I will try to break it down into four points:
1) If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor's DNA sequence.
2) More closely-related species will have a greater fraction of identical sequence when compared to more distantly-related species.
3) Neutral human DNA sequences are approximately 1.2% divergent from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons, etc.
4) So with DNA we can map out an eloquent family tree relating all species on Earth, and this exactly matches what you would expect by studying the fossil record and geographic distribution of animals.
1) If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor's DNA sequence.
2) More closely-related species will have a greater fraction of identical sequence when compared to more distantly-related species.
3) Neutral human DNA sequences are approximately 1.2% divergent from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons, etc.
4) So with DNA we can map out an eloquent family tree relating all species on Earth, and this exactly matches what you would expect by studying the fossil record and geographic distribution of animals.