Forty thousand years ago, during the so-called Laschamp geomagnetic excursion, when Earth’s magnetic field intensity abruptly declined, enhanced ultra-violet radiation (UVR) as a result of low field strength resulted in the selection of our Cro-Magnon ancestors to the detriment of the Neanderthals. A genetic variant of a protein, known as the Aryl Hydrocarbon Receptor (AhR), that is sensitive to UVR, was apparently the difference between Cro-Magnon (modern humans) and Neanderthals. During a short time interval (about 2000 years) of very low field intensity (about 25% of today’s field) about 40,000 years ago, the Neanderthals apparently succumbed whereas modern humans prospered. The study conducted by the Cnr-Ismar and the University of Florida is published in Reviews of Geophysics
Why did Neanderthals become extinct 40 thousand years ago? One of the great mysteries of paleoanthropology is the subject of a research published by the journal Reviews of Geophysics and the result of collaboration between geologists of the Institute of Marine Sciences of the Italian National Research Council (Cnr-Ismar) in Bologna and the University of Florida in Gainesville. Combining the dating on the disappearance of the Neanderthals (41,030-39,260 years ago) from paleolithic sites with genetic data, Luigi Vigliotti and James Channell identified the so-called Laschamp excursion, an excursion of Earth's magnetic field, which took place at this time (41,300 +/-600 years ago) as an important contributing factor. The Earth's magnetic field protects the integrity of the ozone layer (thereby preventing particles transported by the solar wind from destroying it), and the effects of UVR resulted in the selection of our Cro-Magnon ancestors to the detriment of Neanderthals. The cause is to be found in a genetic variant of a protein known as the aryl hydrocarbon receptor (AhR), sensitive to UVR, which was apparently fatal to Neanderthals during a short time interval (about 2000 years) of low magnetic field intensity.
"Neanderthals and Cro-Magnon lived together in Europe for several thousand years, even interbreeding, as shown by the imprints left in our DNA and the somatic traits of some contemporary individuals," explains Luigi Vigliotti of Cnr-Ismar. The reason for the extinction has been subject of several hypotheses including the fratricidal instinct of our ancestors. However, no widely accepted explanations had been found up to now. The discovery by molecular biologists in 2016 of the existence of the variant Ala-381 in the AhR of the Neanderthals compared to the Val-381 variant in modern humans (and fossil Cro-Magnon) was interpreted as a biological advantage for Cro-Magnon in the absorption of environmental toxins, perhaps produced by cooking-smoke exacerbated by troglodytic lifestyles. AhR is fundamental in regulating the effect of a range of environmental toxins including UVR. The timing of Neanderthal extinction implies that oxidative stress produced by the effect of UVR during the Laschamp excursion was the reason for extinction. "The genetic variant of this protein, that exercised control on stress produced by unshielded UVR, appears to have been the cause of Neanderthal demise”, recalls the researcher at Cnr-Ismar.
Many aquatic and terrestrial organisms developed strategies to limit the damage that UVR can impart on the structure of DNA, but there are also positive effects of UVR in the synthesis of vitamin D as well as in fighting viruses, bacteria and parasites. It is not mere coincidence that the Laschamp excursion marks the exit of the Neanderthals and the expansion of Cro-Magnon" continues Vigliotti. "The Laschamp was not fatal only to the Neanderthals. In Australia, 14 genera of large mammals, became extinct at the same time, as shown not only by fossil data but also by the abrupt reduction in concentration in sediments of sporormiella, a coprophilous fungus that lives on the dung of large herbivores. An additional field intensity minimum ~13 thousand years ago is implicated in the disappearance of large mammals in North America and Europe. In North America, where the timing of extinction is best documented, the demise of ~35 genera of large mammals occurred a “geological instant” at about this time. The foci of extinction (e.g. Australia and Europe at ~40 ka, and North America and Europe at ~13 ka) depended on the specific geometry of stratospheric ozone depletion during episodes of low field strength. The authors believe that the role of UVR in the disappearance of these animals was far more important than “overkill” by humans or changing climatic conditions.
The research just published also analyzes the relationships between magnetic field intensity and human evolution during the last 200 thousand years. "We integrated existing fossil data with dating of the main branches of human evolution based on the analysis of mitochondrial DNA and Y-chromosomes. Despite the scarcity of fossils and the large margins of error in methodologies used to reconstruct the age of branching in the various haplogroups (groups with the same genetic profile), we observe interesting relationships with magnetic field strength" concludes Vigliotti. "An age of 190 thousand years for early fossils of modern humans (in Ethiopia) and for the age of the so-called Mitochondrial Eve, our most recent common (maternal) ancestor, coincide with another episode of low magnetic field intensity known as the Iceland Basin excursion. Various developments in human evolution concentrated between 100 and 125 thousand years ago, an interval of the last interglacial, correspond with another minimum in Earth's magnetic field strength at the Blake excursions (125-100 kyr ago). Knowledge of the past geomagnetic field and its role in modulating UVR, and the role of AhR with respect to UVR, as well as more accurate dating of new fossil finds and improvements in human phylogeny, will clarify the role of the intensity of Earth’s magnetic field in the evolution of mammals.
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