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DNA is a molecule that does two things. First, it acts as the hereditary material, which is passed down from generation to generation. Second, it directs, to a considerable extent, the construction of our bodies, telling our cells what kinds of molecules to make and guiding our development from a single-celled zygote to a fully formed adult. These two things are of course connected. The DNA sequences that construct the best bodies are more likely to get passed down to the next generation because well-constructed bodies are more likely to survive and thus to reproduce. This is Darwin’s theory of natural selection stated in the language of DNA.