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Microfossils preserved in ancient rocks of Australia open a window to a little-known period of Earth, when complex cells began to occupy specific environments in the primitive oceans, under environmental conditions still investigated by science.
Microscopic fossils preserved in ancient layers of marine mud in northern Australia indicate that some of the earliest known eukaryotes, a group that includes animals, plants, fungi, and many microorganisms, already lived in oxygenated environments between about 1.75 billion and 1.4 billion years ago.
According to researchers involved in the study, the material helps to investigate an early stage in the history of life on Earth: the presence of more complex cells in specific niches of the ancient oceans.
The study, published in the journal Nature, analyzed microfossils found in fine sedimentary rocks of the Northern Territory, a region that, at that time, was part of a vast inland sea.
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Although invisible to the naked eye, these organisms show signs of more elaborate cellular organization than that observed in bacteria and archaea, prokaryotic life forms predominant in the earliest biological records of the planet.
The researchers observed that these ancient eukaryotes do not appear uniformly in the analyzed samples.
The occurrence of the fossils was recorded almost exclusively in rocks formed in oxygenated environments in the waters of the seabed.
In samples associated with areas without oxygen, the team identified only simple prokaryotic forms.
For the authors, this pattern supports the interpretation that oxygen has been linked to the presence of these organisms since early phases of eukaryotic evolution.
Microfossils of Australia and the ancient inland sea
The rocks analyzed came from drilling cores extracted by mineral exploration companies decades ago and stored in geological collections in Darwin, Australia.
This type of material is usually used to study the composition of the subsoil, but it can also preserve remains of microscopic organisms that lived in environments that disappeared billions of years ago.
At the time these microorganisms existed, part of the current northern Australia was covered by shallow waters, coastal lagoons, tidal flats, and areas further from the coast.
The terrestrial environment was also different from today: the atmosphere had lower levels of oxygen, and the oceans showed an irregular distribution of this gas, with oxygenated areas next to regions poor in or without oxygen.
This context helps explain the scientific relevance of the distribution of fossils.
Instead of indicating a widespread presence of the first eukaryotes in all marine environments, the samples point to an association with specific zones.
According to the team’s interpretation, this restriction may have influenced how these organisms maintained themselves and diversified during a long stage of the Proterozoic.
Eukaryotes and the Origin of Complex Life
Eukaryotes are organisms formed by cells with specialized internal structures.
The main difference compared to prokaryotes is the presence of cellular compartments, such as the nucleus, where genetic material is stored, and organelles that perform specific functions.
Among them are mitochondria, associated with energy production in most current eukaryotes.
This cellular architecture is the foundation of groups that would emerge much later, such as animals, plants, algae, and fungi.
Therefore, understanding where and how these organisms developed is one of the research lines of evolutionary biology.
The most accepted hypothesis currently is that the eukaryotic lineage arose from a symbiotic association between ancestral microorganisms.
In this process, one cell would have started to house another, originating a relationship that, over evolutionary time, contributed to the formation of more specialized cellular structures.
The study does not seek to explain this process alone but adds data about the environment in which these organisms were already present when they appear in the fossil record.
Oxygen and Evolution of the First Eukaryotes
One of the questions investigated by researchers was whether the first eukaryotes depended on oxygen or could live in environments poor in this gas.
The question is relevant because aerobic respiration, which uses oxygen to release energy, is associated with maintaining more complex cellular structures in many current organisms.
To analyze this relationship, the authors combined paleontology, sedimentology, and geochemistry.
The shale samples were crushed and partially dissolved, allowing the examination of organic residues preserved inside the rocks.
In total, the team identified more than 12,000 fossils during the process.
After this stage, the scientists studied minerals and chemical elements present in the rocks to reconstruct the conditions of ancient waters.
Elements sensitive to the presence of oxygen, such as iron, vanadium, molybdenum, and uranium, can indicate whether the sediment was formed in an oxygenated or anoxic environment.
The comparison between the fossils and the composition of the rocks allowed the association of eukaryotes with seafloor areas with available oxygen.
According to Galen Halverson, a professor at McGill University and one of the study’s authors, the oldest eukaryotic fossils analyzed by the team were found mainly in coastal, oxygenated, and benthic environments, that is, linked to the seafloor.
Leigh Anne Riedman, a researcher at the University of California in Santa Barbara, stated that the data indicate that oxygen availability influenced eukaryotic evolution from early stages.
Complex life on the ancient seafloor
The location of the organisms is another point observed by the research.
The distribution of the fossils suggests that many of these eukaryotes lived on or within the seafloor, rather than floating freely in the water column as part of the plankton.
This interpretation is based on the way the fossils appear in the samples.
If these organisms lived dispersed in the surface waters, their remains could sink and also be found in sediments formed in areas without oxygen.
Since this occurrence was not widely observed, the authors consider it more likely that these eukaryotes directly occupied oxygenated seafloor environments.
Maxwell Lechte, a researcher at the University of Sydney and co-author of the work, related the distribution of the fossils to the hypothesis that these organisms remained associated with the seafloor before expanding to other oceanic environments.
According to the team, understanding when this change occurred can help explain the later diversification of eukaryotes in marine ecosystems.
Ancient fossils and clues about the origin of living beings
The discovery does not indicate the existence of plants, animals, or humans during that period.
The record shows something earlier: the presence of cellular lineages with more complex organization in specific environments on the planet.
These organisms were unicellular but had characteristics that differentiated them from simple prokaryotic forms.
In the geological record, they represent an ancient stage of the evolutionary trajectory that, much later, would allow the emergence of multicellular organisms and forms of life visible in current ecosystems.
The data also help investigate why eukaryotic evolution may have advanced slowly over hundreds of millions of years.
If these organisms depended on oxygenated areas at the bottom of the sea, their expansion would have been conditioned by the distribution of oxygen in ancient oceans.
On a planet where this gas was limited and irregular, the suitable environments for this type of life could have been restricted.
Even so, fossils show that cellular complexity was already part of marine ecosystems in a very remote period.
Preserved in rocks that were once mud at the bottom of an ancient sea, they allow us to observe a period before the emergence of animals, plants, and modern ecosystems.
From microscopic fragments, researchers seek to reconstruct how complex cells occupied the first favorable environments on Earth.
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