That opened the way for a group of evolutionary biologists, led by William F. Martin of Heinrich Heine University in Düsseldorf, Germany, to try to discern the nature of the organism from which the bacterial and archaeal domains emerged. By comparing the genomes of simple bacteria with eukaryotes, LUCA has the most universal genes in all life.

“We didn’t even have a complete ribosome,” admits Martin. Image credit: Weiss et al/Nature Microbiology. LUCA is the common ancestor that unites us all. A growing bacteria or archaea can take in genes from the environment around them by ‘recombining’ new genes into their DNA strand.

Presumably, life may have existed even before that. Bill Martin and six of his Düsseldorf colleagues (Madeline Weiss, Filipa Sousa, Natalia Mrnjavac, Sinje Neukirchen, Mayo Roettger and Shijulal Nelson-Sathi) published a 2016 paper in the journal Nature Microbiology describing this new perspective on LUCA and the two-domain tree with phylogenetics. Serpentinization within hydrothermal vents can produce copious amounts of molecular hydrogen. A refuge in the deep ocean during the bombardment would allow a longer period in which life could have evolved. This was a rain of meteorites that crashed into Earth with such force that the oceans were boiled off into an incandescent mist. Yet, a major question remains: What were the first eukaryotes like and where do they fit into the tree of life? Knowing this, Martin’s team searched for ‘ancient’ genes that have exceptionally long lineages but do not seem to have been shared around by LGT, on the assumption that these ancient genes should therefore come from LUCA. From Dr. Martin’s data, it is clear that Luca could manage the complicated task of synthesizing proteins. A paper that appeared recently in Nature, written by a team led by Thijs Ettema at Uppsala University in Sweden, has shed more light on the evolution of eukaryotes. The Düsseldorf team’s analysis indicates that LUCA used molecular hydrogen as an energy source.

There are six known carbon-fixing pathways and work conducted over many decades by microbiologist Georg Fuchs at the University of Freiburg has shown that the Wood–Ljungdahl pathway is the most ancient of all the pathways and, therefore, the one most likely to have been used by LUCA. But it has a grand name, or at least an acronym. “The two-domain tree of life, where the basal split is between the archaea and the bacteria, is now the best supported hypothesis,” he says. “While we were going through the data, we had goosebumps because it was all pointing in one very specific direction,” says Martin. Hydrothermal vents that were home to LUCA turn out to be remarkably common within our solar system. Anaerobic and autotrophic, it didn’t breath air and made its own food from the dark, metal-rich environment around it. “I think that if we find life elsewhere it’s going to look, at least chemically, very much like modern life,” says Martin. However, a new picture has emerged that places eukarya as an offshoot of bacteria and archaea.

During the 500 million years that separates LUCA and the origin of life, DNA had to evolve into a somewhat functional system. 30 Environmental Science Careers: How To Be an Earth Scientist, Plate Tectonic Types: Divergent, Convergent and Transform Plates, Mass Extinctions: The 5 Biggest Dying Events in History, Ice Ages: Glaciations in Geologic History, Human Evolution: A Timeline of Early Hominids [Infographic], Theory of Evolution: Charles Darwin and Natural Selection. The biochemistry results in part from the geology and the materials that are available within it to build life, says Martin Embley.

To make the cut, the ancient gene could not have been moved around by LGT and it had to be present in at least two groups of archaea and two groups of bacteria. So it seems unlikely that it could not also synthesize simpler components, even though the genes for doing so have not yet been detected, said Steven A. Benner of the Foundation for Applied Molecular Evolution. Phylogenetics help narrow this down, but Martin Embley isn’t sure our analytical tools are yet capable of such a feat. Dr. Sutherland, working from basic principles of chemistry, has found that ultraviolet light from the sun is an essential energy source to get the right reactions underway, and therefore that land-based pools, not the ocean, are the most likely environment in which life began. “It’s marrying up a geological context with a biological scenario, and it has only been recently that phylogenetics has been able to support this.”. “What I think has been missing from the equation is a biological point of view,” he says. The latter two— the prokaryotes— share similarities in being unicellular and lack a nucleus, and are differentiated from one another by subtle chemical and metabolic differences. After all, says Martin, biochemistry at this early stage in life’s evolution was still primitive and all the theories about the origin of life and the first cells incorporate chemical synthesis from their environment.

Universal History Archive/UIG, via Getty Images. Embley believes this is why the three-domain tree hypothesis lasted so long – we just didn’t have the tools required to disprove it. Jupiter’s moon Europa has a subterranean ocean, a rocky seabed, and geothermal heat produced by Jupiter’s gravitational tides. Indeed, this is corroborated by the findings of Bill Martin’s team. Eukarya, on the other hand, are the complex, multicellular life forms comprised of membrane-encased cells, each incorporating a nucleus containing the genetic code as well as the mitochondria ‘organelles’ powering the cell’s metabolism. It is known as Luca, the Last Universal Common Ancestor, and is estimated to have lived some four billion years ago, when Earth was a mere 560 million years old. William Martin, a professor of evolutionary biology at the Heinrich Heine University in Dusseldorf, is hunting for LUCA. “The problem with phylogenetics is that the tools commonly used to do phylogenetic analysis are not really sophisticated enough to deal with the complexities of molecular evolution over such vast spans of evolutionary time,” he says.

But the origin of life existed long before LUCA. Water, rock and heat were all that were required by LUCA, so could similar life also exist on Europa? Previous studies of LUCA looked for common, universal genes that are found in all genomes, based on the assumption that if all life has these genes, then these genes must have come from LUCA.

The fact that Luca depended on hydrogen and metals favors a deep sea vent environment for the origin of life, Dr. Martin concludes, rather than the land environment posited in a leading rival theory proposed by the chemist John Sutherland of the University of Cambridge in England. Genes found in both archaea and bacteria could have been shared through LGT and hence would not necessarily have originated in LUCA.

Deep sea vents are surrounded by exotic life-forms and, with their extreme chemistry, have long seemed places where life might have originated.

The field of hydrothermal vents known as Loki’s Castle, in the North Atlantic Ocean, where scientists found archaea believed to be related to the archaea that created eukaryotes through endosymbiosis with bacteria. Any reference in this website to any person, or organization, or activities, products, or services related to such person or organization, or any linkages from this web site to the web site of another party, do not constitute or imply the endorsement, recommendation, or favoring of the U.S. Government, NASA, or any of its employees or contractors acting on its behalf. But he has taken a further step that has provoked considerable controversy. This method has identified 11,000 common genes that could potentially have belonged to LUCA, but it seems far-fetched that they all did: with so many genes LUCA would have been able to do more than any modern cell can. All that’s needed is rock, water and geochemical heat. Consequently, eukaryotes are not one of the main branches of the tree-of-life, but merely a large offshoot. Life could have originated anywhere and later been confined to a deep sea environment because of some catastrophic event like the Late Heavy Bombardment, which occurred 4 billion to 3.8 billion years ago.

Life is so complex it seems to need many millions of years to evolve. LGT involves the transfer of genes between species and even across domains via a variety of processes such as the spreading of viruses or homologous recombination that can take place when a cell is placed under some kind of stress.

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