Life could have emerged from just eight simple compounds – none of which contain phosphate, a molecule that forms the backbone of our DNA – according to new research from the US.
Phosphate is essential for all living systems: it helps us move energy around our cells and is an essential part of the structure of the DNA double-helix. But on early Earth there was very little phosphate around, so researchers combed through all known biochemical reactions and found that it is possible life formed in the absence of phosphate, using sulfur to break down food and keep cells alive instead.
Narrowing it down, the researchers discovered that this would require up to 315 chemical reactions to make key molecules that could sustain life from combinations of hydrogen sulfide, ammonium, carbon dioxide, formate, acetate, water, bicarbonate and nitrogen gas.
"Before our study, other researchers had proposed a sulfur-based early biochemistry, with hints that phosphate may not have been necessary until later," Senior researcher Daniel Segre of Boston University says. "What was missing until now was data-driven evidence that these early processes, rather than scattered reactions, could have constituted a highly connected and relatively rich primitive metabolic network."
Although this non-experimental evidence does not definitively prove that life started without phosphate, it does give compelling support for the iron-sulfur world hypothesis and the thioester world scenario. At the same time, the study calls into question the "RNA world hypothesis," which says self-replicating RNA molecules were the precursors to all current life on Earth.
Instead, the results support the "metabolism-first hypothesis," which says a self-sustaining phosphate-free metabolic network predated the emergence of nucleic acids. In other words, nucleic acids could have been an outcome of early evolutionary processes rather than a prerequisite for them.
"Evidence that an early metabolism could have functioned without phosphate indicates that phosphate may have not been an essential ingredient for the onset of cellular life," says researcher Joshua Goldford of Boston University. "This proto-metabolic system would have required an energy source and may have emerged either on the Earth's surface, with solar energy as the main driving force, or in the depth of the oceans near hydrothermal vents, where geochemical gradients could have driven the first life-like processes."
In future studies, the researchers will continue to apply systems biology approaches to study the origin of life.
"My hope is that these findings will motivate further studies of the landscape of possible historical paths of metabolism, as well as specific experiments for testing the feasibility of a phosphate-free sulfur-based core biochemistry," Segre says. "The idea of analyzing metabolism as an ecosystem-level or even planetary phenomenon, rather than an organism-specific one, may also have implications for our understanding of microbial communities."
Segre also says it will now be interesting to revisit the question of how inheritance and evolution could have worked before the appearance of biopolymers.