A new synthesis by two Santa Fe Institute researchers has provided a coherent picture of how metabolism, and thus all life, arose.
The study offered 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.
In a paper published earlier this year in PLoS Computational Biology, Eric Smith, a Santa Fe Institute External Professor and Santa Fe Institute Omidyar Fellow Rogier Braakman mapped the most primitive forms of carbon fixation onto major, early branching points in the tree of life.
Now, the two researchers have drawn from geochemistry, biochemistry, evolution, and ecology to detail the likeliest means by which molecules lurched their way from rocks to cells.
Their 62-page “Logic of Metabolism” paper 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 believe.
“Think of life like an onion emerging in layers, where each layer functions as a feedback mechanism that stabilizes and improves the ability to fix carbon,” said Braakman.
Carbon fixing and other chemical sub-processes that together constitute metabolism each comprise dozens of steps; some are quick and easy turnkey reactions with simple molecules, others require highly specific chemical helpers, or catalysts.
The parts of metabolism that guide carbon fixation through its unstable intermediate stages fall into the latter category, requiring help. But these seemingly unlikely reactions are remarkably consistent across all living systems.
In fact, said Braakman, their ubiquity and the difficulty with which they .are forged make them the chemical constraints within which all living systems operate – in a sense, the scaffolding for the tree of life.
It’s these dependable regularities of hierarchy and modularity, amid the panoply of reactions comprising metabolism that stabilise the system and enable its complexity.
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 (otherwise unstable) 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 to have developed at all? Extraordinarily so, stated Braakman.
“Metabolism appears to be an ‘attractor state’ within organic chemistry, where it was likely to be selected regardless of earlier stages of chemical evolution” in the chaotic, high-energy conditions of prebiotic Earth, he said.
Can it happen elsewhere? Possibly, even probably, he says. Rocky planets usually have cores chemically similar to ours, so if a planet is volcanically (and perhaps tectonically) active and has an ocean, it will probably have hydrothermal vents that spew chemicals, creating the potential conditions for life, Braakman said.
In fact, the physics of star and planet formation make the chances of such conditions pretty reasonable.
Smith cautions, however, that we still have much to learn about the chemical and physical conditions that might lead to life-like organisation, but he hopes their paper will at least “lead to experimental questions that focus more directly on the key functions that link metabolism to geochemistry.
The research was recently published in the journal Physical Biology.