One of the greatest mysteries facing humans is how life originated on Earth. Scientists have determined approximately when life began (roughly 3.8 billion years ago), but there is still intense debate about exactly how life began. One possibility — that simple metabolic reactions emerged near ancient seafloor hot springs, enabling the leap from a non-living to a living world — has grown in popularity in the last two decades.
Recent research by geochemists Eoghan Reeves, Jeff Seewald, and Jill McDermott at Woods Hole Oceanographic Institution (WHOI) is the first to test a fundamental assumption of this ‘metabolism first’ hypothesis, and finds that it may not have been as easy as previously assumed. Instead, their findings could provide a focus for the search for life on other planets. The work is published in Proceedings of the National Academy of Sciences.
In 1977, scientists discovered biological communities unexpectedly living around seafloor hydrothermal vents, far from sunlight and thriving on a chemical soup rich in hydrogen, carbon dioxide, and sulfur, spewing from the geysers. Inspired by these findings, scientists later proposed that hydrothermal vents provided an ideal environment with all the ingredients needed for microbial life to emerge on early Earth. A central figure in this hypothesis is a simple sulfur-containing carbon compound called “methanethiol” — a supposed geologic precursor of the Acetyl-CoA enzyme present in many organisms, including humans. Scientists suspected methanethiol could have been the “starter dough” from which all life emerged.
The question Reeves and his colleagues set out to test was whether methanethiol — a critical precursor of life — could form at modern day vent sites by purely chemical means without the involvement of life. Could methanethiol be the bridge between a chemical, non-living world and the first microbial life on the planet?
Carbon dioxide, hydrogen and sulfide are the common ingredients present in hydrothermal black smoker fluids. “The thought was that making methanethiol from these basic ingredients at seafloor hydrothermal vents should therefore have been an easy process,” adds Reeves.
The theory was appealing, and solved many of the basic problems with existing ideas that life may have been carried to Earth on a comet or asteroid….
“What we essentially found in our survey is that we don’t think methanethiol is forming by purely chemical means without the involvement of life. This might be disappointing news for anyone assuming an easy start for hydrothermal proto-metabolism,” says Reeves. “However, our finding that methanethiol may be readily forming as a breakdown product of microbial life provides further indication that life is present and widespread below the seafloor and is very exciting.”
The researchers believe this new understanding could change how we think about searching for life on other planets.