Describing how living organisms emerged from Earth's abiotic chemistry has remained a conundrum for scientists.
The study offers new insights into how the complex chemistry of metabolism cobbled itself together, the likelihood of life emerging and evolving as it did on Earth, and the chances of finding life elsewhere.
"We're trying to bring knowledge across disciplines into a unified whole that fits the essentials of metabolism development," said co-author Eric Smith, a Santa Fe Institute External Professor.
Creating life from scratch requires two abilities: fixing carbon and making more of yourself. The first, essentially hitching carbon atoms together to make living matter, is a remarkably difficult feat.
Carbon dioxide (CO2), of which Earth has plenty, is a stable molecule; the bonds are tough to break, and a chemical system can only turn carbon into biologically useful compounds by way of some wildly unstable in-between stages.
As hard as it is to do, fixing carbon is necessary for life. A carbon molecule's ability to bond stably with up to four atoms makes it phenomenally versatile, and its abundance makes it suitable as a backbone for trillions of compounds.
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Once an organized chemical system can harness and manipulate carbon, it can expand and innovate in countless ways.
Researchers Smith and Rogier Braakman mapped the most primitive forms of carbon fixation onto major, early branching points in the tree of life.
They have drawn from geochemistry, biochemistry, evolution, and ecology to detail the likeliest means by which molecules lurched their way from rocks to cells.
Their study presents a new, coherent picture of how this complex system fits together. What started as wonky geochemical mechanisms were sequentially replaced and fortified by biological ones, the researchers said.
"Think of life like an onion emerging in layers, where each layer functions as a feedback mechanism that stabilises and improves the ability to fix carbon," said Braakman.
Braakman and Smith describe specific features of metabolism and sub-divide helper metabolites by their functions. For example, vitamin B9, a complex molecule in the 'cofactor' class, facilitates the incorporation of one-carbon compounds into metabolism.
In mapping the chemical pathways to life's emergence, the researchers touch on a more existential question: How likely was it for life.
The study was published in the journal Physical Biology.