The last common ancestor of all living organisms on Earth was a microorganism that lived 4.2 billion years ago, and its genome was quite large, encoding approximately 2600 proteins. It feeds on hydrogen and carbon dioxide and has a basic immune system to resist virus invasion. This is the conclusion of a new study published on July 12th in Nature Ecology and Evolution. This study compared the genomes of 700 modern microorganisms to identify commonalities and determine which features first appeared. Although the study did not reveal how life began, it suggests that a complex cellular organism similar to modern microorganisms evolved several hundred million years after the formation of Earth. I am very excited Evolutionary biologist Betl Kacar from the University of Wisconsin Madison said, "This is a comprehensive analysis and a great example." This is not the first time scientists have attempted to outline the last common ancestor (LUCA) of all species before differentiation. For example, in 2016, a study led by evolutionary biologist William Martin from the University of Dusseldorf in Germany compared known microbial genomes, providing the most convincing genetic evidence to date that LUCA may be an anaerobic bacterium. Martin's genetic analysis also found evidence that it is a thermophilic microorganism that feeds on hydrogen gas and may live near underwater volcanic vents. The Martin team did not provide a specific timeline for LUCA in their 2016 study, but other studies have placed LUCA's existence at 3.8 billion years ago. In a new study, Edmund Moody, a genomics expert at the University of Bristol in the UK, has developed a method that can more accurately predict the existence time of LUCA. By utilizing the known mutation rates of different genes in microbial species and the speed of gene transfer between species, a molecular clock can be created. By constructing a family tree, identifying which organisms may have evolved from other organisms, and tracking genetic changes in conserved genes, researchers can roughly estimate the differentiation time of two adjacent branches, thereby determining the "age" of their common ancestor. Moody and colleagues further studied five sets of "parallel" or repetitive genes found in various bacteria and archaea, which suggests that gene duplication occurred before LUCA split these offspring. Moody said that tracking whether a mutation occurs on two copies of these genes or only on one copy can make it easier to determine when they replicate and thus determine the "age" of LUCA. Their analysis indicates that LUCA lived approximately 4.2 billion years ago. Rika Anderson, an evolutionary microbiologist from Carleton College in the United States who was not involved in this study, said, "This may be a bit earlier than other estimates, but the difference is not significant." In order to explore the lifestyle of LUCA like Martin, the Moody research team tracked 57 "marker" genes from 350 bacteria and 350 archaea to construct a tree of life. The Moody team tracked the evolutionary patterns of individual genes and gene families of these bacteria and archaea separately, and these genes were cataloged in a commonly used genomic database. By comparing the evolutionary history of individual genes with species genes, they can better determine which genes have been replicated, lost, or undergone horizontal gene transfer. From this, they can infer the substances present in LUCA. Analysis by British scientists shows that LUCA uses carbon dioxide and hydrogen as its' fuel '. But they also found evidence that LUCA has a gene that can protect it from UV damage, meaning this microorganism may live in surface water - where it can capture carbon dioxide and hydrogen from the atmosphere, rather than living in deep-sea volcanic vents. But like Martin, they discovered the characteristics of an enzyme called reverse transcriptase, which is typically present in thermophilic bacteria, and they acknowledged that this means LUCA may also reproduce around volcanic vents. Moody also discovered that LUCA may have 19 CRISPR-Cas9 genes, which are devices used by modern bacteria to cleave the genetic material of invading viruses. This excited Kacar as it hinted at a thriving ecosystem composed of microorganisms and pathogens that existed a long time ago. Anderson pointed out that the CRISPR-Cas9 system is "a bit complex". This means that in just a few hundred million years, early life on Earth evolved complex microorganisms that interacted with each other and quickly formed a simple ecosystem. (New Society)