The Ancestor Of All Life On Earth Is Even Older Than Previously Thought

In the beginning, there was no life on Earth. At one point in its 4.5 billion-year existence, one microbe sparked all cellular life in the planet.

Scientists call this mystery microbe the "last universal common ancestor" or LUCA. It is widely believed that all life that has ever existed in the planet sprung from this ancient single cell.

While the age of LUCA hasn't been definitively proven in the past, Inverse reports that scientists have generally agreed on a period between 3.5 to 3.8 billion years ago. However, a new study published in the journal Nature Ecology and Evolution suggests that the ancestor of all life on Earth emerged 100 million years earlier than initially believed.

The Molecular Clock Technique

To arrive to this conclusion, researchers from the University of Bristol and the University of Bath used a method called the "molecular clock" instead of relying on fossils. This technique uses differences in individual species' genomes to determine how much time has passed since a pair shared a common ancestor.

"No fossil can ever tell us to the true age of a clade because the clade must have existed already for the fossil to have been formed," Holly Betts, study author and Ph.D. candidate at the University of Bristol, points out to Inverse. "This is partly why the molecular clock is so useful as we can use it to look at the time prior to the fossil, when the lineages actually appeared."

The team tweaked the molecular clock and applied it to a number of the oldest fossils ever found in the world. A total of 29 genes from 102 living organisms, plus nine fossils, were analyzed for the study.

"We used a relaxed clock framework, which means that the branches across the [evolutionary] tree can have differing rates of evolution," Holly Betts, study author and Ph.D. candidate at the University of Bristol, explains to Inverse. This is because the age differences that the method provides are relative.

After reading the genes of long-dead species with the molecular clock technique, the researchers then used fossils to provide a guide in real time.

The Results

Betts and the rest of the team discovered that bacteria and archaea came 1 billion years after LUCA, followed by eukaryotes 1.8 billion years after.

This discounts a theory that life originated from three primary lineages: eukaryotes, bacteria, and archaea.

"Instead, the eukaryotes arose from within the archaea and were formed via an endosymbiosis event between an archaeal cell and a bacterial cell," says Betts. "So the main hypothesis now has two major lineages of life — the bacteria and the archaea, with the eukaryotes diverging as a lineage within."

The findings imply that the two lineages may have descended from LUCA, which means the age of LUCA could be determined by using bacteria and archaea. With the molecular clock method, the scientists were able to reach an estimation of LUCA's age: 3.9 billion years old.