The oldest evidence of photosynthetic organisms has been identified inside a microfossil from Australia. This discovery can provide better insight into the evolution of oxygenic photosynthesis and in understanding how life emerged on our planet.
Evolution of Photosynthesis
Photosynthesis, the process of harnessing sunlight to convert water and carbon dioxide into glucose and oxygen, has been the basis of survival of almost all living things on Earth. Photosynthetic organisms not only form the foundation of most food webs, but their metabolic processes also fill the atmosphere with the breathable oxygen needed by most organisms to survive.
In the early history of the Earth, there was not a lot of oxygen floating freely in the atmosphere and oceans. However, various lines of geochemical evidence suggest that oxygen levels increased suddenly 2.4 billion years ago in what is now known as the Great Oxidation Event. It remains unclear what caused it, although one possibility is the emergence of photosynthetic organisms.
The earliest undisputed evidence of cyanobacteria is an organism known as Eoentophysalis belcherensis which lived 2.018 billion years ago. However, their fossils are not easy to interpret and their internal structures do not always survive intact. Aside from this, not all cyanobacteria species have thylakoids, a structure inside chloroplasts and cyanobacteria which serves as the main site for the light-dependent reactions of photosynthesis.
The principal production of both oxygen and organic matter on Earth is attributed to oxygenic photosynthesis. It is a process in which sunlight catalyzes the conversion of carbon dioxide and water into glucose and oxygen. Today, this process is unique to cyanobacteria and their plastid relatives within the domain eukarya.
As an active organism during the Great Oxidation Event around 2.4 billion years ago, cyanobacteria played an important role in the evolution of early life. However, the timings of the origins of oxygenic photosynthesis are still under debate due to lack of evidence.
Oldest Evidence of Oxygenic Photosynthesis
In the McDermott Formation in the desert of northern Australia, scientists discovered thylakoids.
These fossils came from the Grassy Bay Formation which dated to up to 1.01 billion years ago, while the McDermott Formation stretched as far back as 1.75 billion years ago. This means that the fossil record of thylakoids was extended back by 1.2 billion years, an indication that oxygenic photosynthesis must have evolved before that time.
These membrane-bound compartments were located inside the cells of organisms called cyanobacteria which are known for having the pigment chlorophyll that absorbs light for photosynthesis. According to paleomicrobiologist Catherine Demoulin of the University of Liège, their study provides direct evidence for the presence of metabolically active cyanobacteria which perform oxygenic photosynthesis.
Demoulin and her team used various high-resolution microscopy techniques in investigating the external and internal structures of microfossils of a species called Navifusa majensis, thought to be cyanobacteria. Within the bodies of these single-celled organisms from two fossil beds, the scientists found thylakoid membranes.
Their findings of the study "Oldest thylakoids in fossil cells directly evidence oxygenic photosynthesis" suggest that a detailed analysis of other fossils could possibly identify more similar structures. This could pinpoint the moment when photosynthetic structures were utilized by the earliest forms of complex algal cells.
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