then a molecular sieving technique was used to separate the intact collagen chains from broken fragments and other molecular debris. And when this material was radiocarbon dated the results showed that it was in fact over 50,000 years old, not 35,000 as the mineral bone samples suggested.
“The situation is highly complex and we’ve only been able to study a relatively small number of sites, but from what we’ve seen it looks very much like the reason Homo Sapiens and Neanderthals didn’t seem to interact much in Southern Iberia is that the sites we thought were young are actually much older. In these sites there is no chance that the Neanderthals even met Homo sapiens!” Dr Wood says.