Final answer:
DNA structure is the most direct evidence of the common ancestry of all life because it is consistent across all known life forms, indicating that all organisms descended from a common ancestor with DNA as its genetic material.A is the correct answer.
Step-by-step explanation:
The question relates to the topic of evolutionary biology, specifically the evidence for the common ancestry of all life.
Among the options provided, a. DNA structure is the most direct evidence of the common ancestry of all living organisms. This is because the DNA molecule, the carrier of genetic information, is remarkably consistent across all known life forms, suggesting a common origin. The structure of DNA, including its four nucleotide bases (adenine, thymine, cytosine, and guanine), the double helix formation, and the genetic code, is universal.
This universality indicates that all life on Earth has descended from a common ancestral life form that used DNA as its genetic material.
While fossilized footprints (b), atmospheric composition (c), and rock formations (d) can provide information about past life, environments, and geological history, they do not directly imply genetic relationships among different organisms.