The earliest known eukaryotes, the ancestors of all complex life on Earth, lived in oxygenated, shallow marine environments nearly 1.7 billion years ago, according to a new study led by researchers at McGill University and the University of California, Santa Barbara. The findings cast doubt on the long-held belief that early complex life emerged 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 first evolved is central to understanding how life on Earth became diverse and complex.
"We wanted to know what environments earliest eukaryotic life inhabited, in particular as a test of whether early eukaryotic fossils had already acquired mitochondria, giving them the ability to occupy aerobic environments," said Galen Halverson, professor in McGill's Department of Earth and Planetary Sciences and a senior author on the study.
The research team studied microscopic fossils preserved in fine-grained rocks from northern Australia, dating from about 1.75 to 1.4 billion years ago. To understand the habitats of these organisms, they analyzed the chemistry of the rocks themselves. Using oxygen‑sensitive elements such as iron, they were able to determine that the seawater in which these early eukaryotes lived contained oxygen, even though at this time, most of the oceans lacked oxygen.
"We found that the earliest eukaryotes for which we have fossils lived in predominantly near-shore, oxygenated, benthic (on the seafloor) settings," Halverson said.
"This shows that the availability of oxygen was dictating eukaryote evolution from its early stages," said Leigh Anne Riedman, a researcher at the University of California, Santa Barbara and a co-author of the study.
Many scientists had assumed early eukaryotes lived without oxygen or drifted through the water. The finding that oxygen was part of early life on Earth calls long-held assumptions into question.
The location of the fossils provided further clues about how these early organisms lived.
"The distribution of the fossils also shows that the eukaryotes likely lived on the seafloor, and probably didn't expand out into the open oceans until about a billion years later, which would have transformed the biosphere once more," said Maxwell Lechte, another co-author now at the University of Sydney who conducted this research while a postdoctoral fellow at McGill.
The findings align with recent studies of micro-organisms closely related to the ancestors of eukaryotes, which suggest an ability to use oxygen.
"Eukaryotes represent most of the visible life around us," Halverson said. Understanding how they originated, he added, "is a longstanding major question in science that links to making sense of the biodiversity present today on Earth and possible on other habitable planets."
About the study
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. The research was supported by the Simons Foundation.
About McGill University
Founded in Montreal, Quebec, in 1821, McGill University is Canada's top ranked medical doctoral university. McGill is consistently ranked as one of the top universities, not only nationally, but internationally. It is a world-renowned institution of higher learning with 12 faculties, 14 professional schools, 700 programs of study, over 40,000 students, including more than 10,000 graduate students, and research activities spanning three campuses. McGill attracts students from nearly 150 countries around the world, its 11,000 international students making up 27% of the student body. Over half of McGill students claim a first language other than English, including approximately 20% of our students who say French is their mother tongue.
