Who or what is LUCA?

Is there an evolutionary starting point and a primordial organism from which all modern life descended?

Over billions of years life has diversified and adapted to Earth’s changing environment. From the earliest microbes to the complex life we see today, the hand of evolution has shaped the trajectory of life. But what would happen if we ran the tape backwards? If we descend down the evolutionary tree, what organism would we find at its roots?

Charles Darwin proposed the existence of an evolutionary starting point and a primordial organism from which all modern life descended. This started the search for a last universal common ancestor or ‘LUCA’. In the 20th century the theory gained weight after the genetic code was deciphered and found to be universal across all life on Earth.

So what sort of beast was LUCA? Several scenarios have been proposed by molecular evolutionists. LUCA was most likely a single-celled organism that lived between three and four billion years ago. It may have used RNA both to store genetic information like DNA, and to catalyse chemical reactions like an enzyme protein. Similar to some species of archaea – ancient and very tough microbes – LUCA would have been highly resistant to extreme environments. Some scientists have even suggested that LUCA emerged close to deep sea hydrothermal vents (below).

Still, there may not be one LUCA as such, but an indefinable evolutionary starting point for contemporary life. One person who believed this was legendary late microbiologist Carl Woese. He suggested that all life evolved through horizontal gene transfer between ancient organisms as opposed to solely vertical evolution. And so the question of where we ultimately came from may never be answered exactly, but the glimpses that science gives us will no doubt continue to inspire – as with any good family story.