Our understanding of early life on Earth often seems as murky as the primordial soup from which the first organic compounds are theorized to have originated.
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It is clear, though, that photosynthesis is critical to complex life. All of the oxygen on our planet comes from this process, which allows green plants and some other organisms to harness energy from sunlight and convert it into the chemical energy that can be used for essential functions like synthesizing nutrients from carbon dioxide and water.
There are two types of photosynthesis: oxygenic and anoxygenic. The former uses light energy to split water molecules, thereby releasing oxygen, electrons, and protons. The latter utilizes compounds instead of water, like hydrogen sulfide or minerals such as iron or arsenic, and does not result in oxygen.
The traditional view holds that anoxygenic photosynthesis evolved long before oxygenic photosynthesis, meaning oxygen was not present on the planet until about 2.4 to 3 billion years ago.
Two new studies — one published in the journal Heliyon and the other in the preprint biology server bioRxiv — instead find that oxygenic photosynthesis likely originated simultaneously with anoxygenic photosynthesis around 3.4–3.8 billion years ago. The findings could dramatically change our understanding of how and when complex life on Earth began.
"I think it is likely that, 3.8 billion years ago, cellular life had already evolved and possibly diversified into bacteria and archea," author Tanai Cardona, a research associate in Imperial College London's department of life sciences, told Seeker. Archea are microorganisms that resemble bacteria but differ somewhat in their chemical structure.
He added, "I also think at that time ancient forms of photosynthesis had already emerged in ancestral forms of bacteria."
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Cardona first became fascinated by photosynthesis when he was an undergraduate at the University of Los Andes in Colombia.
"As I started to become more and more interested in the evolution of photosynthesis, I realized that we really don't know a lot about it," he said. "I also realized that some of the things we thought we knew about the origin of photosynthesis did not really make perfect sense."
"For example," he continued, "there are scenarios for the evolution of oxygenic photosynthesis that suggest it is a very late event, or that cyanobacteria obtained photosynthesis via the transfer of genes from anoxygenic photosynthetic bacteria."
Prior investigations of photosynthesis attempted to detect oxygen in ancient rocks. Cardona took a different approach. He instead analyzed the molecular machines that carry out photosynthesis: complex enzymes known as photosystem I and photosystem II.
Curtis Neveu, Wikimedia Commons
Using a technique called the Bayesian relaxed molecular clock, he studied how long ago the genes underlying the photosystems evolved to be different. Cardona explained that DNA sequences change over time due to naturally occurring mutations.
"So, if I can find out at what rate the sequences are changing, the rate of evolution — how many mutations happen in the sequence per unit of time — then I can figure out when two different sequences started to differ from each other," he said.
Bayesian statistics, a data analysis tool, allowed him to plug in known data as calibration points. For example, it is known that flowering plants emerged around 100 million years ago, red algae evolved before 1.5 million years ago, and so on.
"Relaxed," in terms of Bayesian inference, means that the resulting molecular clock assumes that sequences, or organisms as represented by their DNA or protein sequences, evolve at different rather than fixed rates.
This is important, because when Cardona first plugged in his data and considered a more static rate of evolution, the data analysis tool showed that the photosystems emerged before Earth formed 4.6 billion years ago. The initial finding would seem to indicate that photosystems originated on another planet or celestial body.
"Although it is fun to think about that, I am quite reluctant to go in that direction," Cardona said.
"We would need to rule out quite a few other scenarios that are more likely: in this case, that the rate of evolution was initially faster," he continued. "Faster rates could have occurred for many different reasons, like more ultraviolet light in the absence of an ozone layer, a hotter world, or simply the origin of photosynthesis itself."
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He explained that when a gene evolves a new function, it is known that the subsequent rate of evolution accelerates for a time.
The idea that a photosynthesizing organism from another planet landed on Earth and jumpstarted all complex life here has therefore been put aside in favor of the more plausible scenario that photosystems evolved at a faster rate in the past than they have over the past 2.4 billion years.
Cyanobacteria convincingly date to at least 2.4 billion years ago and could even be much older. These are aquatic and photosynthetic bacteria. Also known as blue-green algae, they consist of various types that can produce toxic algal blooms, which often pose risks for humans and animals.
Some scientists have speculated that the earliest life forms were cyanobacteria. Still others have suggested that the distinction goes to green sulphur bacteria (Chlorobi) or the so-called purple bacteria (Proteobacteria).
Cardona thinks these ideas are flawed.
He points out that no one would think that the ancestor of chimps and humans was a chimp or a human; it was another species of primate. Similarly, he believes that today's bacteria were preceded by another, as yet unknown, ancestral form.
As for the emergence of oxygen, he does not think that the air and oceans were suddenly rich with the gas 3.8 billion years ago. It is more likely that there were "whiffs of oxygen" in localized environments before the Great Oxygenation Event (GOE) took place around 2.45 billion years ago.
Geological, isotopic, and chemical evidence all support the GOE as being a time when distinct, measurable levels of oxygen were present in Earth's atmosphere.
Graeme Churchard, Wikimedia Commons
"Before 2.4 billion years ago, it is thought that the level of oxygen was 0.00001 percent of the current level," Cardona said. "The GOE was a change from 0.00001 percent to about 0.01–1 percent of the current level."
This condition remained for a very long, rising only to current levels about 500 million years ago.
Cardona, however, is more interested in the origin of photosynthesis and the corresponding first emergence of oxygen on the planet. He and colleagues Bill Rutherford and Peter Nixon recently received a grant from the Leverhulme Trust to reconstruct the ancestral genetic sequence of the earliest photosystems.
To do so, they will use computational methods to calculate the gene sequences of these first photosystems. They can then make the genes using commercially available DNA synthesis technologies and exchange the photosystems of a model cyanobacterium with the ancient versions.
"We are going to recreate the earliest photosystems of oxygenic photosynthesis, those that could be more than 3 billion years old," Cardona said. "It sounds far-fetched, but it is actually quite possible and the technology is well developed. It just has not been used in photosynthesis research."