A new UK study challenges current views on the sequence of events that led to the origins of life on Earth. Researchers from the University of Cambridge suggest chemical reactions that allow cells in today's living organisms to make essential building blocks like amino acids, nucleic acids and lipids, may have started spontaneously billions of years ago in Earth's oceans, before life began.
The study questions the idea that the reactions required to produce these metabolites can only occur with the help of enzymes - highly complex biological molecules that catalyze the thousands of metabolic processes that enable living cells to stay alive and function.
Instead, the findings suggest that, in the era before oxygen, the oceans were rich in soluble iron and other metals, and these concentrations were high enough to act as catalysts for the reactions.
Study leader Dr. Markus Ralser of Cambridge's Department of Biochemistry and the National Institute for Medical Research, says:
"Our results show that reaction sequences that resemble two essential reaction cascades of metabolism - glycolysis and the pentose-phosphate pathways - could have occurred spontaneously in the Earth's ancient oceans."
He and his colleagues report their findings in the journal Molecular Systems Biology.
When they reconstructed the chemical and physical conditions of the Earth's earliest oceans in their laboratory, the Cambridge team showed that the complex metabolic reaction sequences, also known as metabolic pathways, occurred in the absence of enzymes.
They observed how their reconstruction gave rise to 29 reactions, including the formation of one particular noteworthy metabolite - ribose 5-phosphate - a precursor of RNA, that, like DNA, codes, decodes, regulates and expresses genes, replicates and catalyses chemical reactions. Molecules like ribose 5-phosphate could in theory give rise to RNA, say the researchers.
In pre-oxygen era, iron was more soluble and able to act as a catalyst
In the era before oxygen, the oceans were rich in soluble iron and other metals
Based on the composition of early sediments, they replicated the conditions that prevailed in the iron-rich "Archean" oceans that clad the surface of the planet around 4 billion years ago, before the first single-celled organisms existed.
As this predates photosynthesis, there was no oxygen, and iron was more soluble and able to act as a catalyst, as Dr. Ralser explains:
"In our reconstructed version of the ancient Archean ocean, these metabolic reactions were particularly sensitive to the presence of ferrous iron, which was abundant in the early oceans and accelerated many of the chemical reactions that we observe. We were surprised by how specific these reactions were."
Co-author Dr. Alexandra V Turchyn, of Cambridge's Department of Earth Sciences, says:
"We are quite certain that the earliest oceans contained no oxygen, and so any iron present would have been soluble in these oxygen-devoid oceans. It's therefore possible that concentrations of iron could have been quite high."
The researchers say, as well as iron, the presence of other metals and phosphates enabled a series of chemical reactions that are similar to the metabolic processes inside living organisms to take place without the need for enzymes.
Further studies are now needed to determine how the first enzymes may have adopted these metal-catalyzed reactions.
In August 2011, Medical News Today reported how scientists proposed a plausible scenario for the origin of life on Earth based on a relatively simple combination of naturally occurring sugars and amino acids. Their research also demonstrated how precursors to RNA could have formed before life existed.