An exciting new study reveals that life on Earth might have already flourished within a few hundred million years of the planet’s formation.

Everything alive today derives from a single common ancestor, known as LUCA (Last Universal Common Ancestor). LUCA is the hypothesized progenitor of all modern cellular life, from bacteria to redwood trees and, of course, humans. It represents the root of the tree of life before it splits into the groups recognized today: Bacteria, Archaea, and Eukarya.

The team compared genes from the genomes of living species, counting the mutations in their sequences over time since they shared an ancestor in LUCA. By using the fossil record to date the separation of some species, the researchers employed a genetic equivalent of the equation used to calculate speed in physics to estimate when LUCA existed. Their results indicated LUCA lived around 4.2 billion years ago, only about 400 million years after Earth formed.

“We did not expect LUCA to be so old, within just hundreds of millions of years of Earth’s formation. However, our results fit with modern views on the habitability of early Earth,” said co-author Dr. Sandra Álvarez-Carretero of the University of Bristol’s School of Earth Sciences.

LUCA’s Complex Biology

Biologists use the DNA sequences of modern organisms to reconstruct the Tree of Life and to figure out the likely characteristics of the most recent common ancestor of all living things — the “trunk” of the Tree of Life.

All life forms, no matter how simple or complex, use the same basic genetic code. The nucleotides composing DNA and RNA are universal. Whereas, if multiple ancestors had independently evolved, we would expect to see significant variations in the genetic codes across different forms of life.

Life shares remarkably similar biochemical pathways for fundamental processes, such as DNA replication, transcription, and translation. The enzymes and molecular machinery involved in these processes are highly conserved. Additionally, studies of genomes across different species reveal a striking similarity in the core set of genes involved in essential cellular functions.

The same 20 standard amino acids have been used in proteins throughout the history of life on Earth. This set was likely “canonicalized” or standardized during early evolution. Before this, smaller amino acid sets gradually expanded as organisms developed new synthetic proofreading and coding abilities. Moreover, all living things use adenosine triphosphate, or ATP, as energy to power their cells.

All of these different lines of evidence, drawing from genetic, biochemical, and fossil data, point towards the same conclusion. It seems that all life on Earth can trace its origin to LUCA. Perhaps other types of life forms with radically different biology appeared before our LUCA, but they didn’t survive.

From Bacteria to Humans: Tracing the Tree of Life Back to LUCA

While LUCA’s exact form is still a mystery, researchers suggest it was a simple cell with key components found in all life forms today, such as ribosomal proteins and ATP synthase.

The earliest known microfossils date back to around 3.5 billion years ago. So, we know that life is at least that old. However, since LUCA’s fingerprints are still visible in the genes of modern organisms, scientists can use special techniques to wind back the genetic clock and travel in reverse on the tree of life until they reach the trunk.

To understand LUCA’s biology, the new study modeled the physiological traits of living species back through LUCA’s genealogy. The evolutionary history of genes is complicated by their exchange between lineages. However, using the right model may reconcile these differences and even reveal LUCA’s features.

“One of the real advantages here is applying the gene-tree species-tree reconciliation approach to such a diverse dataset representing the primary domains of life Archaea and Bacteria. This allows us to [speak] with some confidence and assess that level of confidence on how LUCA lived,” said co-author Dr. Tom Williams

The study showed LUCA was a complex organism, similar to modern prokaryotes (single-cell organisms whose cell lacks a nucleus and other membrane-bound organelles), with an early immune system indicating interactions with viruses. Professor Davide Pisani noted that LUCA’s immune system suggests it engaged in an arms race with viruses even 4.2 billion years ago.

LUCA”s waste likely fed other microbes, such as methanogens, creating a recycling ecosystem. This insight implies that an ecosystem was quickly established on early Earth.

A Window to the Early Earth

During the time of LUCA, the early Earth was a dramatically different place compared to today. The planet’s atmosphere lacked oxygen and was instead covered in a thick haze of methane, ammonia, hydrogen, and carbon dioxide. The surface was dominated by oceans, dotted with volcanic islands and hydrothermal vents. These vents spewed hot, mineral-rich water into the sea, likely providing the essential energy and chemical building blocks for early life. Meanwhile, the Earth’s crust was still cooling and solidifying. And the planet was frequently bombarded by comets and meteorites.

Early Earth was hell — but somehow life found a way only four hundred million years after the planet formed. The Earth is 4.53 billion years old. An earlier study that appeared in late 2023 suggests an even earlier timeline for the appearance of LUCA, sometime between 4.32 and 4.52 billion years ago.

The study’s authors emphasize that the study combined data and methods from multiple disciplines to reveal insights into early Earth and life unachievable by any one field alone. The rapid establishment of an ecosystem on early Earth suggests that life may also be thriving on Earth-like biospheres elsewhere in the universe.

The findings appeared in the journal Nature Ecology & Evolution.