Homo neanderthalensis is an extinct species of archaic humans that lived in Eurasia until roughly 40,000 years ago. The cause of their extinction is an ongoing matter of debate. Now a new study has suggested that the weakening of Earth’s magnetic field may have been what pushed them over the edge.

The magnetic field of our planet protects us directly from the solar wind, high-energy particles that flow outward from the Sun, and indirectly from the Sun's ultraviolet light (UV). The ozone layer, our shield against the UV, is susceptible to high-energy particles (among other things) so a period of lower magnetic field strength could lead to a thinning of the ozone layer, allowing more UV to reach the surface.

As reported in Reviews of Geophysics, the researchers claim that changes to Earth's magnetic field, specifically when it has appeared weakest, coincides with the extinctions of mammals across the world.

Roughly 41,000 years ago, the magnetic field of the planet reversed for a few hundred years. This is known as the Laschamp event, and it's thought the magnetic field fell to just 5 percent of its current strength. According to the researchers' genetic analysis, Neanderthals were particularly susceptible to ultraviolet radiation, and being exposed to the harmful UV that got through could have led to health problems that may have contributed to their decline.

Not only that, but they posit changes in the magnetic field over the last 200,000 years could possibly be linked to other episodes in our human ancestors' family tree by damaging DNA and leading to mutations that were not beneficial to their survival.

“This tells us more about how our ancestors evolved within the mammal population,” lead author Professor James Channell, from the University of Florida, said in a statement. “Ultra-violet radiation was an important influence on mammal evolution.”

“The effect was not a blitzkrieg, but a process that affected the genome over time,” Channell added.

The claim UV radiation played a part in hominins' evolutionary history will certainly start a debate. Most mammals have developed ways to protect themselves from the harmful UV rays, fur being one of them. It's also not as straightforward as genetic mutations in the skin being passed on to the next generation. We know so little when it comes to Neanderthals, there is much to consider. 

We have their genome, but the interpretation of it, just like the interpretation of modern genomes, rests on understanding that is often inadequate, and occasionally contradictory. For example, other researchers have found evidence in regards to UV light that demonstrates the opposite property in Neanderthals. This study published in 2013, showed that a gene related to the UV response in modern Euroasians was lost when their ancestors moved out of Africa and then it was reintroduced by interbreeding with Neanderthals and another species of ancient humans called Denisovan. The authors argued that Neanderthals' genes played a role in protecting modern Eurasians from UV damage.

It's also important to consider that the Laschamp event was not the only dramatic change that our planet was experiencing when the Neanderthals died out. Earth was experiencing a complex climate pattern with dramatic temperature fluctuations, and the disappearance of Neanderthals also coincides with a particularly cold period. That might have been too much for a species who is believed to have already been in decline.