Did meteorites deliver the critical ingredients for life on Earth?

Synthetic biologist Sheref Mansy wonders how life could have emerged in a time before biological mechanisms.

One of humanity’s great remaining mysteries is where we came from. Nearly everyone, scientist and non-scientist alike, has wondered about our origin story. Many think the answer is unknowable — that this mystery will endure forever.

It’s true that we may never know the exact sequence of events that led to life as we know it, as there’s no fossil record of Earth’s first life forms to tell us with certainty. We can, however, demonstrate plausible scenarios. After all, we’ve known that the laws of science are the same both inside and outside of a living cell since at least the days of Friedrich Wöhler — a German chemist who in the early 1800s accidentally synthesized a biological molecule (urea) in the laboratory from non-biological starting material.

With that knowledge, scientists have made tremendous progress in the last few decades demonstrating plausible ways that the building blocks of life could have been assembled into genomic material, protein and the types of fat molecules needed to make membranes.

One area has remained largely unexplored, though. Despite the natural tendency of things to degrade, life fights back by continually repairing and replacing damaged material — but the mechanisms that allow living things to persist over time require a constant supply of energy. We call the process of energy being extracted from fuel to help sustain life metabolism. Remarkably, origins-of-life researchers have neglected to test out theories about how metabolic chemistry could have played a role in the emergence of life.

DNA alone is not sufficient to maintain life. Modern-day metabolism relies heavily on metals like iron, copper and zinc. It’s a chicken-and-egg problem: As metals are so important to life, and Earth is rich in metals, it seems plausible that the chemistry that led to life heavily exploited these metals. At the same time, most of metabolism is mediated by large, complex, sophisticated proteins that are the products of evolution. To put it another way, it’s highly unlikely that such proteins existed on the Earth before there was life! So how could these metal ions have been exploited if there were no proteins present?

The leading theory addressing this question involves rocks — minerals that contain metals. Rocks were thought by many to have played the same role on prebiotic Earth as contemporary proteins. While it’s true that rocks can participate in chemical reactions and likely impacted the types of molecules that existed on prebiotic Earth, there’s no explanation from such theories on how proteins (or even life) could have emerged from rock.

Could the building blocks of life have been delivered to Earth by meteorites? Or could natural, abiotic chemistry either here on Earth or in interstellar space have led to the formation of the components of proteins?

In my laboratory here at the University of Trento, my team and I are developing an alternative theory that fits better. Experimental evidence suggests that the building blocks of proteins and even short protein fragments (called peptides) existed on prebiotic Earth. Amazingly, some meteorites that have fallen to Earth from space have been shown to contain these same protein-building blocks.

This begs the question: Could the building blocks of life have been delivered to Earth by meteorites? Or could natural, abiotic chemistry either here on Earth or in interstellar space have led to the formation of the components of proteins?

My team of students and researchers — including Claudia Bonfio, Simone Scintilla, and Luca Valer — working with collaborators at the MRC Laboratory of Molecular Biology and Harvard, recently demonstrated that, under simulated early Earth conditions, short peptides plus iron and sulfur spontaneously assemble into chemical arrangements that display properties similar to modern-day proteins. These tiny prebiotic metal-binding peptides may be the ancestors to the proteins that you and I are made up of today.

In some ways, this isn’t surprising. Iron and sulfur are among the most abundant elements in the universe, so it makes sense that the natural chemistry arising from these two elements would be found at the heart of biology. Once formed, it seems likely that this arrangement of iron and sulfur can function as a type of scaffolding that positions short peptides to facilitate their growth into longer peptides — which resemble the kinds of proteins we find in contemporary living cells.

Finally, an answer to that chicken-and-egg question: the chemical reactions we discovered are facilitated by sunlight in a way that mimics contemporary metabolism. Instead of exploiting highly evolved biological mechanisms that couldn’t have existed on Earth before life, this chemistry depends on the most basic, widely available energy source: the sun.

So maybe the building blocks of life did originate in asteroids, or even right here on Earth. At the very least, life may have been kickstarted in a reaction between peptides, iron, sulfur, and sunlight — which, to us at least, seems more likely than metabolism or living cells arising from rock. We’re one step closer to knowing.

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