How Earth changed from a lifeless ball of rock, water, and gases to a blue planet teeming with organisms is one of science’s greatest puzzles. Life had to assemble out of chemical building blocks about 4 billion years ago. Some scientists have invoked spectacular sources of energy for life’s spark, such as a comet impact or lightning strike into a pool of primordial ooze; more recently, others have suggested life’s beginning didn’t require a large infusion of energy. Now, researchers have reconstructed the chemical reactions that would have allowed life to assemble. They found that as life was evolving, it only needed water, hydrogen, carbon dioxide (CO2), a dash of salt, and a little geothermal heat to get started.
In 2016, evolutionary biologist William Martin at Heinrich Heine University (HHU) and colleagues compared the genomes of two types of evolutionarily ancient single-celled organisms, bacteria and archaea. By looking at the genes bacteria and archaea share, the researchers reconstructed the genome of the last universal common ancestor (LUCA) of all living things, they reported in Nature Microbiology. Having LUCA’s genome was a key step in understanding how nonliving chemicals organized themselves into living organisms.
To further understand how LUCA came to exist, the researchers next explored LUCA’s metabolism—the set of chemical reactions it used to transform simple compounds into the building blocks of life. Martin and his colleagues compared the chemical reactions of bacteria and archaea, relying on the Kyoto Encyclopedia of Genes and Genomes, to find the reactions they had in common. Martin, lead author Jessica Wimmer, a graduate student in bioinformatics at HHU, and their colleagues uncovered 402 chemical reactions at the core of LUCA’s metabolism, they report this week in Frontiers in Microbiology. Many of the reactions only required hydrogen and CO2 to assemble more complex chemicals.
Wimmer ran computer simulations of the 402 reactions, varying temperature and alkalinity. She and the team found that in an alkaline environment at 80°C to 100°C, more than 95% of the reactions would release energy. Because each reaction needs a heat boost to get started, one reaction can feed the next, keeping a protocell’s metabolism going. “The energy for life is in life itself,” Martin says. “It’s unfolding in these reactions.”
Those findings narrow the number of places where life may have gotten its start, says Shino Suzuki, a microbiologist at the Japan Aerospace Exploration Agency who was not involved in the study. A nonvolcanic hydrothermal vent on the ocean floor seems like a good candidate. (Hydrothermal vents powered by molten rock would be too hot.) At those warm hydrothermal vents, certain iron-rich minerals react with water and produce the right amount of heat and alkalinity, and a steady stream of hydrogen gas for LUCA’s metabolism. Suzuki, Martin, and Wimmer think LUCA arose in these environments.
In addition, such warm vents have rocks with microscopic pores that could have kept a set of metabolic reactions together in one place, before cell walls evolved. LUCA itself may have simply been a set of reactions happening within rock pores, only “half-alive,” Martin says. “It’s not a free-living cell,” he says. “It’s just a very advanced [combination of] chemical reactions.”
Judit Šponer, a chemist at the Institute of Biophysics of the Czech Academy of Sciences is impressed with Martin’s reconstruction of LUCA’s metabolism and she agrees that a hydrothermal vent was probably where it all came together. But she doesn’t think the other components of life necessarily arose there. The necessary pieces for life—cell membranes, metabolic reactions, a genome—could have evolved in different places over millions of years, she says, and then somehow come together. “Life emerged in a variety of conditions,” she says.
Martin’s work means life might also have arisen on other planets without dramatic heat or electric sources. “You need water, you need rocks, and a little bit of energy,” he says. “If you give life what it needs—namely hydrogen and CO2—that’s all you need.”