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How are all living things related?
One of Darwin's most revolutionary ideas was that all living things on Earth are related, connected to each other like branches on a giant "tree of life." If its structure could be worked out, the tree would give us a snapshot of evolutionary history.
Scientists have already worked out much of the tree's basic structure. For instance, we know that at the root of the tree lies a 3.8 billion year old ancestor. This ancestor, which probably resembled the present-day organisms we call bacteria, gave rise to every other life form that has ever lived on Earth. Fairly recently, scientists also realized that there are really only three main branches off the ancestral trunk. All life forms (organisms) fall into one of these groups. Two of the branches contain only bacteria. The third branch holds everything else -- that includes you, your cat, your potted plant, the fruit you ate for lunch, the last mosquito that bit you, and the cotton that's woven into your shirt.
Even though scientists know many of the tree's larger features, others are still under study, and the details -- out in the smallest branches -- are still being worked out. No matter what branch they're working on, they use the same method to figure out evolutionary relationships.
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