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.
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.