The clearest picture yet of our “last universal common ancestor” suggests it was a relatively complex organism living 4.2 billion years ago, a time long considered too harsh for life to flourish.

If you follow any path of ancestry back far enough, you’ll reach the same single point. Whether you begin with gorillas or ginkgo trees or bacteria that live deep in the bowels of the Earth — or yourself, for that matter — all roads lead to LUCA, the “last universal common ancestor.” This ancient, single-celled organism (or, possibly, population of single-celled organisms) was the progenitor of every varied form that makes a life for itself on our planet today.

LUCA does not represent the origin of life, the instance whereby some chemical alchemy snapped molecules into a form that allowed self-replication and all the mechanisms of evolution. Rather, it’s the moment when life as we know it took off. LUCA is the furthest point in evolutionary history that we can glimpse by working backward from what’s alive today. It’s the most recent ancestor shared by all modern life‚ our collective lineage traced back to a single ancient cellular population or organism.

“It’s not the first cell, it’s not the first microbe, it’s not the first anything, really,” said Greg Fournier(opens a new tab), an evolutionary biologist at the Massachusetts Institute of Technology. “In a way, it is the end of the story of the origin of life.”

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A Probabilistic Portrait

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