Complex life may have started before Earth had much oxygen at all. A University of Bristol-led Nature study found that the ancient archaeal lineage behind all plants, animals and fungi began developing complex cellular machinery in anoxic oceans almost 2.9 billion years ago — nearly a billion years earlier than some scientists thought.

Complex life is usually told as an oxygen story. First Earth changed, then life became more complicated: oxygen rose, cells gained new energy options, and the long route toward plants, animals and fungi opened up.

A University of Bristol-led Nature study makes that order less tidy. In the paper, Christopher J. Kay and colleagues argue that the ancient archaeal host lineage from which eukaryotes emerged was already developing parts of the cellular machinery associated with complex cells in largely anoxic oceans almost 2.9 billion years ago, long before oxygen became abundant in Earth’s atmosphere.

The finding does not mean animals, plants or fungi existed 2.9 billion years ago. It means the cellular toolkit that later made eukaryotic life possible may have begun assembling much earlier than the fossil record of recognisable complex organisms would suggest.

The lineage behind complex cells

The organisms at the centre of the question are archaea, one of life’s major domains. Within that domain, Asgard archaea have become central to debates over eukaryogenesis: the evolutionary process that produced cells with nuclei, internal compartments and mitochondria.

In a 2017 Nature paper, Katarzyna Zaremba-Niedzwiedzka and colleagues described how Asgard archaea illuminate the origin of eukaryotic cellular complexity. Their genomes contained proteins once thought to be specific to eukaryotes, including components associated with membrane trafficking and other cell-organisation systems.

That finding did not show a fully eukaryotic cell hiding inside an archaeon. It showed something subtler: pieces of the later eukaryotic toolkit already existed among archaeal relatives. The new Bristol-led study asks when those pieces came together.

The Nature paper, “Dated gene duplications elucidate the evolutionary assembly of eukaryotes”, uses dated gene duplications to reconstruct the sequence of events. The University of Bristol release described the result as support for a complexified-archaean, late-mitochondrion model, or CALM.

This is one study, not settled consensus. It relies on evolutionary reconstruction, molecular dating and comparison of modern genomes, rather than a fossil cell that can be pointed to under a microscope.

Why the oxygen timing matters

Earth’s oxygen history is not a simple switch from empty to full. In a 2008 Nature feature, Lee Kump described the emerging picture of the rise of atmospheric oxygen, from an early atmosphere almost devoid of oxygen to the oxygen-rich world of today. The Great Oxidation Event is usually placed around 2.4 billion years ago, though details of local oxygen oases and ocean chemistry remain debated.

If the Bristol-led date is right, the archaeal host line leading toward eukaryotes was already accumulating complex-cell machinery before that major atmospheric transition. That matters because eukaryotes are often discussed in connection with oxygen: mitochondria use oxygen efficiently, larger cells have higher energy demands, and later multicellular life depends heavily on oxygen availability.

The paper therefore separates two questions that are often blurred together. One is when the molecular parts of complex cells began to appear. The other is when oxygen-rich environments allowed those parts to be used in the kind of cells and organisms that later dominated the visible biosphere.

Those two dates may not be the same.

What cellular machinery means here

The phrase can sound vague, so precision matters. The Nature paper says the inferred host already had complex cellular features before mitochondrial endosymbiosis, including an elaborated cytoskeleton, membrane trafficking, endomembrane systems, phagocytotic machinery and a nucleus. Those are the kinds of systems eukaryotic cells use to organise themselves internally, move material and manage compartments.

The Bristol-led study adds the time dimension. It suggests that these kinds of cellular systems were not late decorations added only after oxygen became plentiful. Some may have been part of an older archaeal experiment in cellular organisation.

Anoxic does not mean simple

One tempting misread is that life without oxygen must have been primitive in every sense. That is too blunt. Anoxic oceans can still host complex microbial ecologies, chemical gradients, syntrophic partnerships and elaborate metabolic strategies. Low oxygen limits some options, but it does not stop evolution from producing intricate molecular systems.

That point is important for early Earth, and also for astrobiology. If key parts of cellular complexity began in oxygen-poor environments, then oxygen may not be the only environmental signpost for the beginnings of more complicated biology. It may still matter enormously for later large, active organisms, but the first steps toward eukaryotic cell architecture may have been less dependent on an oxygen-rich planet than many simple accounts suggest.

The finding also sits alongside evidence that Asgard archaea themselves are metabolically varied. A 2023 Nature paper led by Laura Eme placed eukaryotes within Asgard archaea, as a sister lineage to Hodarchaeales within Heimdallarchaeia, and inferred that the lineage toward eukaryotes acquired the genetic potential to support a heterotrophic lifestyle. That study, on the heimdallarchaeial ancestry of eukaryotes, treated eukaryotic origins as a reconstruction problem rather than a single fossil discovery.

The new oxygen-poor timing belongs in that same cautious category. It is a reconstruction of a deep evolutionary branch, not a recovered ancient organism.

What the study does not prove

The paper does not show that complex multicellular life began 2.9 billion years ago. It does not show that oxygen was irrelevant to later eukaryotic success. It does not settle exactly what the last archaeal host looked like, or whether it resembled any Asgard archaeon living today.

It does suggest that the foundation was older and stranger than a simple oxygen-first story allows. Some of the machinery that later became central to complex cells may have been taking shape in archaeal lineages while Earth’s oceans were still largely anoxic and long before eukaryotes became obvious in rocks.

That changes the emphasis. Oxygen may have helped complex life expand, diversify and become energetically demanding. But the first molecular steps toward cellular complexity may have begun in a world that did not yet look hospitable to the life forms that eventually inherited it.

The next tests will come from better molecular clocks, more Asgard archaeal genomes, structural work on the proteins themselves, and, where possible, living cultures that let researchers move from inferred machinery to observed cell biology.