Final answer:
The fact that all organisms have DNA indicates they share a common ancestor. The similarity in DNA sequences among more closely related species supports evolutionary relationships, while the study of genetics supports the idea of a shared tree of life for all organisms.
Step-by-step explanation:
The presence of DNA in all organisms suggests common genetic material and points to the idea of a common ancestor. All living things are composed of cells, which contain DNA that holds the genetic information passed onto daughter cells. This universal genetic code is strong evidence for evolution and indicates that all life on Earth shares a connection. The fact that DNA sequences are more similar in more closely related organisms supports this notion as well. Thus, when scientists compare the genomes of different species, they can assess the evolutionary relationships among them. For example, the human genome is not entirely unique and shares many similarities with other organisms, suggesting that even if we find a gene unique to humans, it does not refute our shared ancestry with other forms of life.What's more, the study of the genetic code has revealed a shared branch of life for all species, reflecting a tremendous amount of diversity, particularly at the microbial level. This is visualized in the 'tree of life', which shows the evolutionary relationships between different organisms based on genetic data. The almost identical nature of genomes across organisms, especially within mammals, reinforces the idea of evolution from a common progenitor.