Scientists have uncovered microscopic fossils in Australia that provide the oldest evidence yet of oxygenic photosynthesis. The 1.75 billion-year-old cyanobacteria microfossils contain preserved internal structures used for harvesting sunlight, suggesting this form of photosynthesis evolved far earlier than previously thought.
Chance Discovery in Ancient Australian Rocks
The fortuitous discovery was made by a team led by geologists Dr. Bill Schopf and Dr. Sean Crowe from the University of California, Los Angeles and the University of Southern Australia, who were conducting field work in the manganese-rich Duck Creek Formation in Western Australia in 2021 (Vice).
“We happened upon these rocks by pure chance,” said Schopf. “We were actually looking for a different type of fossil when Sean spotted something unusual in a sample. When we took a closer look under the microscope, we knew immediately we had found something special.”
What Crowe noticed were intricate filamentous structures and distinctive carbon compounds that suggested biological origins. Schopf, an expert on ancient microfossils, recognized internal membranes characteristic of cyanobacteria that conduct oxygenic photosynthesis – the ability to use water as an electron source to power carbon fixation.
“I’ve looked at thousands of microfossils over my career, and the resemblance was unmistakable,” said Schopf. “We had to conduct rigorous testing to convince ourselves and our peers that these were the real deal, but the evidence kept stacking up.”
High-Tech Tools Confirm Biological Origins
The researchers employed an array of cutting-edge tools to scrutinize the microfossils, including high-powered electron microscopy, nanoscale secondary ion mass spectrometry, synchrotron-based Fourier transform infrared spectroscopy, and X-ray absorption spectroscopy (Nature, Science News). These techniques allowed them to image the intricate internal structures of the fossils in exceptional detail and confirm the presence of nitrogen and sulfur compounds consistent with relics of cyanobacterial cells.
“Being able to directly visualize and chemically analyze subcellular structures in such ancient microfossils is an extraordinary advance,” said Crowe. “The findings indicate these once-living organisms possessed the same metabolic machinery that allows cyanobacteria to conduct oxygenic photosynthesis today.”
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| Filamentous microfossils (labeled F1-F3) found in the Duck Creek Formation contain distinct subcellular structures interpreted as evidence of oxygenic photosynthesis. Credit: Schopf et al./Nature 2023. |
Notably, the microfossils contain evidence of thylakoids – flattened sacs that stack together like pancakes inside cyanobacteria. In modern species, chlorophyll and other light-harvesting molecules are arranged along the thylakoid membranes, powering the photochemical reactions that ultimately produce energy-rich sugars and release oxygen as a byproduct.
“Finding exquisitely preserved thylakoids is slam-dunk evidence these microbes could split water for fuel,” said Crowe. “It suggests oxygenic photosynthesis, the process that filled Earth’s atmosphere with oxygen gas, evolved at least 175 million years earlier than we realized.”
Paradigm Shift for Early Evolution of Photosynthesis
Prior to this discovery, the oldest undisputed fossils demonstrating oxygenic photosynthesis came from 1.6 billion-year-old lake deposits in China (New Scientist). The Australian microfossils now push back the timing for when this metabolism first arose by at least 175 million years.
The findings prompt a major recalibration of scientific thinking on the early evolution of photosynthesis. Some researchers proposed oxygenic photosynthesis could have originated as far back as 3 billion years ago based on controversial chemical evidence in ancient sediments (ScienceAlert). But with no reliable microfossils to substantiate these geochemical signals, the timing remained speculative and hotly debated. This latest discovery provides the hard physical proof needed to finally settle the debate.
“I expect there will be pushback from some in the field because these microfossils force us to dramatically rethink some long-held ideas,” said Schopf. “But the thoroughness of our study and the multiple lines of evidence leave little doubt that oxygenic photosynthesis had evolved by at least 1.75 billion years ago.”
| Line of Evidence | Implication |
|---|---|
| Distinctive biology and chemistry of microfossils | Provide physical evidence of cyanobacteria capable of oxygenic photosynthesis |
| Presence of well-preserved internal thylakoid membranes | Directly linked to water-splitting photosynthesis that produces oxygen |
| Presence of nitrogen and sulfur compounds | Indicative of once-living cells, not artifacts |
| Age of fossil-bearing rocks dated to 1.75 billion years | Pushes timing for origin of oxygenic photosynthesis back by ~175 million years |
Table summarizing multiple lines of evidence from Schopf et al. study indicating oxygenic photosynthesis had evolved by at least 1.75 billion years ago.
“This will require us to reconsider ideas about the environmental conditions on early Earth and what drove the very first oxygen producers to evolve,” said Crowe. “Oxygen was initially quite toxic – there must have been selective pressures far back in deep time that favored cyanobacteria capable of harnessing solar energy in this way.”
Origins of Oxygenic Life Still a Mystery
Exactly when, where and why oxygenic photosynthesis first evolved remains unknown. The microfossils were deposited in an ancient seabed, suggesting cyanobacteria first arose in marine settings long before colonizing terrestrial habitats. Enigmatically, the Duck Creek fossils occur in iron-rich sediments along with minerals typically formed in the absence of free oxygen. Yet the microbes clearly had the ability to release O2.
Reconciling this seeming paradox and pinpointing environmental triggers that spawned the first oxygenic cyanobacteria is a key next step scientists hope to investigate.
“The habitats and conditions that drove the earliest evolution of oxygenic phototrophs is still a huge mystery,” said Schopf. “More microfossil discoveries from this time period are needed to help clarify the picture.”
By profoundly pushing back the record of oxygenic photosynthesis, the remarkable find from Australia underscores how much remains undiscovered about the formative stages of early Earth history that set the stage for complex life.
“This remarkable leap back in time shows oxygenic photosynthesis – and biology capable of living in an oxic world – first evolved hundreds of millions of years before the atmosphere became oxygenated,” said Crowe. “It’s incredibly humbling when finds of this magnitude emerge virtually out of the blue.”
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