September Scientists studying ancient rocks from northern Australia have uncovered more than 12,000 microscopic fossils, providing new evidence about the environments in which some of Earth’s earliest complex cells lived.

The fossils are between approximately 1.75 and 1.4 billion years old and belong to early eukaryotes—the group of organisms whose descendants eventually gave rise to animals, plants, fungi and many other forms of complex life. The study was published in Nature.

Ancient Life Hidden in Australian Rocks

The fossils were recovered from mudstone preserved in rock cores stored by the Northern Territory Geological Survey in Darwin.

These rocks formed from sediments deposited on the floor of an ancient sea that covered parts of northern Australia more than a billion years ago.

Researchers analysed the geological material to determine not only what organisms were present, but also the conditions in which they lived.

More Than 12,000 Fossils Examined

Scientists dissolved samples of the ancient mudstone and examined the remaining organic material under microscopes.

They identified more than 12,000 fossil specimens, allowing them to compare the distribution of early eukaryotes across different ancient environments.

The fossils were found in environments ranging from coastal mudflats to offshore marine settings. However, an important pattern emerged: the eukaryotic fossils were overwhelmingly associated with sediments that had oxygenated bottom waters.

In contrast, samples deposited in oxygen-free environments contained mainly simpler prokaryotic organisms.

Oxygen May Have Played a Major Role

The findings support the idea that oxygen was important for the survival and development of some of Earth’s earliest eukaryotes.

Modern eukaryotic cells generally rely on mitochondria and oxygen-dependent metabolism to generate large amounts of energy. The researchers suggest that the ancient organisms may already have possessed mitochondria or comparable mechanisms that allowed them to use oxygen.

This is significant because scientists have long debated whether the earliest eukaryotes could thrive in oxygen-poor environments.

Life on the Seafloor

The distribution of the fossils also suggests that these early eukaryotes probably lived on or near the seafloor, rather than floating freely throughout the open ocean.

According to the research team, eukaryotes appear to have remained largely associated with oxygen-rich benthic environments for a long period before eventually expanding into planktonic habitats much later in Earth’s history.

That ecological expansion may have contributed to the later diversification of complex life.

Why the Discovery Matters

Eukaryotes represent one of the most important transitions in Earth’s evolutionary history.

Unlike bacteria and archaea, eukaryotic cells contain complex internal structures, including a nucleus and specialised organelles. The emergence of these cells ultimately created the biological foundation for multicellular organisms.

Understanding where the earliest eukaryotes lived can therefore help scientists reconstruct the environmental conditions that preceded the evolution of animals and other complex organisms.

A Window Into Earth’s Ancient Environment

The study demonstrates how fossils and geological chemistry can be combined to reconstruct environments that existed more than a billion years ago.

The researchers say further examination of ancient microfossils could reveal more about how early eukaryotes adapted to changing oxygen levels and eventually expanded into new ecological environments.

The discovery does not provide a complete explanation for the origin of complex life, but it adds important evidence that oxygen availability was closely connected with the early evolution of eukaryotic organisms.