Scientists Recreated The Ancient Chemical Reactions That May Have Sparked Life
It's difficult to imagine how life kicked off on our planet. In ecosystems today, life is so deeply entwined with itself that very few creatures live directly off Earth's raw materials. That has been the case for a very, very long time.
But the first organisms on an otherwise lifeless planet would have had to make do with what the mineral environment had to offer. There was little to no oxygen, and no photosynthesis. As you can see in the video below, some deep sea organisms still live this way, surviving on hydrothermal vents at depths where the sun don't shine.
Borrowing electrons from hydrogen as it spews from the Earth's core, the deep-sea microbes follow a recipe more ancient than the genes they use to conduct it, called the acetyl CoA pathway. It is the only method for carbon fixation – processing inorganic carbon into organic compounds – that can be re-created without enzymes.
But when this recipe was first written, in Earth's early years, seawater contained a whole lot more dissolved iron than it does today. A team led by geochemist Vanessa Helmbrecht of Ludwig Maximilian University of Munich in Germany wanted to test how much of a difference this dissolved iron would have made, by simulating these ancient ocean conditions in the lab.
"The ancient occurrence of hydrothermal iron-sulfide rich deposits in the geological record extend into the early Archaean eon (4 to 3.6 billion years ago) and exhibit fossil features interpreted as some of the oldest signatures for life on Earth," the team writes in their paper describing the experiment.
"However, links between abiotic H2 [dihydrogen] production in iron-sulfide chemical gardens simulating [primordial] hydrothermal systems and early life are scarce."
A single-celled microbe of the order archaea, Methanocaldococcus jannaschii, was selected as the test subject for these simulations. It was first collected from a hydrothermal vent off the western coast of Mexico, where, using the acetyl CoA pathway, it relies on carbon dioxide and hydrogen as its primary sources of energy.
"Abiotic H2 was a potentially important electron donor and CO2 served as a key electron acceptor for the first cells," the team explains. "Anaerobic organisms that use the H2-dependent reductive acetyl CoA pathway for CO2 fixation are modern representatives that have preserved vestiges of the first metabolisms."
The experiments placed M. jannaschii into a miniature version of the deep sea hydrothermal vents, neatly contained in a glass vial. By injecting sulfidic fluid into water devoid of dissolved oxygen, they formed a black precipitate that grew into a chimney structure within 5-10 minutes.
At high temperatures, the iron and sulfur in this microcosm formed the iron sulfide minerals mackinawite (FeS) and greigite (Fe3S4). When iron sulfide is hydrated, H2 is released.
Though quite different from its modern home, M. jannaschii thrived in this strange environment.
"At the beginning, we expected only slight growth, as we did not add any extra nutrients, vitamins, or trace metals to the experiment," Helmbrecht says. "As well as over-expressing some genes of the acetyl CoA metabolism, the archaeans actually grew exponentially."
The M. jannaschii cells tended to hang out right beside the mackinawite particles, in a scene much like some of the earliest traces of life found in fossil specimens. These chemical gardens, the scientists think, fuelled Earth's first microbes.
This is evidence that the recipe for acetyl CoA metabolism emerged from the extreme and energy-limited environments where Earth life may have struck its first sparks.
"Our study points to mackinawite and greigite chemical gardens as potential hatcheries of life, primordial environments that could theoretically support a continuous evolution of the first metabolizing cells," the authors conclude.
The research is published in Nature Ecology & Evolution.
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