Scientists reveal the oldest direct evidence of life on Earth from 1.75 billion years ago
Scientists have unearthed the oldest direct evidence of oxygenic photosynthetic structures within fossilized bacteria. Cyanobacteria, like these ocean-dwelling Prochlorococcus, invented photosynthesis billions of years ago. (CREDIT: LUKE THOMPSON/CHISHOLM LAB, NIKKI WATSON/WHITEHEAD/MIT)
The origins of life on Earth have long been shrouded in mystery, with scientists continuously piecing together the puzzle of how complex organisms emerged on our planet. In a groundbreaking study, researchers have uncovered a remarkable find, shedding light on the evolution of photosynthesis and the pivotal role it played in shaping our world.
According to a recently published study, scientists have unearthed the oldest direct evidence of oxygenic photosynthetic structures within fossilized bacteria discovered in Australia and Canada.
These fossils, estimated to be a staggering 1.75 billion years old, have pushed back the known origin of photosynthesis by at least 1.2 billion years. This breakthrough discovery challenges our previous understanding of when this critical process, which transforms sunlight, water, and carbon dioxide into energy and oxygen, first emerged.
Photosynthesis, as we understand it today, is believed to have evolved approximately 3.5 billion years ago, albeit in a more primitive form that did not produce oxygen, known as anoxygenic photosynthesis.
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In the present day, nearly all photosynthesizing organisms, with the exception of a few algae, bacteria, and plants, generate oxygen as a byproduct. However, the exact timing of this transition remains uncertain.
Cyanobacteria, among the earliest organisms on Earth, are thought to have been pioneers in oxygenic photosynthesis. These tiny life forms utilized specialized structures called thylakoid membranes, which are dense stacks of proteins and fats where photosynthesis occurs. When cyanobacteria initiated oxygenic photosynthesis, they ushered in a transformative period in Earth's history known as the Great Oxygenation Event.
Emmanuelle Javaux, a co-author of the study from the University of Liège in Belgium, explains the profound impact of this transition, stating, "Their production of oxygen led to accumulation of oxygen and profoundly modified the chemistry of the Earth's oceans and atmosphere, and the evolution of the biosphere, including complex life."
Image of Navifusa majensis, a microfossil from the McDermott Formation in Australia. This 1.75 billion-year-old microfossil contains thylakoids, which identify it as a cyanobacterium. (CREDIT: Emmanuelle Javaux)
Over time, these cyanobacteria evolved into the chloroplasts found in modern-day organisms, becoming the primary factories for harnessing sunlight. Robert Blankenship, a distinguished professor at Washington University in St. Louis, remarks on the sophistication of photosynthesis within cyanobacteria, emphasizing that this process closely mirrors what occurs in plants today, suggesting that it was perfected eons ago.
Previous research had identified thylakoid membranes in fossils ranging from 150 to 550 million years old, along with indirect evidence of oxygen-producing photosynthesis through genetic and chemical studies. However, the latest study, published in the journal Nature, provides the oldest direct evidence of thylakoid membranes within fossils dating back 1.75 billion years in the McDermott Formation in Australia and around one billion years in the Grassy Bay Formation in Arctic Canada.
Images of N. majensis microfossils. (CREDIT: Nature)
This remarkable temporal leap in evidence is not entirely surprising, given the scarcity of fossilized bacteria. Blankenship explains, "It's a problem with the preservation of the fossils being really difficult and there not being very many of them." The challenges arise from the fact that soft-bodied organisms like bacteria, lacking mineral or bone structures, do not preserve well in the fossil record.
Moreover, these fossils are minuscule, measuring less than a millimeter in size, and the internal structures are even smaller, making their discovery among layers of compressed sediment a formidable task. Over millions of years, as sediment and rock compress, they can obliterate the very structures researchers seek to unearth. In some cases, like the fossils from the Democratic Republic of the Congo analyzed by Javaux's team, no thylakoids were found.
TEM pictures of a specimen of Navifusa majensis, from the Grassy Bay Formation (Shaler Supergroup, Canada). (CREDIT: Nature)
The thylakoid membranes discovered in this study are about as wide as a human hair and required a meticulous process to observe, involving encasing the bacteria in resin, slicing them into ultra-thin sections, and examining them under an electron microscope.
Javaux underscores the significance of this find, explaining that it offers valuable insights into the evolution of complex life on Earth. Blankenship takes it a step further, suggesting that understanding the origins of photosynthesis on our planet could provide clues for identifying this life-sustaining and world-altering process on other celestial bodies.
Schematic drawings showing how compressed microfossils were cut transversally for TEM observations. (CREDIT: Nature)
In collaboration with NASA, Blankenship has theorized what photosynthesis might look like on planets with stars emitting different wavelengths of light than our sun.
Nonetheless, many questions remain unanswered, including the pivotal inquiry of whether oxygen-producing photosynthesis evolved before, during, or after the Great Oxygenation Event. Blankenship proposes that it could have occurred prior to the event, with oxygen levels taking time to accumulate. Further meticulous analysis of older microfossils, akin to the methodology employed in this study, may hold the key to unraveling these mysteries, as Javaux suggests.
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