Ancient complex life depended on oxygen, study of early fossils finds

Thursday 21 May 2026 - 08:02

By: Dakir Madiha

Ancient complex life depended on oxygen, study of early fossils finds

The earliest known eukaryotes on Earth, the ancestors of all complex life including animals, plants and fungi, lived in shallow oxygen-rich seafloors nearly 1.75 billion years ago, according to a new study published in Nature. The findings challenge long-standing theories that early complex organisms first evolved in low-oxygen waters or drifted freely through ancient oceans as plankton.

Researchers from McGill University and University of California Santa Barbara analyzed ancient microfossils recovered from drilling cores in Australia’s McArthur and Birrindudu basins. The region, located in the Northern Territory, preserves some of the oldest generally accepted eukaryotic fossils ever discovered. Scientists said the area once consisted of shallow inland seas filled with lagoons, mudflats and calm coastal waters between 1.75 billion and 1.4 billion years ago.

The study found that ancient eukaryotes appeared almost exclusively in rocks formed in oxygenated marine environments. Researchers linked fossils to four separate habitats, including lagoons, tidal zones, coastal waters and offshore environments. They also used mineral indicators such as iron pyrite to estimate oxygen levels within each setting. The evidence suggests that these organisms depended on oxygen during at least part of their life cycle.

Co-author Leigh Anne Riedman said the fossil distribution showed that the earliest known eukaryotes likely lived on or within seafloors rather than floating in open water. The research also supports the idea that eukaryotes acquired mitochondria early in their evolutionary history. Scientists believe close contact with bacteria on the seafloor may have enabled the biological integration that eventually produced mitochondria, the energy-producing structures found in all modern eukaryotic cells.

Researchers said this restricted habitat could explain why eukaryotic life remained relatively simple and limited in diversity for nearly one billion years after its emergence. Fossils dating from 800 million years ago still closely resemble forms that appeared around 1.7 billion years ago, suggesting slow biological development during much of Earth’s early history.

The study also proposes that eukaryotes did not expand widely into open oceans until roughly one billion years later, a transition that may have transformed Earth’s biosphere. Co-author Maxwell Lechte, now at the University of Sydney, said the movement into open marine environments likely marked a major ecological turning point for complex life on Earth.