PLANET EARTH
The earliest direct evidence of photosynthesis has been discovered in fossils dating back to 1.75 billion years ago. Scientists collected fossils from Australia, Canada and the Democratic Republic of Congo and found that the samples from Australia and Canada contained evidence of cyanobacteria, the oldest known life form on Earth. Scientists believe that cyanobacteria first emerged 2 to 3 billion years ago before evolving to be capable of oxygen-producing, or oxygenic, photosynthesis. In a recent study, researchers revealed these cyanobacteria fossils featured photosynthetic structures, known as thylakoid membranes, which contain pigments like chlorophyll that convert light into chemical energy via photosynthesis. The cyanobacteria were preserved in a mud clay that was compacted over time to become rock. The researchers used a technique called transmission electron microscopy (TEM) to see the membranes and other tiny details preserved in the fossils.
Instead of using light to image objects, TEM uses electrons, which have a much smaller wavelength than light, allowing us to see much finer details down to the atomic level. To achieve this, scientists bombard a sample with an electron beam. Some electrons will pass through, while some will be absorbed or scattered off more dense parts of the object. “Finding these membranes tells us that [these cells] are indeed cyanobacteria that are performing oxygenic photosynthesis,” said Emmanuelle Javaux, a palaeobiologist from the University of Liège in Belgium. “This pushes back the fossil record of such membranes by 1.2 billion years,” Javaux said. Identifying the exact time in which cyanobacteria evolved the ability to produce oxygen is an important milestone in Earth’s natural history.
The concentration of oxygen in Earth’s atmosphere rose dramatically around 2.45 billion years ago in what is known as the Great Oxidation Event. The rise in atmospheric oxygen transformed life on Earth. It unlocked aerobic respiration for many life forms and increased the rate at which minerals weathered and provided nutrients to different environments. However, scientists don’t know whether the Great Oxidation Event was triggered by the evolution of oxygenic photosynthesis or whether other ecological or geological events occurred first. The exact biological and physical drivers of the Great Oxidation Event are still deeply debated among scientists. Though cyanobacterial photosynthesis is generally accepted as the key reason why oxygen concentrations increased, drivers like volcanic eruptions or a decreased level of iron in the oceans may have also played a part.
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