Final answer:
The strongest evidence for common ancestry among all organisms is the similarity in DNA sequences and the existence of homologous structures, as both DNA and anatomical features reflect the shared genetic heritage and developmental patterns stemming from a common ancestor.
Step-by-step explanation:
The strongest evidence that all organisms on Earth evolved from a common ancestor is that all organisms inherit instructions for their development in DNA. When scientists compare DNA sequences, they find that more similarities in these sequences suggest a closer evolutionary relationship. This evidence is particularly compelling because DNA is the molecular basis of heredity in all life forms.
Another strong piece of evidence comes from the existence of homologous structures, which are similar structures found in different species that originated from a common ancestor. For example, the bone structure in the limbs of mammals, birds, and other species shows a common pattern of organization despite their diverse forms and functions.
While the fossil record provides valuable insights into the history of life, the fact that the DNA sequences of different organisms are highly homologous is a more direct and robust line of evidence for common descent because it reflects the underlying genetic information that governs the development and functioning of all living things.