Did the Ancestor of All Complex Life Already Breathe Oxygen?

Yes — the Last Eukaryotic Common Ancestor (LECA), the single cell that gave rise to every animal, plant, and fungus on Earth, was already running on oxygen. New research using genomic analysis has overturned the long-held assumption that early complex cells struggled to tolerate oxygen, showing instead that LECA had fully developed aerobic machinery from the very beginning.

What Is LECA?

LECA stands for the Last Eukaryotic Common Ancestor — the single-celled organism that sits at the root of all eukaryotic life. Every one of the estimated 8.7 million complex species on Earth, including humans, traces its lineage back to this one cell. LECA appeared roughly 1.8 billion years ago, and while it left no fossil record, scientists can reconstruct its biology by comparing genes across thousands of living species — a method called phylogenomics.

The Old Assumption: Oxygen Was the Enemy

For decades, many researchers assumed the first complex cells were anaerobic — meaning they lived without oxygen and could barely tolerate it. The thinking went that the merger between an archaeon (the host cell) and the bacterium that would become the mitochondrion was partly motivated by the need to handle a toxic, increasingly oxygenated world. It was a tidy story. It was also wrong.

Oxygen Arrived Long Before LECA Did

The Great Oxidation Event, roughly 2.4 billion years ago, pushed atmospheric oxygen to around two percent — far below today’s levels, but enough to fundamentally change Earth’s chemistry. That event happened at least 600 million years before LECA ever existed. Oxygen wasn’t a new threat LECA had to adapt to. It was already a stable feature of the world LECA was born into.

What the New Research Found

Researchers using phylogenomics — comparing gene sequences across a wide range of living species — found that LECA already carried the proteins needed for oxidative phosphorylation, the energy-conversion process that powers virtually all complex life today. These weren’t late adaptations. They were inherited from the bacterium that became the mitochondrion, a member of the Alphaproteobacteria, which brought its full aerobic toolkit into the merger. LECA didn’t evolve the ability to use oxygen — it was born with it.

Then Why Did the Merger Happen?

This is where the science gets genuinely unsettled. The old answer — that the archaeal host needed the bacterium to survive an oxygenating world — no longer holds. The new findings strip that motivation away. The leading alternative is syntrophy: the idea that the proto-mitochondrion and its archaeal host formed a metabolic partnership, exchanging useful molecules and energy in a way that gave the merged cell a competitive edge over everything around it.

A prominent hypothesis from 1998, the hydrogen hypothesis, argued that the merger was driven by anaerobic hydrogen exchange between the two microbes. That idea hasn’t been discarded — it’s been folded into a broader model in which both hydrogen-based and oxygen-based metabolism coexisted in the shared ancestor. The merger was likely driven by metabolic cooperation, not environmental crisis.

Why This Matters

Reconstructing LECA isn’t just an exercise in deep history. It tells us something fundamental about the conditions under which complex life becomes possible — and what kind of cell was capable of eventually producing everything from oak trees to octopuses to us. The cell at the root of it all wasn’t gasping for air. It was already thriving on it.