Jan. 8 (UPI) -- Scientists at the Scripps Research Institute have come up with a plausible recipe for how life began on early Earth. The recipe of life-yielding chemical reactions includes a list of ingredients all found on Earth some 4 billion years ago.
The researchers published their recipe Monday in the journal Nature Communications.
"This was a black box for us," senior author Ramanarayanan Krishnamurthy, an associate professor of chemistry at TSRI, said in a news release. "But if you focus on the chemistry, the questions of origins of life become less daunting."
Krishnamurthy and his colleagues began by identifying the chemical reactions essential to the citric acid cycle, the conversion of carbohydrates, fats, and proteins into carbon dioxide and adenosine triphosphate, the chemical energy used by all anaerobic organisms.
Previous attempts to formulate a recipe for early life have looked to replicate the chemical reactions that form today's citric acid cycle, but Krishnamurthy believes the ingredients and reactions used to store and access cellular energy were likely too fragile to exist on early Earth. Some of today's ingredients were likely nowhere to be found when Earth was just a billion years old.
The researchers looked to replicate the chemical reactions using ingredients that were found on planet Earth 4 billion years ago. They identified ingredients among two non-biological cycles of chemical reactions, the HKG cycle and the malonate cycle.
Both cycles resemble the citric acid cycle in several ways, including -- most importantly -- the ability to introduce new source material into the cycle. The cycles also feature oxidative decarboxylations, which produce CO2.
In models, researchers showed the two cycles could combine to jumpstart a primitive version of the citric acid cycle, capable of producing amino acids and CO2. Such a cycle would have provided the chemical architecture necessary to kickstart life on Earth.
Their analysis also showed one of the elements central to the early citric acid cycle, glyoxylate, is still an essential component today.
"Modern metabolism has a precursor, a template, that was non-biological," said first author Greg Springsteen, associate professor of chemistry at Furman University.
Once biological molecules like enzymes showed up on early Earth, they could have inserted themselves into the cycle of reactions, modernizing the citric acid cycle.
"The chemistry could have stayed the same over time, it was just the nature of the molecules that changed," said Krishnamurthy. "The molecules evolved to be more complicated over time based on what biology needed."
Krishnamurthy and his colleagues hope to do additional research to determine how a citric acid cycle became stable and sustainable on early Earth.