A study in Nature reveals that the earliest eukaryotes known to science lived primarily on or in the seabed, in environments where there was oxygen, and not in the water column as previously thought.
The first complex life on Earth was probably much more limited than previously thought. A new study, led by researchers from the University of California, Santa Barbara and McGill University, finds that the earliest eukaryotes known to science lived for hundreds of millions of years mostly on or in the seabed, and only in areas where oxygen was available. The study, published in the journal Nature, changes the conventional picture of the beginnings of eukaryotic life, the group that includes animals, plants, fungi and most visible multicellular organisms.
Eukaryotes are organisms whose cells contain a nucleus, membrane-bound organelles, and usually mitochondria – the powerhouses of the cell. Understanding their emergence and spread is one of the keys to understanding how the Earth became so rich in life. For years, researchers believed that ancient eukaryotes resembled modern plankton and lived mainly in the water column. However, the new findings point to a different scenario: for a very long time, they were actually tied to the sea floor.
Microfossils from ancient Australia
The researchers focused on sediments from the McArthur and Birrindudu basins in Australia’s Northern Territory. Today, the region features desert, savannah, forest and wetland landscapes, but 1.75 to 1.4 billion years ago it was a shallow inland sea, with lagoons, tidal flats, coastal mudflats and calm waters near the coast. At that time, oxygen was already accumulating in the oceans, but its distribution was patchy. The concentration of oxygen in the atmosphere was one percent or less of what it is today – a level at which humans could not have survived.
Dr. Leigh Ann Ridman of the University of California, Santa Barbara, mined and prepared microfossils from drill cores, identifying the eukaryotes. At the same time, researchers from McGill University analyzed the types of sediments and environments in which the rock layers were formed. This allowed the researchers to associate the fossils with four ancient environments: lagoons, tidal areas, coastal areas, and more open water.
To understand the oxygen levels in each environment, the team examined the mineral composition of the rocks. The appearance of iron pyrite, for example, indicates an oxygen-deficient environment. Concentrations of metals such as vanadium, molybdenum and uranium provided further clues about ancient oxygenation conditions. Combining the fossil, sedimentary and geochemical information allowed the researchers to reconstruct where early eukaryotes lived and under what conditions.
Oxygen at the bottom, not just at the surface of the sea
The result was surprising: The eukaryotic fossils appeared almost exclusively in rocks formed in oxygenated seafloor environments. They were found not only in shallow water, but also in areas further from the coast – as long as the seafloor contained oxygen. According to the researchers, this strong association suggests that the ancient eukaryotes required oxygen for at least part of their life cycle.
The find is also important because it likely indicates their lifestyle. If eukaryotes lived primarily in the oxygenated upper waters, their remains should have also sunk to and been preserved in oxygen-depleted seabeds. The fact that they are almost never found in such environments strengthens the possibility that they lived on or in the seabed itself.
The researchers note that the finding fits with the idea that eukaryotes acquired mitochondria very early on. According to the accepted theory, mitochondria evolved from free-living bacteria that were engulfed by an ancient cell and became part of it. Living on the seafloor would have placed early eukaryotes in close proximity to other creatures, a situation that could have facilitated such a process. The structural complexity seen in the 1.75 billion-year-old fossils also supports the hypothesis that the group had already undergone a deeper evolutionary history before the appearance of the known fossils.
Why did they remain confined for so long?
One of the big questions in evolutionary biology is why eukaryotes remained so rare and relatively diverse for so long. According to the new study, the answer may have to do with their limited habitat. If early eukaryotes could only thrive in oxygenated seafloors, they would have been confined to small environmental islands within oceans that were still largely oxygen-poor.
This pattern could explain why, for nearly a billion years, the fossil record has shown fairly similar groups of creatures, without a major burst of diversity. The researchers note that fossils that are 800 million years old and fossils that are 1.7 billion years old show, for the most part, “the same set of actors.” The big change came later, after the Cryogenian period, which began about 720 million years ago, when the Earth went through extreme freezing events known as “Snowball Earth.”
When the ice retreated about 635 million years ago, new ecological niches opened up. The Ediacaran period that followed marked the first appearance of large-scale complex multicellular life—all eukaryotic. In this sense, the new study not only describes where the first eukaryotes lived, but also offers a possible explanation for the long delay between their appearance and the emergence of complex life.
The researchers are now continuing to examine even older rock layers in Australia’s McArthur Basin and Minnesota’s Animikie Basin. Their goal is to figure out when eukaryotes first appeared, how they reached the level of complexity already seen in the ancient fossils, and when they moved from the seafloor into the water column. As Ridman concluded, such studies allow us to see these tiny creatures not just as names on a fossil record, but as organisms that lived in a particular environment, did certain things, and were part of the pathway that ultimately led to the vast biodiversity of the Earth—and to us.
Short FAQ:
What are eukaryotes??
Eukaryotes are organisms whose cells contain a nucleus and membrane-bound organelles. This group includes animals, plants, fungi, and many single-celled organisms.
What did the new study reveal??
The study found that the earliest eukaryotes known to science lived primarily on an oxygenated seabed, rather than in the water column as previously believed.
Why was oxygen important to ancient eukaryotes??
The findings indicate that they required oxygen for at least part of their life cycle, probably because they already used energy production mechanisms associated with mitochondria.
One response
Interesting and informative, as usual.
Beautiful and clear writing. Thank you!