Scientists just discovered a 1.75 billion-year-old secret about the origin of life: ScienceAlert

Tiny fossils that spent nearly 2 billion years locked in chunks of ancient rock give us the earliest evidence of photosynthesis on Earth.

In the McDermott Formation in the northern Australian desert, tiny structures called thylakoids have been found in what are believed to be fossilized cyanobacteria dating back 1.75 billion years.

These structures are found in the cells of photosynthetic organisms today, which contain the pigment chlorophyll used to absorb light for photosynthesis.

This means that microfossils represent the oldest direct evidence for photosynthesis, giving us a new minimum age for the appearance of thylakoid-bearing cyanobacteria and a new tool for understanding Earth’s early ecosystems and how life on our planet arose.

Images of microfossil specimens of Navifusa majensis. (Demoulin et al., Nature2024)

“Our study provides direct evidence for the presence of metabolically active cyanobacteria performing oxygenic photosynthesis,” wrote a team led by paleomicrobiologist Catherine Demoulin of the University of Liège.

The findings suggest that detailed analysis of other fossils could identify more structures like it, pinpointing the moment when photosynthetic structures were absorbed and put to work by the earliest forms of complex algal cells.

Photosynthesis, which uses sunlight to convert water and carbon dioxide into glucose and oxygen, may seem like something that plants and algae do quietly over there, but it is the basis for the survival of almost all living things.

Photosynthesizing organisms not only form the basis of most food webs, their metabolic processes fill the atmosphere with the breathable oxygen most of us need to survive.

We know that early in Earth’s history there wasn’t much oxygen floating freely in the atmosphere and oceans. However, various lines of geochemical evidence reveal that oxygen levels suddenly skyrocketed about 2.4 billion years ago in what is known as the Great Oxidation Event. It is not clear what caused it, but one possibility is the appearance of photosynthetic organisms.

The earliest indisputable microfossil evidence for cyanobacteria is an organism called Eoentophysalis belcherensis, dated to 2.018 billion years ago. But fossils are often difficult to interpret, and their internal structures do not always survive intact. And not all species of cyanobacteria have thylakoids.

Transmission electron microscopic images of specimens from Navifusa majensis from the McDermott Formation. (Demoulin et al., Nature2024)

Demoulin and her colleagues used a variety of high-resolution microscopy techniques to examine the external and internal structures of microfossils from a species known as Navifusa majensis, considered cyanobacteria. And in the bodies of single-celled organisms from two fossils, they found thylakoid membranes.

These fossils are from the Grassy Bay Formation in Canada, dated to 1.01 billion years ago; and the McDermott Formation, stretching back as far as 1.75 billion years ago. This extends the thylakoid fossil record back as much as 1.2 billion years – and means that oxygenic photosynthesis must have evolved before that time.

But what we don’t yet know is whether it evolved over time to contribute to the Great Oxidation Event. Only the finding and careful study of even older fossils can give us an answer to this burning question.

“The discovery of preserved thylakoids within N. majensis reported here provides direct evidence for a minimum age of about 1.75 billion years ago for the divergence between thylakoid-bearing and non-thylakoid-bearing cyanobacteria,” the researchers wrote.

“We predict that such ultrastructural analyzes of well-preserved microfossils can expand the geologic record of oxygenic photosynthesizers and the early, low-oxygen ecosystems in which complex cells evolved.”

The study was published in Nature.

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