Modern humans evolved in Africa approximately 200,000 years ago (Campbell and Tishkoff 2010). As groups migrated out of Africa they underwent bottlenecks leading to sharp reductions in population size and genetic diversity (Amos and Hoffman 2010; Harpending and Rogers 2000; Ramachandran et al. 2005). To this day, African populations retain the most genetic diversity globally (Auton et al. 2015). In order to survive both within and out of Africa, early human populations had to adapt to their novel environments, including new food resources, colder climates, higher altitudes, and, especially, infectious diseases (Balaresque et al. 2007; Fumagalli et al. 2011). These adaptive requirements, facilitated by natural selection, led to an increased frequency of alleles that were beneficial in that environment. Due to the fact that these adaptive requirements were driven by local environmental pressures, some of these evolutionarily advantageous alleles display geographic and ancestral specificity, as observed in the genomes of present-day humans (Fumagalli et al. 2011).
The geomagnetic field extending outward beyond Earth’s solid surface encounters a strong, highly variable flow of hot ionized gas from the Sun called the solar wind. This compresses and shapes the dayside of Earth’s magnetic field. On the night (anti-sunward) side of the Earth, the magnetic field gets drawn out into a long, comet-like tail. Present evidence is that this magnetotail region extends to hundreds or thousands of Earth radii. Research over the past six decades has revealed much about the various current systems that shape the Earth’s "magnetosphere". This chapter is devoted to providing a broad overview of the individual current systems that, acting together, generate the complex and fascinating geomagnetic field.
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