It’s been well accepted by the scientific community that all life on Earth arose from a single primitive ancestor, which was the starting point for all the biodiversity we observe today. Previous genetic studies were consistent with the idea that life began around hydrothermal vents, but other possible habitats, such as tidal flats, could not be excluded. Now a comprehensive new study advances our understanding by suggesting how the Last Common Universal Ancestor, or LUCA for short, made its living.
A team of researchers from the Heinrich Heine University in Düsseldorf, Germany. analyzed more than six million protein-encoding genes, and was able to link 355 protein families to the common ancestor. Taken together, they provide new evidence that life originated at a hydrothermal vent. The research was published in the journal Nature Microbiology.
Based on the proteins it used, LUCA appears to have obtained energy from hydrothermal vents by oxidizing hydrogen gas. It got carbon and nitrogen—the building blocks of cells—from carbon dioxide and nitrogen gas, and it used iron near the vents to construct enzymes. Hydrothermal vents were strictly anaerobic early in Earth’s history, meaning that oxygen was in extremely short supply, and that trace metals such as iron were readily available as a result.
Even more interesting is what LUCA was not able to do. Based on the protein families identified by the researchers, LUCA appeared to harvest chemicals across a gradient between the hot vent water and colder sea water to make ATP (life’s currency of energy). That would have confined primitive life to the vicinity of the hydrothermal vents—it would have been unable to survive away from them, because it would not have been able to pump ions across a membrane to make ATP, a feat that every organism today can do. The research team also found no genes to make amino acids, the essential building blocks of proteins and life as we know it. Thus, the first organism may have had to rely solely on amino acids that formed naturally at hydrothermal vents. Depending on their definition of life, many scientists might not even call LUCA a form of life.
Even with this comprehensive study, there remains, of course, considerable uncertainty. The researchers’ methodology didn’t allow all of LUCA’s protein families to be identified. Some later genetic additions—for example by horizontal gene transfer, the swapping of genes between different microbes, may have been misinterpreted as dating from the time of LUCA. Having said that, the new results make sense in terms of the processes we believe are ancient, such as the oxidation of hydrogen for metabolism (especially with the use of carbon dioxide). We also would expect the first cell to have been rather primitive, somewhere between a chemical system and what we would today call life.
If the authors are correct, their research has far-reaching consequences, in that it helps us to rule out alternate hypotheses for the origin of life. For example, it counters the idea that LUCA used organic compounds for its metabolism. It may be too early to say that LUCA’s physiology and likely habitat are now resolved, but anyone suggesting an alternative hypothesis will have to address these new findings.