Scientists have discovered that some of Earth’s earliest complex life forms lived on the seafloor and depended on oxygen, challenging long-standing ideas about how advanced life evolved.
The study, published in Nature, examined some of the oldest known fossils of eukaryotes—organisms whose cells contain a nucleus and other specialized structures.
Eukaryotes include animals, plants, fungi, and many microscopic organisms.
Understanding how they first evolved is key to understanding how life on Earth became so diverse.
Researchers from the University of California, Santa Barbara and their collaborators studied fossil-rich rock formations in Australia’s Northern Territory.
These rocks were formed between 1.75 and 1.4 billion years ago, when the region was covered by a shallow inland sea.
At that time, oxygen levels on Earth were much lower than they are today. Scientists estimate that atmospheric oxygen was less than 1% of modern levels, meaning humans would not have been able to breathe.
For many years, researchers assumed that early eukaryotes lived mainly in the open water, similar to modern plankton.
Some scientists also questioned whether these ancient organisms had mitochondria, the tiny structures inside cells that use oxygen to produce energy.
To investigate, the research team analyzed fossil distributions, sediment layers, and the chemical composition of surrounding rocks. Different minerals form under different oxygen conditions, allowing the scientists to reconstruct ancient environments and estimate oxygen levels in the water.
The results revealed a surprising pattern. The oldest known eukaryotes were found almost exclusively in oxygen-rich seafloor environments. They appeared in shallow coastal areas and deeper offshore regions, but only where oxygen was present near the seabed.
This suggests that these organisms likely needed oxygen to survive and were probably living on or within the seafloor rather than floating freely in the water above.
The findings also support the idea that early eukaryotes had already acquired mitochondria. Since mitochondria require oxygen to generate energy, their presence would explain why these organisms were strongly linked to oxygen-rich habitats.
The researchers were surprised by how limited the distribution of these early life forms appeared to be. Despite existing for hundreds of millions of years, eukaryotes remained relatively rare and showed little change in their overall appearance.
According to the scientists, this may be because they occupied a very narrow ecological niche on the seafloor. If they were confined to such specialized environments, opportunities for diversification would have been limited.
That situation may have changed much later during the Cryogenian Period, often called “Snowball Earth,” when much of the planet became covered in ice. The extreme conditions likely caused widespread extinctions, opening new ecological opportunities when the climate warmed again.
Soon afterward, during the Ediacaran Period about 635 million years ago, large and complex multicellular organisms began appearing in the fossil record.
The researchers believe the fossils show that even the oldest known eukaryotes were already surprisingly sophisticated. This suggests that the group may have originated even earlier than current fossil evidence indicates.
By uncovering where these ancient organisms lived and how they survived, scientists are gaining new insights into one of the most important events in Earth’s history: the rise of complex life that eventually led to animals, plants, and ultimately humans.
