DNA replication isn’t 100% reliable, there are errors during copying. You typically have ~70 mutations from your parent’s DNA. Most will be benign or no effect (there is a lot of redundancy in DNA amino acid encoding), eg GCU, GCC, GCA, GCG all code to the same amino acid. Some mutations maybe beneficial. If there were mutations that were fatal, you wouldn’t be around to notice.
Add up 70 mutations over thousands of generations and 3 billion DNA bases and you eventually get 1-2% differences.
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