Could Life’s Earliest Stages Have Survived Without a Key Ingredient?

By Jeffrey Marlow | March 21, 2017 5:25 am
During the early Earth's heavy bombardment, could metabolism have started without phosphate, one of life's key ingredients? (Image: NASA/Simone Marchi)

During the early Earth’s heavy bombardment, could metabolism have started without phosphate, one of life’s key ingredients? (Image: NASA/Simone Marchi)

“CHNOPS” is one of science’s most revered acronyms, an amalgamation of letters that rolls of the tongues of high school biology students and practicing researchers alike. It accounts for the six elements that comprise most biological molecules: carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur.

Biologists have traditionally assumed that all six elements were prerequisites, as each one is found in several of life’s most essential molecules. But what if earlier life forms weren’t quite so demanding? Could a sustainable metabolism actually exist without one of these seemingly essential elements? To explore this revolutionary possibility, Joshua Goldford, a graduate student in Boston University’s Bioinformatics Program, led a theoretical study taking aim at phosphorous and its most biologically utilitarian derivative, phosphate.

There are reasons to believe that phosphate’s availability could have been a limiting factor in life’s early stages: it’s scarce on a geological scale, and even when it is present, it’s often tightly bound to rocks and minerals. In phosphate-free thought experiments, previous researchers have proposed bigger roles for sulfur and iron, as well as thioesters (relatively reactive molecules that contain a carbon, oxygen, and sulfur-associated functional group).

Goldford and his colleagues wanted to see how a phosphate-free world would map onto the contours of microbial biochemistry: could you generate everything a cell might have needed? To find out, they adopted a “reaction network” approach, essentially lumping all known metabolic reactions together regardless of the modern organism they came from. This broad umbrella is a bit of a leap, as modern reactions have had billions of years of evolution to explore new functional space, but the study posits that “many of such reactions could have been initially catalyzed to a much weaker and less specific extent by a number of small molecules,” and that this scenario “is not new to origin of life research.”

Remarkably, a core, phosphate-independent network of 315 reactions and 260 biomolecules emerged. It could build organic carbon through the reductive TCA cycle, which is believed to be an ancient carbon fixation pathway. Ten amino acids could be made, and the rest could be harvested from natural, abiotic sources. Genes in this core network were more highly represented among modern microbial genomes, pointing to their foundational role. Proposed enzymes were smaller, on average, than modern life’s mean enzyme – a finding consistent with the scientists’ proposal that ancient catalysts may have consisted of smaller, more primitive amino acid chains.

This core set of reactions may provide the raw materials for cells, but what’s powering them? After all, the energy currency of all biological reactions – adenosine triphosphate, or ATP – contains, you guessed it, three phosphate groups. Tear one off and approximately 50 kJ / mol of energy is yours to do with what you will, driving energy-requiring reactions forward to build many of the more complex biomolecules.

Goldford proposes that thioesters – and a molecule called pantetheine in particular – could play a similar role: when an acetyl group is stripped off, 33 kJ / mol of energy is generated. It’s not as much as an ATP molecule, but it’s enough to ease the energetic burden of the hypothetical cell and enable a more luxurient way of life.

While phosphate may not have been required for a core set of proto-biological needs, inheretance of genetic material isn’t part of the equation: RNA and DNA both involve a phosphate molecular backbone, without which genetic material as we know it wouldn’t be possible. Nonetheless, by kickstarting biochemical processes in the absence of phosphate, metabolic networks may have been poised to jump into an RNA world once phosphate became more readily accessible.

As useful as a phosphate-free metabolic network could be in helping to explain a longstanding disconnect between geology and biology, we probably won’t be seeing “CHNOS” in textbooks any time soon.

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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Go the analog route. Set up a paired line of Miller Urey abiotic synthesis experiments (no addenda, igneous rock, black smoker deposit), half with and half without phosphate sources, plus an assortment of energy sources (thermal, UV, radioactive, electric discharge). Let them cook for a year. See if it matters.

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  • OWilson

    What is that elusive “Key ingredient?

    So far, the myth that an explosion of nothing from nothing gave me my family, my pets, my trees and flowers, is running about even with Michelangelo’s depiction of the Godly “touch of life”.

    I prefer the more romantic of the creation myths. Why not?

    • StanChaz

      Why not, you ask? Because you will have wasted your life, or part of it, immersed in a myth of self-deception, rather than honestly seeking and openly exploring reality, and truly fulfilling your life’s potential. Of course you’re free to do that –but at what cost? YOU are the creator my friend – the creator of the meaning of your own short life. If not, if you abdicate that responsibility, then you’re living your short precious life through someone else’s eyes, with their pre-packaged choices, and following somebody’s else’s “sacred” commands – you’re living their lives, not yours. As I said, what a waste of man’s creative potential. My “romance” consists of my deep love for the world and the people around me, not a love and hope for some mythical pie-in-the-sky that requires me to starve myself in the precious present. My goal is to learn about life by living it, not by trying to figure out a cryptic plan that some supposed creator had in store for me. I would urge you to get off your knees. Stand up like the proud Men and Women that you truly are, and choose to live your own lives fully and responsibly, in your own ways. Walk tall on your own path – with your own meanings and mistakes and methods. Break the chains of dependency, religion and magic, and breathe the fresh and healing air of freedom: deeply …fully … finally.

      • OWilson

        Since no one knows how life came to be (which is my point) your advice is well taken.

        I would only add my own advice for a full, rich life.

        Strive to leave the world a better place than you found it, and do not incur debts that must be paid by others after you are gone.

  • Paddy Ryan

    “luxurient” “inheretance” – anybody heard of a spell-checker here?

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The Extremo Files

The Extremo Files traces the science that is pushing the boundaries of biology, from the deep sea to outer space to the brave new world of synthetic biology.

About Jeffrey Marlow

Jeffrey Marlow is a geobiologist exploring the limits of life, from the role of microbes in global elemental cycles to the possibility of life beyond Earth and the brave new world of synthetic biology. He received his PhD from the California Institute of Technology and is currently a Postdoctoral Scholar at Harvard University, where he studies the inner workings of methane-metabolizing organisms.

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