Once upon a time, Earth was a barren planet. That changed dramatically when, out of the chemistry present during its early history, something began to squirm processing matter to survive, reproduce, and thrive. The nature of that something, and when it first appeared, have puzzled humanity for as long as we have been able to ask, “What am I?”
Now, a groundbreaking study has provided some answers, revealing that life emerged surprisingly early in Earth’s history. By analyzing the genomes of organisms alive today, scientists have determined that the last universal common ancestor (LUCA) the first organism that spawned all current life on Earth emerged as early as 4.2 billion years ago.
Given that Earth is around 4.5 billion years old, this suggests that life began when the planet was still a relative newborn. “We did not expect LUCA to be so old, within just hundreds of millions of years of Earth formation,” says evolutionary biologist Sandra Álvarez-Carretero of the University of Bristol in the UK. “However, our results fit with modern views on the habitability of early Earth.”
In its early days, Earth had an atmosphere vastly different from today’s, one that would be extremely toxic to current life forms. Significant amounts of oxygen did not emerge until around 3 billion years ago. Nevertheless, life managed to emerge much earlier; fossilized microbes from 3.48 billion years ago have been discovered, and scientists believe conditions on Earth may have been stable enough to support life around 4.3 billion years ago.
However, due to Earth’s erosional, geological, and organic processes, finding evidence of life from that time is nearly impossible. Thus, a team of scientists, led by phylogeneticist Edmund Moody of the University of Bristol, turned to the genomes of living organisms and the fossil record to seek answers.
Their study utilized a molecular clock, a method that estimates the rate at which mutations occur and counts them to determine the time elapsed since organisms diverged from a common ancestor. All organisms, from microbes to fungi, share common traits such as a universal genetic code, a common protein-making process, and the use of adenosine triphosphate (ATP) as an energy source in cells.
Based on these similarities and differences, Moody and his colleagues calculated how long it has been since LUCA’s descendants started to diverge. Using complex evolutionary modeling, they uncovered more about LUCA itself its nature and survival mechanisms on an inhospitable early Earth.
The study found that LUCA was likely similar to a prokaryote, a single-celled organism without a nucleus. It was not reliant on oxygen, which aligns with the low oxygen levels of early Earth. LUCA’s metabolic processes likely produced acetate. Interestingly, LUCA appeared to possess an early immune system, indicating an arms race with viruses as early as 4.2 billion years ago.
“Our study showed that LUCA was a complex organism, not too different from modern prokaryotes,” says phylogenomicist Davide Pisani of the University of Bristol. “But what is really interesting is that it’s clear it possessed an early immune system, showing that even by 4.2 billion years ago, our ancestor was engaging in an arms race with viruses.”
The research implies that LUCA was not alone; other life forms could have emerged shortly after it, using its metabolic waste products. This suggests that it takes relatively little time for a full ecosystem to develop, a finding with profound implications for the potential of life on other planets.
“Our work draws together data and methods from multiple disciplines, revealing insights into early Earth and life that could not be achieved by any one discipline alone,” explains paleobiologist Philip Donoghue of the University of Bristol. “It also demonstrates just how quickly an ecosystem was established on early Earth. This suggests that life may be flourishing on Earth-like biospheres elsewhere in the Universe.”
The research has been published in Nature Ecology & Evolution.