By tracking the evolution of proteins, scientists at the Weizmann Institute of Science have outlined the history of oxygen availability on Earth and solved a long-standing scientific riddle.
Photosynthetic bacteria (cyanobacteria). The source of oxygen emitted into the atmosphere during the oxygen revolution
About two and a half billion years ago, our planet underwent one of the greatest upheavals in its history: from zero oxygen concentration in the atmosphere to the availability of the molecule everywhere. The “oxygen revolution” – also known as the “oxygen disaster” – wiped out most of the non-aerobic (anaerobic) species, but at the same time allowed the evolution of breathable creatures, including humans. Although this is a groundbreaking event in Earth’s history, there is debate among scientists as to whether it was a revolution, that is, a rapid, extreme and sudden change, or whether in fact living things on Earth already knew how to produce and use oxygen, to one degree or another, many years before. Scientists at the Weizmann Institute of Science recently solved the long-standing riddle and showed that the ability of living things to process oxygen is more than half a billion years ahead of the oxygen revolution.
Prof. Dan Tawfiq, from the Department of Biomolecular Sciences of the Weizmann Institute of Science, explains that the date of the oxygen revolution, which is backed up by geological findings, is undisputed. There is also no arguing that the oxygen emitted into the atmosphere during the oxygen revolution originated in the development of bacteria that used photosynthesis to produce energy and emitted oxygen as a by-product of the process. The focus of the scientific debate is, therefore, whether life on Earth was exposed to oxygen and knew how to use it even before the oxygen revolution. In this debate there are those who argue that molecular oxygen (O2), I.e. free oxygen molecules, could not be available before the oxygen disaster, since the chemical composition of the atmosphere and the oceans before the oxygen revolution was such that any oxygen molecule released in photosynthesis immediately produced a chemical reaction, i.e. did not remain free. Others argue that some oxygen may still remain free long enough for living things to learn to use – even before the oxygen revolution.
Although Prof. Tawfiq’s research group specializes in the evolution of proteins, research student Yagoda Jablonska suggested that this specialization could help solve the scientific riddle of the history of oxygen use. In other words, she hypothesized that using methods to track how and when different enzymes evolved, it was also possible to find out when living things began to process oxygen.
Jablonska first created a list of about 130 families of enzymes that use or produce oxygen in bacteria and archaea (ancient bacteria), as these are the life forms that characterized the archaic – the geological era between the appearance of life on Earth about 4 billion years ago and the oxygen revolution. After blowing up about half of them, Jablonska was left with families of enzymes in which the use or emission of oxygen characterizes most or all of the family members. From the remaining families she chose 36 whose evolutionary pedigree can be fully delineated. “Of course it was far from simple,” says Prof. Tawfiq. “Genes can ‘get lost’ in some organisms, creating the false impression that they later evolved in other organisms. In addition, bacteria pass genes horizontally between them, thus cluttering the family trees and leading to overestimation of the enzyme’s age.”
The Oxygen Revolution left its geochemical record in the iron deposits in the Earth’s mantle and rocks
The pedigrees created by the researchers clearly showed an evolutionary eruption of the appearance of oxygen enzymes about three billion years ago – more than half a billion years before the oxygen revolution; Some of the enzymes even evolved earlier. “The opinion is that in order for enzymes that use oxygen to appear, the presence of oxygen is necessary,” says Prof. Tawfiq. “Thus our findings confirm the hypothesis that oxygen appeared and persisted in the biosphere long before the oxygen revolution. Although it took time to reach the higher oxidation level that characterized the oxygen revolution and left its geochemical record, by the time it happened, oxygen was already known to many Earth creatures.”
Oxygen is one of the most chemically reactive elements. This fact makes it very useful for many life forms, but also has great potential for harm. Jablonska and Prof. Tawfiq speculate that bacteria that produced oxygen, and other microorganisms that lived in their environment, were forced to quickly develop ways to get rid of oxygen efficiently. Thus appeared enzymes designed to remove oxygen molecules from the cells. However, the waste of one creature is the potential life source of another creature, and enzymes that use oxygen to break down “durable” molecules such as aromatic compounds and lipids soon began to develop.
Jablonska concludes: “Our study presents an entirely new means of dating the appearance of oxygen in the atmosphere, and helps us to understand how life evolved as we know it.”
