Evidence of Early Life on Earth
There are two scientific disciplines that could provide evidence of early life on earth: genomics and geology. Both preserve information that survives for billions of years, telling us clues of possible life on the early earth. Unlike prebiotic chemistry, which relies mostly on testing hypothesis in the lab, genomics and geology have developed a historical approach of life origins with strong concrete evidence. Dubious claims and incorrect interpretations happened in the past, but science could always repair itself.
Genomics
The universal feature of genetic code among all living things imply that we are all related. Genomic analysis between two different species could show how closely related the two of them are. And with all genomic data that we have now, we could make a tree of life, where all living things genomes are compared and placed in each branch. Based on 16s RNA, Carl Woese developed a tree of life that consists of three large branches as the domains of life: Archaea, Eukaryotes, and Bacterias. From this tree too, Woose tried to infer the characteristics of the Last Universal Common Ancestor (LUCA) of the tree domain(Koonin 2014).
According to Eugene Koonin, a computational biologist from NCBI, Woose proposed that LUCA might not be a modern cell as we know it. It was likely a community of protocells with high “genetic temperature”, which means that it experienced frequent mutations and horizontal genetic transfers. This condition required the universal genetic code to crystalize before the LUCA. “Horizontal genetic transfer selectively maintains the universality of genetic code…because the code is an evolutionary lingua franca required for an essential “genetic commerce” among lineages,” says Woese.
Besides the genetic code, the cellular translation system and energy metabolism also likely locked in before the LUCA. The evidence is the diverse genes encoding the ATP synthetase, a protein that is involved in energy production in cell, and paralogs of aminoacyl ARS, a protein that attaches to specific amino acid in translation process. These are the oldest information of early life that we could get from comparative genomics(Fournier et al. 2011).
Geology
Analyzing the age and composition of sedimentary rocks could also give us direct evidence of early life, but cautious interpretation and analysis need to be made. In 1987, Schopf and Parker claimed that they found evidence of photosynthetic life as early as 3.5 billion years ago. By comparing the size of modern cyanobacteria and something that look like a chain of circles from the Apex Basalt in Warawoona, Australia, he made a conclusion of discovering the world’s oldest fossil of life(Schopf and Packer 1987). In 2006, Brasier challenged Schopf’s statement by arguing that Schopfs cannot falsify the abiogenic explanation for his specimen. The microfossils was likely from “hydrothermal dykes vein quartz”. Brasier had also showed that Schopf’s specimen was irregular, branched, and disoriented. Something that is likely abiogenic.(Brasier et al. 2002)
Biogeneicity is an important feature to classify something as an evidence of living organism. Size must fall around the same measurements and it should demonstrates its flexible folding ability (some might say “gooey). The oldest evidence of life was from Tice 2011 that shows a worm-like fossil from 3.42 billion years ago (possibly a bacteria, but not cyanobacteria). The oldest cyanobacteria was found by Hoffman (1976) which are very clear, — -uniform size, preserved silica, and tougher cell walls. This fossil was 2 billion years old. The other evidence of life is the stromatolites, a structure that represents the existence of cyanobacteria in the past.
But geology doesn’t only give us evidence of living organism, it also gives us evidence of what life could do: metabolism! In the case of methanogenic organisms, geological research by Ueno (2006) shows direct evidence of methanogenesis from more than 3.5 billion years ago. By analyzing the origin of methane composition in quartz from Western Australia, the team confirmed the existence of methanogens in that era(Ueno et al. 2006). Carbon isotop fractionation had alsoconfirmed that the methane was indeed biogenic and it could contribute to solve the Faint Young Sun paradox in early Archaean era.
The existence of methanogens could also give insight on earth’s habitability. Relying mostly on nickels as its metal cofactor, methanogenesis could survive for long as the nickel supply was reduced due to earth’s evolutio)(Konhauser et al. 2009). This gives rise to the Great Oxidation process (2.5 billion years ago), a condition beneficial for oxygenic organisms. In this case, another form of metabolism flourished: photosynthesis. This kind of metabolism is important for the development of earth’s biosphere, because this when life started to harness what’s available around: the energy from the sun and water!
Brasier, Martin D., Owen R. Green, Andrew P. Jephcoat, Annette K. Kleppe, Martin J. Van Kranendonk, John F. Lindsay, Andrew Steele, and Nathalie V. Grassineau. 2002.
“Questioning the Evidence for Earth’s Oldest Fossils.” Nature 416 (6876): 76–81. https://doi.org/10.1038/416076a.
Fournier, Gregory P., Cheryl P. Andam, Eric J. Alm, and J. Peter Gogarten. 2011. “Molecular Evolution of Aminoacyl TRNA Synthetase Proteins in the Early History of Life.” Origins of Life and Evolution of Biospheres 41 (6): 621–32. https://doi.org/10.1007/s11084-011-9261-2.
Konhauser, Kurt O., Ernesto Pecoits, Stefan V. Lalonde, Dominic Papineau, Euan G. Nisbet, Mark E. Barley, Nicholas T. Arndt, Kevin Zahnle, and Balz S. Kamber. 2009. “Oceanic Nickel Depletion and a Methanogen Famine before the Great Oxidation Event.” Nature 458 (7239): 750–53. https://doi.org/10.1038/nature07858.
Koonin, Eugene V. 2014. “Carl Woese’s Vision of Cellular Evolution and the Domains of Life.” RNA Biology 11 (3): 197–204. https://doi.org/10.4161/rna.27673.
Schopf, J. W., and B. M. Packer. 1987. “Early Archean (3.3-Billion to 3.5-Billion-Year-Old) Microfossils from Warrawoona Group, Australia.” Science (New York, N.Y.) 237 (July): 70–73. https://doi.org/10.1126/science.11539686.
Ueno, Yuichiro, Keita Yamada, Naohiro Yoshida, Shigenori Maruyama, and Yukio Isozaki. 2006. “Evidence from Fluid Inclusions for Microbial Methanogenesis in the Early Archaean Era.” Nature 440 (7083): 516–19. https://doi.org/10.1038/nature04584.