In the search for answers on how life might have evolved on Earth some four billion years ago, researchers have turned the problem on its head and looked at key chemical reactions instead of concentrating on the availability of the right molecules, and found that life could have found a way with a crude replacement to todays metabolic processes.
To survive, every oxygen-breathing organism on Earth, from tigers to toadstools, relies on a process known as the citric acid cycle (CAC) to release stored energy in cells. In simplistic terms, this occurs when reactions in the cycle eat up a substance called acetyl coenzyme A (or acetyl-CoA) which can be found in carbohydrates, fats and proteins and transforms it into carbon dioxide as a waste byproduct.
The cycle, also known as the tricarboxylic acid cycle (TCA), is so important to many biochemical pathways that scientists suggest that it was one of the earliest established components of cellular metabolism and that it may have originally occurred in non-living matter, such as simple organic compounds.
In the past, researchers have concentrated on finding the right type of molecule for the citric acid cycle as life uses it today, however these molecules are particular fragile and may not have lasted long enough for life to take hold. Turning the situation on its head, chemists at The Scripps Research Institute (TSRI), looked at the chemical reactions first, then determined which molecules that were present on early Earth could have worked to facilitate the reactions.
The team, led by Ramanarayanan Krishnamurthy, associate professor of chemistry at TSRI, found that two non-biological cycles – called the HKG cycle and the malonate cycle – could have used reactions that perform the same fundamental chemistry of organic compounds known as a-ketoacids and b-ketoacids found in the citric acid cycle, to kick-start a simplified version of this evolved process.
The citric acid cycle also helps to create compounds that can be used to form certain amino acids – the building blocks of protein. Amino acids give cells their structure and they also play a key role in the transport and the storage of nutrients. As they ran their reactions, Krishnamurthy and team found that in addition to CO2, the chemistry also produced these much needed amino acids.
The researchers think that as biological molecules like enzymes became available, they could have led to the replacement of non-biological molecules in these fundamental reactions to make them more efficient (and elaborate).
"The chemistry could have stayed the same over time, it was just the nature of the molecules that changed," says Krishnamurthy. "The molecules evolved to be more complicated over time based on what biology needed."
To add weight to this new hypothesis, the reactions also utilise a molecule called glyoxylate (whose chemical formula is C2HO3-). Glyoxylate is part of the citric acid cycle today (and is a variation of the tricarboxylic acid cycle) and previous studies have indicated that the molecule could have been available on early Earth.
Although this initial success helps to explain how life could have taken hold on early Earth, more research needs to be done to see how these chemical reactions could have become as sustainable as the citric acid cycle is today, added lead author Krishnamurthy.
