Short summary
DNA is like a biological record of inheritance. Because all living things use DNA, scientists can compare that record across species. Those comparisons strongly support the idea that life is related through common ancestry.
Why DNA matters so much
Before genetics, scientists already noticed that related species often resemble one another in body structure. DNA gave them a more direct way to test relationships.
Instead of only comparing bones, feathers, leaves, or shells, researchers could compare inherited sequences themselves.
The basic logic
If two species share a more recent common ancestor, they should usually have more similar DNA than two species whose shared ancestor is much older.
That is exactly the broad pattern scientists find.
This does not mean every gene changes at the same speed or in the same way. But across whole genomes, the family-tree pattern is clear.
A simple analogy
Imagine copying a long document over and over for many generations. Copies made from a recent common version will usually match each other more closely than copies whose shared source is much older.
DNA comparisons work in a similar way. They are not the whole story, but they give strong clues about branching history.
Shared genes and shared biological tools
One striking fact about life is that many very different organisms use many of the same basic genetic tools. That includes genes involved in growth, metabolism, and development.
This does not mean all organisms are nearly identical. It means common ancestry leaves a deep signature.
DNA can test evolutionary predictions
Evolution does not simply allow any pattern. It predicts branching relationships. DNA can be used to test whether those relationships make sense.
For example, if anatomy suggests that certain groups are close relatives, scientists can ask whether DNA fits the same pattern. Again and again, it often does.
That matching is powerful because the evidence comes from different sources.
Genetics also helps explain human relationships with other primates
When people ask whether humans are related to other primates, genetics provides a very strong answer. Humans share substantial DNA similarity with other primates, which fits the broader evidence for shared ancestry.
That does not mean humans came from modern monkeys. It means the lineages are related.
DNA evidence does not replace fossils
Some people talk as if fossils and genetics compete. They do not. Fossils reveal extinct forms and time order. Genetics reveals inherited relationships among living things. Together they provide a much fuller picture.
Why this evidence is especially convincing
DNA is persuasive because it is difficult to explain away as coincidence. When many genes, many species, and many methods all support the same broad tree-like pattern, common ancestry becomes the simplest explanation.
A good next step
If you want to see the historical side of the story, read fossil evidence. If you want to see how change can be measured in the present, read observed evolution.