Scientists Say Life on Earth May Have Begun Not Once, but Twice

3 min readHere’s what you’ll learn when you read this story:One of the most popular theories of life’s origins centers on the hydrothermal vents that line convergent plate boundaries in the depths of the ocean. These chemical and metal-rich environments possibly provided the necessary ingredients for life to emerge not once, but twice. A new study finds that certain enzymes used to catalyze life’s necessary chemicals are not conserved across bacteria and archaea, suggesting the groups developed independent methods to achieve the same metabolic reactions.How life began on Earth is a question that’s puzzled scientists, philosophers, and theologians since humanity was conscious enough of its own existence to wonder how we came into being. While cultures across the world developed their own wide array of mythic origin stories, scientists began investigating more grounded explanations of how life came to be in the 20th century. That explanation took a major leap forward in 1977 when scientists from Woods Hole Oceanographic Institution (WHOI) first discovered hydrothermal vents—underwater hot springs that form at spreading ridges and convergent plate boundaries—near the Galapagos Islands. Scientists found never-before-seen organisms clinging to these rocks, and a few years later, they published a paper first detailing a hypothesis that life on Earth possibly originated from these newly discovered submarine structures.But since the beginning, humans have conceived of a singular moment when life emerged from a series of chemical reactions. Now a new paper argues that maybe life actually emerged twice, with the two groups of prokaryotes—bacteria and archaea—devising different-yet-successful methods for catalyzing the essential metabolic reactions. The results of the study were published by an international team of researchers in the journal Science Advances.“The new data leave only one conclusion,” University of Düsseldorf biologist William Martin, senior author of the study, said in a press statement. “The bacteria and archaeal lineages made the transition to the free-living state independently. Only free-living cells are alive. Let’s call it by name: we are looking at one origin of the genetic code, but two origins of life.”The Tree of Life is separated into two major groups. The first, prokaryotes, are single-celled organisms that lack a membrane-bound nucleus and internal organelles, and these include bacteria and archaea. The second group, eukaryotes, can be larger, multi-celled, and comprise the rest of life on Earth, including human beings. The typical hydrothermal hypothesis is that these ecosystems provided just the right chemicals and metals necessary to form the building blocks of life. Those blocks eventually formed prokaryotes that, billions of years later through a process called endosymbiosis, formed eukaryotes, which then really poured gas on the smoldering fire that was early life on Earth.In the new study, scientists investigated genomes, protein structures, and chemical reactions that make up the earliest phases of microbial evolution. They looked at the 420 chemical reactions, collectively called “metabolism,” that form building blocks such as essential amino acids, RNA bases, and vitamins—all derived from the hydrogen gas, ammonia and carbon dioxide present on early Earth. First, the team suggested that nearby metals played an absolutely vital role for life to truly begin. They found that the Last Common Universal Ancestor, or LUCA, only possessed the ability to process enzymes for half of these reactions—the rest came from the metals themselves.“Metals that naturally occur in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism,” Max Planck Institute inorganic chemist Harun Tüysüz, a co-author of the study, said in a press statement.With this in mind, the team constructed four phases of early life: metal-only, a metal-enzyme hybrid in LUCA, and then divergent evolution toward the ancestors of bacteria and archaea.According to Martin, the big surprise is that the enzymes used to catalyze certain chemical reactions are not conserved across bacteria and archaea, suggesting that life independently discovered two ways to distinctly catalyze the same metabolic reactions, and thus formed the two major groups of prokaryotes today. The team also theorized that reactions between phosphite and palladium, which are abundant around hydrothermal vents, replaced adenosine triphosphate (ATP) and enzymes as the engine that drove these early reactions.Life is so exceedingly rare in the cosmos that we have yet to find any evidence of it in the thousands of worlds we’ve discovered beyond our own. But Earth continues to display an embarrassment of riches, as life possibly found not one but two methods for taking hold in an otherwise cold and (as far as we know) lifeless universe.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.