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Origin & Evolution of Life on Earth

Life’s Engine: The Early Ocean
How the early ocean lead to modern life. Credit: Molecular Systems Biology

Tipping the scales of a long-standing debate, researchers at the University of Cambridge in England have modeled ancient oceans and discovered that basic mechanisms used in metabolism of Earth’s organisms arise spontaneously when certain ingredients are put together.

In their paper, published today in Molecular Systems Biology, Drs. Markus Keller, Alexandra Turchyn and Markus Ralser explain how building blocks of life were generated when they added together elements believed to have made up the ancient oceans.

The geologic record gives us a recipe for archaic ocean sediment. That recipe includes elements such as iron, cobalt, nickel, molybdenum and phosphate. Turchyn, Keller and Ralser combined these elements into a mimic of ancient ocean sediments, called a mimetic. “Mimetic in this case refers to the metal mixture that was used in the experiments,” said Turchyn, “We designed the metal mixture to mimic a hypothesized metal combination for the Archean ocean (based on work by Ariel Anbar, Andy Knoll, Olivier Rouxel, among others).”

They then added water and sugars with phosphoric acid attached. At certain temperatures, sugar-phosphate molecules were spontaneously metabolized, as they might be inside a living cell, though there were no cells in the system.

“What we found was that the carbon compounds that are formed by all cellular metabolism were stabilized and accelerated by the absence of oxygen and the presence of the metals, Turchyn said, “We then tested for the importance of the various metals in the mimetic and were able to show that iron was the most important one.”

Cells assembling. Credit: DigitalSpace
Cells assembling. Credit: DigitalSpace

Remarkably, not only did models mimicking the ancient ocean show that cellular metabolic reactions took place, but they also showed that the reactions were stabilized and even accelerated by the elements present in the sediment.

What may be most notable about this occurrence -and most hopeful from the perspective of astrobiology- is that all of this took place in the absence of enzymes. Enzymes are the modern-day workmen of cells. Enzymes are produced by living organisms and were believed to be necessary for even the most basic biologic reactions to occur. DNA, RNA, and all of the building blocks of life as we know it depend upon enzymes-or so we thought. As a result, it been generally accepted that enzymes might be both necessary and sufficient for life, launching a quest to discover how enzymes evolved, so that we could discover how life evolved.

However, Keller, Ralser and Turchyn’s experiments occurred in an enzyme-free environment. Without so much as a single enzyme in the sight, 29 distinct, basic biologic reactions took place, including those forming ribose 5-phosphate, which constitutes the backbone of RNA. In the presence of heat, water and mimetic sediment, basic sugar and protein precursors were processed from intermediate to final forms identical to the ones found in modern cells without the aid of more complex molecules like enzymes.

“Our results demonstrate that the conditions and molecules found in the Earth’s ancient oceans assisted and accelerated the interconversion of metabolites in a similar fashion as [sic] in two of the essential and most centrally placed reaction cascades in modern organisms ” says Ralser, describing what occurred in the reconstructed version of the Archean ocean, “In the presence of iron and other compounds found in the oceanic sediments, 29 metabolic-like chemical reactions were observed, including those that produce some of the essential chemicals of metabolism, for example precursors of the building blocks of proteins or RNA.”

Ralser’s results imply that the structure of modern metabolism might have arisen directly from the geochemistry that existed in the very early days of Earth. In other words, in this model, and perhaps on ancient Earth, geology made metabolic processes that were later incorporated into the first cells.

The evolution-of-life debate -or metabolism first hypothesis- is two-sided. One side holds that life evolved, and then evolution selected the cells with the chemistry we see today.

These new results add weight to the other side of the metabolism first hypothesis debate: which point out that life, or genetics, did not yet have to be in place to form the central reaction cascades used in modern cells metabolism. Metabolic processes may have developed before, and potentially facilitated, life itself.

These results also gives astrobiology a new focus: to look for the elements and intermediates in oceans of far-off worlds like Enceladus that can power life.

“Its [sic] difficult to provide a definite proof of a metabolism first hypothesis, as on Earth we have lost all primitive forms/precursors of modern metabolism,” said Ralser, “Astrobiology might indeed be able to help one day in answering this question.”

If simple sediments swimming in the presence of sugar and phosphate can power life’s metabolism, then the key to turning on life’s engine might be far simpler than we previously believed, and life even more abundant in our Solar System and beyond.

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