Life on Earth began somewhere, and scientists believe that “somewhere” is LUCA, the Last Universal Common Ancestor. This prokaryote-like organism is considered the ancestor of all living things, from the smallest bacteria to the largest blue whales.
Although the Cambrian Explosion, which occurred about 530 million years ago, significantly advanced complex life, the timeline of life on Earth extends much further back. Scientists have long estimated that LUCA appeared around 4 billion years ago, only 600 million years after Earth’s formation. However, a new study by an international team of scientists suggests that LUCA might have existed as early as 4.2 billion years ago. The study also reveals intriguing details about LUCA’s life. These findings were published in the journal *Nature Ecology & Evolution*.
To determine LUCA’s appearance on Earth, scientists had to trace backward in time. They compared genes in living species and counted mutations that have occurred since these species shared a common ancestor with LUCA. Using a genetic equation based on the time of separation between species, the researchers concluded that LUCA must have been present as early as 400 million years after Earth’s formation, placing this organism in the middle of the Hadean Eon, a period characterized by extreme geological conditions.
“The evolutionary history of genes is complicated by their exchange between lineages,” said Edmund Moody of the University of Bristol, the study’s lead author. “We have to use complex evolutionary models to reconcile the evolutionary history of genes with the genealogy of species.”
Beyond determining LUCA’s age, the researchers also explored the physiological characteristics of living species to infer what LUCA might have been like 4.2 billion years ago. Their findings revealed that although LUCA was a simple prokaryote, it likely possessed an immune system, suggesting it was already combating primordial viruses.
“It’s clear that LUCA was exploiting and changing its environment, but it is unlikely to have lived alone,” said Tim Lenton of the University of Exeter, a co-author of the study. “Its waste would have been food for other microbes, like methanogens, that would have helped to create a recycling ecosystem.”
While LUCA is recognized as the oldest common ancestor, scientists still seek to understand how life evolved from its very beginnings to the early communities LUCA was part of. Future studies will need to delve deeper into this primordial history to uncover how all living things, including humans, came to exist.