Final answer:
Genetic variation among humans decreases the farther you go from Africa due to the Out of Africa model of human migration, where initial small groups founding new populations had limited genetic variation. This pattern is further influenced by genetic drift and adaptations to new environments. The greatest genetic diversity in Africa supports its role as the cradle of human genetic lineage.
Step-by-step explanation:
The reason why genetic variation among humans decreases the further outside of Africa you go is rooted in our evolutionary history. According to the Out of Africa model, all humans share common ancestors from Africa, where the human species has the longest established genetic lineage, as documented by various types of genetic evidence, including mitochondrial DNA (mtDNA). As humans migrated out of Africa, small groups with limited genetic variance founded new populations, which further underwent genetic drift and the founder effect, leading to reduced genetic diversity compared to the larger, more varied population they left behind in Africa. Statement C provides the best explanation by stating that humans most likely descended from Africa, and this is supported by similar genomes found outside of Africa that still maintain close ties to African genomes. There is also genetic variance in Africa that is not found elsewhere, indicating a longer period for the accumulation of mutations and genetic diversity. This is consistent with the principle that the longer a population exists, the more genetic variation it accumulates due to mutations and adaptations. Furthermore, studies of organisms like lice that have coevolved with humans can help fill gaps in the human fossil record, offering insights into human evolution and migration. As humans spread across the globe, they encountered new environments, resulting in further diversification of genetic characteristics.