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We鈥檙e not the first living beings to drastically alter Earth鈥檚 climate. The earliest photosynthetic microorganisms belched out enough methane to warm the planet by 15掳C. This bout of global warming may have saved Earth from freezing over, and created a comfortable climate for early organisms.

When Earth formed 4.5 billion years ago, the sun was 25 per cent dimmer than it is today. This suggests the early planet should have been a big snowball, but geological evidence indicates it was just as warm as now, if not warmer.

One explanation for this 鈥�faint young sun paradox鈥� is that greenhouse gases like carbon dioxide warmed Earth by trapping the sun鈥檚 heat. But carbon dioxide levels probably weren鈥檛 high enough to fully account for the balmy climate.

Now, and at the Georgia Institute of Technology in Atlanta and their colleagues suggest that a major contributor to this greenhouse effect was methane 鈥� released by primitive microorganisms that had evolved to photosynthesise.

Carbon fixing

Photosynthesis is the process by which green plants and many microbes sustain themselves. It involves using the sun鈥檚 energy to convert carbon into carbohydrates like sugars, which can be used as fuel. It requires sunlight and a source of electrons to 鈥渇ix鈥� the carbon.

Today鈥檚 photosynthetic organisms, which date back at least 2.5 billion years, mostly use water as this source of electrons. The reaction between water and carbon dioxide produces carbohydrate fuel and releases oxygen as a waste product.

But this wasn鈥檛 always the case. A simpler form of non-oxygen-producing or 鈥渁noxygenic鈥� photosynthesis evolved at least 3.5 billion years ago during the Archaean aeon. At this time, the atmosphere was rich in hydrogen and the oceans in dissolved iron. These two elements provided electrons that microbes could use to transform carbon dioxide into methane fuel.

The researchers calculated that anoxygenic photosynthetic microbes would have generated enough methane to increase the average surface temperature of the planet from about 4 to 19掳C.

Their modelling incorporated a range of factors, including the amount of hydrogen gas spewed from volcanoes, carbon dioxide levels in the atmosphere, the burial rate of iron and organic matter on the sea floor, and the breakdown of methane by the sun鈥檚 ultraviolet rays.

The findings are consistent with the fossil record, says at the University of New South Wales in Australia. For example, there is evidence of 3.5-billion-year-old in the Pilbara region in Western Australia, home to some of the oldest-known microbial fossils.

A change in the air

Nowadays, few anoxygenic photosynthetic organisms remain. They include green sulphur bacteria and purple bacteria, which cannot tolerate oxygen and live in anaerobic environments like in Indonesia and Lake Fryxell in Antarctica.

Oxygenic and anoxygenic photosynthetic bacteria probably battled it out for millions of years before oxygenic species eventually won, says Reinhard. The tipping point may have been when these microbes produced enough oxygen to 鈥渞ust out鈥� the dissolved iron that anoxygenic microbes depended on, he says.

Because they are so scarce, anoxygenic photosynthetic microbes no longer have a significant effect on Earth鈥檚 climate, says Ozaki. This may help explain why the average surface temperature is now slightly cooler at 15掳C, he says.

The findings suggest we should widen the search for extraterrestrial life, says Walter. 鈥淲e鈥檝e mostly been looking for life in places that have a similar climate to ours, but this suggests life can evolve in hydrogen-dominated atmospheres too.鈥�

It may even be easier for hydrogen-based anoxygenic microorganisms to evolve because they don鈥檛 require complex enzymes to get electrons out of water, says Ozaki. 鈥淭he Archaean anoxic biosphere is therefore expected to be an extremely useful analogue for the primitive biospheres of other Earth-like planets.鈥�