Thanks to the tools of modern genetics, scientists are working to identify a little bug that they believe was the ancestor of every creature alive today.
They call it LUCA -- shorthand for the ``last universal common ancestor'' -- and they think it inhabited the Earth 3 billion to 4 billion years ago.
LUCA consisted of only a single cell, like a bacterium, scientists say, but its descendants comprise modern humans, animals, plants, fungi and invisible microbes.
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