Those questions are at the heart of research at the Institute of Molecular Evolution at Heinrich Heine University Düsseldorf (HHU). Now, an international team led by biologists in Düsseldorf has reported new findings in Science Advances that shed light on the chemical reaction network used by the earliest cells to produce the basic ingredients of life, as well as the energy sources that powered those reactions. By tracing the origins of enzymes during the earliest split between bacteria and archaea, the researchers found evidence that free-living cells may have originated independently twice.
About 4 billion years ago, Earth looked very different from the planet we know today. If it were possible to watch the earliest cells take shape, the scene may have involved two distinct forms of primitive cellular life. "We would see two very different kinds of cells emerging, pioneer bacteria and pioneer archaea, making their first attempts at life outside the confines of a hydrothermal vent," says Natalia Mrnjavac, biologist at the University of Düsseldorf and lead author on the new publication in Science Advances.
Mrnjavac and an international team examined genomes, protein structures and chemical reactions to investigate some of the earliest stages of microbial evolution, including the period before fully free-living cells existed. "These comparisons are giving us unprecedented insights into the phase of evolution when metabolism catalyzed by enzymes was arising from spontaneous reactions catalyzed by metals in the Earth's crust," says Düsseldorf biologist William Martin, senior author of the study. Reconstructing the Chemistry of Early Life Rather than focusing on only selected parts of early metabolism, the researchers examined the complete group of chemical reactions that cells use to manufacture key biological components (amino acids, RNA bases and vitamins) from materials that were available on the early Earth, including hydrogen gas, ammonia and CO2.
Together, these 420 chemical reactions form the metabolic network known as metabolism. The reactions themselves are extremely ancient and are conserved across life to a degree comparable with the genetic code. What surprised the researchers was that the enzymes responsible for carrying out those reactions do not show the same degree of conservation between bacteria and archaea.
Martin: "The surprise is that the enzymes that catalyze those reactions are not conserved across the evolutionary divide that separates bacteria and archaea. We found that the last universal ancestor of all cells, LUCA, possessed enzymes for only about half of the reactions of metabolism. The other half was catalyzed by metals in the environment where LUCA arose." That finding suggests that the earliest metabolism depended much more heavily on the surrounding environment than modern cellular metabolism does.
"Metals that naturally occur in hydrothermal vents can replace a surprisingly large number of enzymes in metabolism," says Harun Tüysüz, inorganic chemist from the Max-Planck-Institut für Kohlenforschung and the IMDEA Materials Institute in Madrid, and co-author on the study. "The closer we look, the more clearly we can see that early biochemical evolution was a hybrid of enzymatic and metal catalysts," says Joseph Moran from the University of Ottawa, Canada, an international leader in the use of metals to catalyze metabolic reactions, replacing enzymes and cofactors. One of the study's major advances was the reconstruction of four stages in the early evolution of biological catalysis.
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