Final answer:
The exact percentage of individuals with inversions visible via light microscopy is not available in the texts, but chromosome 18 inversion is a significant event contributing to human evolution.
Step-by-step explanation:
The question pertains to the prevalence of chromosomal inversions, specifically the inversion of chromosome 18, which can be detected through light microscopy. While the provided texts do not specify the percentage of individuals possessing visible inversions, they do discuss the chromosome 18 inversion that occurred around five million years ago, following the divergence of early humans from a common ancestor with chimpanzees. This inversion involves the duplication and reinsertion of approximately 19,000 nucleotide bases on chromosome 18p in an inverted orientation. The light microscope is a tool used to identify such structural rearrangements in chromosomes since it magnifies cells to a level where these inversions can be observed.
The chromosome 18 inversion is of particular interest because it does not render individuals nonviable, impaired, or infertile; rather, it has contributed to the evolution of humans, distinguishing us from our closest genetic relatives, the chimpanzees. Human evolution has been marked by such structural rearrangements, including pericentric inversions on several chromosomes and the fusion of chromosomes seen in chimpanzees but represented as chromosome two in humans. The presence of such an inversion attests to the significance of chromosome structure in our evolutionary history.