It’s long been thought that eukaryotic cells emerged in environments that had low oxygen levels, or were free-floating in the open seas; but this study, which was reported in Nature, challenges that hypothesis.
Once eukaryotes emerged, they gave rise to species as diverse as fungi, plants, humans, and tiny organisms like protists.
So for this work, they analyzed microscopic fossils found in rocks in northern Australia, thought to be about 1.75 to 1.4 billion years old.
The majority of the oceans on Earth at that time are thought to have lacked oxygen.
“We found that the earliest eukaryotes for which we have fossils lived in predominantly near-shore, oxygenated, benthic (on the seafloor) settings,” said Halverson.
New research has suggested that the ancestors of complex life, or the most ancient eukaryotic cells to have ever existed on Earth, lived in the shallow marine environments of 1.7 billion years ago, where oxygen was present. It’s long been thought that eukaryotic cells emerged in environments that had low oxygen levels, or were free-floating in the open seas; but this study, which was reported in Nature, challenges that hypothesis.
Once eukaryotes emerged, they gave rise to species as diverse as fungi, plants, humans, and tiny organisms like protists. It’s thought that eukaryotes were created in part after a symbiotic organism was formed, in which one microbe inhabited another, to create the powerhouse of the eukaryotic cell now known as mitochondria. Mitochondria are the only cellular organelles that carry their own little genomes, lending credence to this hypothesis. Another hypothesis suggests that once cells were able to produce the energy made by this symbiont, they could give rise to other organelles.
This studied looked to understand whether ancient eukaryotes could live in places where oxygen was present.
“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 co-study author Galen Halverson, a professor at McGill University.
Researchers obviously cannot directly study the conditions of the planet millions of years ago. So for this work, they analyzed microscopic fossils found in rocks in northern Australia, thought to be about 1.75 to 1.4 billion years old.
The researchers assessed the rocks’ chemistry and analyzed elements like iron that are sensitive to oxygen to determine that these ancient organisms lived in seawater that contained oxygen. The majority of the oceans on Earth at that time are thought to have lacked oxygen.
“We found that the earliest eukaryotes for which we have fossils lived in predominantly near-shore, oxygenated, benthic (on the seafloor) settings,” said Halverson.
“This shows that the availability of oxygen was dictating eukaryote evolution from its early stages,” added co-study author Leigh Anne Riedman, a researcher at the University of California, Santa Barbara.
“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 first study author Maxwell Lechte, now of the University of Sydney.
This work may disagree with previous assumptions, but it confirms other recent work that has suggested that close microbial relatives of eukaryote ancestors known as Asgard archaea were also able to use oxygen.
Sources: McGill University, Nature
