🫧 The first complex life forms evolved in oxygenated seas

The oldest eukaryotes, the ancestors of all complex life on Earth, lived nearly 1.7 billion years ago in shallow, oxygenated marine environments.

This is revealed by a new study led by scientists from McGill University and the University of California, Santa Barbara. These results challenge a long-held belief that the first complex life forms appeared in oxygen-poor environments or floated freely in the open ocean.


Eukaryotes include humans, plants, animals, fungi, and many microscopic organisms. Knowing where and how they appeared is important for understanding the diversity and complexity of life on Earth.

"We wanted to know the habitat of the earliest eukaryotic life forms, particularly to check whether the fossils of the first eukaryotes already showed evidence of mitochondria, which would have allowed them to adapt to aerobic environments," says Galen Halverson, professor in the Department of Earth and Planetary Sciences at McGill University and lead author of the study.

The research team studied microscopic fossils preserved in fine-grained rocks from northern Australia, dating back to about 1.75 to 1.4 billion years ago. To understand the habitats of these organisms, they analyzed the chemical composition of the rocks. Using elements sensitive to oxygen, such as iron, they were able to establish that the seawater in which these early eukaryotes lived contained oxygen, whereas at that time most oceans were devoid of it.

"We discovered that the oldest fossilized eukaryotes lived mainly in oxygenated benthic (seafloor) environments near the coastline," says Galen Halverson.

"These results show that oxygen availability drove the evolution of eukaryotes from their earliest stages," says Leigh Anne Riedman, a researcher at the University of California, Santa Barbara and co-author of the study.

Many scientists have long believed that the first eukaryotes lived without oxygen or drifted with water currents. The role of oxygen from the very beginnings of life challenges assumptions about the earliest life forms on Earth.

The location of the fossils provided additional clues about the lifestyle of these primitive organisms.

"The distribution of the fossils also shows that these eukaryotes likely lived on the seafloor. Their expansion into the open ocean did not occur until about a billion years later, which would have led to a further transformation of the biosphere," says Maxwell Lechte, co-author who now works at the University of Sydney and conducted this research while a postdoctoral fellow at McGill University.

These results align with those from recent studies on microorganisms closely related to the ancestors of eukaryotes, suggesting that they could use oxygen.

"Eukaryotes make up most of the visible life around us," explains Galen Halverson. "A major question remains: how did they arise? We absolutely need to answer this question to understand current biodiversity on Earth and that which might exist on other habitable planets."

The study The article "Early fossil eukaryotes were benthic aerobes ", by Maxwell A. Lechte, Leigh Anne Riedman, Susannah M. Porter, Galen P. Halverson and Margaret Whelan, was published in Nature. This study was funded by the Simons Foundation.

Contact: Claire Loewen Organization: Communications Officer, Science and Engineering Email: claire.loewen [at] mcgill.ca Category: