Magnetic flip UV surge linked to Neanderthal demise

Until 40,000 years ago or so, there were two groups of human ancestors alive on Earth: us - anatomically modern Homo sapiens, and our close relatives, the Neanderthals. They didn’t just overlap in time, either; there’s evidence that the two inhabited some of the same territories and even interbred. But, suddenly, after hundreds of thousands of years, Neanderthals abruptly disappear. A range of theories have been advanced to account for where they went, ranging from competition from our forebears to absorption into a larger, more rapidly expanding anatomically modern human population. But now scientists have another tantalising hypothesis: 41,000 years ago the Earth’s magnetic field flipped. Known as the Laschamps excursion, this temporarily dialled down the magnetic force field that defends us from space radiation, thinning the ozone layer, so UV levels surged. With their more advanced clothes, and a familiarity with iron-based make-up, perhaps our anatomically modern human ancestors were better equipped to deal with it, suggests Raven Garvey, an anthropologist at the University of Michigan…

Raven - Currently, Earth's magnetic field is what's referred to as a dipole. There is a north pole and a south pole, but these poles reverse periodically on roughly a time scale of every 200,000 to 300,000 years. But sometimes Earth's poles will wander a bit and become unstable. This particular group of scientists were interested in a phenomenon about 41,000 years ago where the pole came near to reversal but didn't quite, so the dipolar structure of the magnetic field was quite disrupted. This likely had near-Earth implications, so things that would have been felt by all sorts of life on Earth, which is where I became involved as an archaeologist. I study such things as humans' interactions with their environments.

Chris - What was the population like 41,000 years ago? Who was around and what were they doing and where?

Raven - Just to give a little bit of context, archaeologically, it can be quite difficult to reconstruct demographics to know exactly how many people or even approximately how many people were in a place at a particular time. We believe that populations were quite low. The area where this Laschamps phenomenon, this excursion of the magnetic poles, would have been quite strong was Eurasia. Living in Eurasia at the time were two species of hominins, the Neanderthals and Homo sapiens, which is us.

Chris - Were they overlapping? Were they interacting? What do we know about what they were doing at that time?

Raven - Neanderthals arrived first. They probably developed in Eurasia sometime before 400,000 years ago. Evidence currently suggests that Homo sapiens, anatomically modern humans, arrived in Eurasia probably about 57 or so thousand years ago. Just about the time of this Laschamps excursion, so around 45,000, 43,000 years ago, anatomically modern humans spread westward quite rapidly. There is evidence that they were interacting with Neanderthals. Genetic evidence shows that many living populations of humans, Homo sapiens, us, have between 1% and 4% Neanderthal DNA. There was certainly some amount of interaction going on when these two populations were living side by side in Eurasia.

Chris - Critically, that's also the time point when we know that Neanderthals began to disappear, isn't it? Because if you look much more recently than that, they've gone quite quickly. Are these results suggesting that this phenomenon to do with the reversal of the magnetic field might be linked to that happening?

Raven - That's precisely what we're interested in exploring. Yes, in fact, Neanderthals are no longer around in this area by 40,000 years ago, which is right about the tail end of this geomagnetic phenomenon. One of the things we've wondered is what about that phenomenon changed what people in this place might have been experiencing at the time? One thing that may have occurred is that the dipolar structure of our current magnetic fields creates something like a force field around our Earth, which ordinarily shields us from an awful lot of bombardment of particles from space, including ultraviolet radiation, which you may have heard in other contexts is what causes things like sunburn and skin cancer. And so when the dipolar structure sort of broke up, these magnetic field lines were not creating that shield nearly as effectively. So more of this ultraviolet radiation was making its way to Earth. And so one of the things we wonder is, are there differences in the cultures practiced by Neanderthals versus Homo sapiens that might be significant in this regard? Archaeological sites that we associate with Neanderthals have lots and lots of tools, and some of those tools were probably used for the preparation of hides, probably for things like cloaks or loincloths, maybe some footwear to protect their bodies, because this was also a very cold time in Europe. Sites that we associate with Homo sapiens, conversely, not only have similar kinds of tools that were for dispatching animals and removing their hides and preparing those hides for other purposes, but we also find needles and awls, things that we associate with stitching, with sewing. And so one interpretation of this difference is that Homo sapiens were making tailored clothing, clothing fitted to the limbs like shirts and pants, rather than simple draped clothing that we tend to associate with those Neanderthal sites. And this may have conferred the advantage not only of keeping people warm in what is a cold climate, but if the UV radiation is as we suspect it might have been stronger during the Laschamps excursion, this would have provided a secondary benefit, that of protecting exposed skin from harmful radiation. In addition to the tailored clothing, in sites that we associate with Homo sapiens, there is considerable amount of a substance referred to as ochre, which is iron oxide, that in some modern populations is used as a sunscreen, and there have been experimental studies to show that it is an effective sunscreen. So the fact that the amount of this substance in sites associated with Homo sapiens goes up during this space weather event, the fact that it has been observed as an effective sunscreen in more recent times, is again suggestive that people may have been applying this to the skin as, say, another form of sun protection.

Chris - Is there any evidence that they were doing that, as in artwork or other documentation where you can see people were clearly applying this at that time, which would add kind of weight to the idea that people were self-medicating almost in that way?

Raven - Sadly no, I wish we had such evidence, but the depictions at this time, depictions in rock art, are primarily of animals or abstract designs, and that I know of, there are no representations of hominins, human-like forms, applying a substance to the skin.