Which describes oxygen content as earth evolved over time? oxygen levels sharply declined about 400 million years ago. oxygen levels stay above 15 percent starting 200 million years ago oxygen levels remain below 5 percent starting 1000 million years ago. oxygen levels raise significantly between 1000 and 700 million years ago.

Astudy by solloch and et. al., in 2017, gives the map above which shows the frequency of alleles with a ccr5-delta32 mutations over 87 different countries. this mutation deletes the presence of a co-receptor (ccr5) on the outside of human t-cells (lymphocytes). some viruses, such as the one responsible for the black death and human immunodeficiency virus (hiv), require this receptor for attachment to host cells during the infection process. the black death was an epidemic that passed over northern europe during the 14th century killing nearly 60% of europeans. according to this information, which explanation best explains why northern europeans show a greater immunity for hiv than some other parts of the world?