A recent study by climate scientist Aixue Hu, reported yesterday by Science Now, explains the role of the Beringian land bridge in violent climate swings over last ice age, which ultimately led to the extinction of the Neanderthals.
Earth’s climate has been relatively stable since end of the last ice age, but between 80,000 and 10,000 years ago average temperatures in the North Atlantic rose or fell by 10°C or more over the course of a decade or two. The cause of these climate swings has been the subject of debate by scientists.
80,000 years ago, vast ice sheets expanding over northern Eurasia and North America captured more and more of Earth’s water, causing global sea levels to drop around 50 meters lower than they are today. This lead to land masses around the world becoming exposed, including a huge land bridge connecting Siberia and Alaska at what is know the Bering Straits. This land bridge, christened ‘Beringia’, eventually became settled by modern humans of Siberian origin 15–20,000 years ago and was used as a stepping stone for the first migration into North America.
Hu’s team ran two sets of climate simulations: one where the Bering Strait was open, as it is now, and one where it was closed, and Beringia separated the North Pacific from the Arctic Ocean. In each simulation, the team added large amounts of fresh water to the North Atlantic, spanning latitudes from southern Cuba to southern England, to simulate the meltwater from northern ice sheets during occasional warm spells. In both sets of simulations, the fresher surface waters never got denser than the underlying salty water, so never sank to the seafloor to flow southward. This temporarily shut down ocean circulation, including the Gulf Stream, and plunged areas around the North Atlantic into a cold spell. When the Bering Strait was closed, it took as long as 1,400 years for ocean circulation to recover; when the strait was open, the circulation took less than 400 years to recuperate. The study confirmed that ocean circulation is more stable when the strait is open.
This ties in with recent findings on the extinction of Neanderthal Man in Europe. Many scientists previously thought a relatively stable population of Neanderthals were killed off after being out-hunted, killed off or even ‘bred out’ by modern humans after their appearance in Europe from around 35,000 years ago. However DNA research showed that climate change probably pushed Neanderthals to the verge of extinction long before modern humans arrived on the scene.
The study of Neanderthal mitochondrial DNA was carried out by an international research team led by Anders Götherström and Love Dalén and found that Neanderthal specimens from Western Europe older than 50,000 years, as well as specimens from sites in Western Asia and the Middle East, showed a high level of genetic variation, while Western European Neanderthals younger than 50,000 years showed an extremely low amount of genetic variation.
This suggested that western European Neanderthals went through a population bottleneck, which happened to coincide with a period of extreme cold in Western Europe. This small group then recovered and repopulated the region. The fact that this occurred long before they came into contact with modern humans came as a complete surprise to Dalén and the team, as it indicated that the Neanderthals were more sensitive to dramatic climate change than previously thought and that their populations were already demographically stressed when modern humans arrived in Europe. Neanderthals, with their heat-storing barrel chests and stocky limbs, were probably biologically adapted to deal with wild fluctuations between extreme cold to mild cold and back again in a matter of a few decades. However, rapid climate change would have caused ecological changes which the Neanderthals were slower to adapt to, such as the replacement of their favoured woodland hunting grounds with open grasslands, better suited to hunting by longer-limbed modern humans.
The land bridge of Beringia, and the unstable ocean currents it created, ultimately proved to be the undoing of the Neanderthals.