Home / Science / “Happy or inevitable” – Life may have relied on nothing but oxygen for the first 1.5 billion years

“Happy or inevitable” – Life may have relied on nothing but oxygen for the first 1.5 billion years

The origin of life

“Is the existence of life on earth a matter of luck or an inevitable consequence of the laws of nature? Is it easy for life to appear on a newly formed planet, or is it the virtually impossible product of a long series of unlikely events? asks Nobel Laureate and Professor of Genetics at Harvard Medical School, Jack Szostak. “Progress in areas as diverse as astronomy, planetary science and chemistry now promises that answers to such in-depth questions may be around the corner. If life turns out to have occurred several times in our galaxy, as scientists hope to discover, the road to it may not be so difficult. In addition, if the path from chemistry to biology proves to be easy to cross, the universe can be full of life. ”

Albert Einstein stated that “one can best feel in dealing with living things how primitive physics still is.” Perhaps the same can be said about the origin of life theories. Scientists now ask that early life on earth relied on arsenic during the first 1.5 billion years before oxygen was present?

“We really do not know how these systems worked”

During half the time that life has existed on earth, we really do not know how these systems worked, says University of Conn professor of marine and geoscience Pieter Visscher, in a guess as to a possible precursor to a key component of the oxygen cycle are plants and certain types of Bacteria mainly take sunlight, water and CO2 and convert them into carbohydrates and oxygen, which are then cycled and used by other organisms that breathe oxygen. This oxygen acts as a means of electrons, receiving and donating electrons as it is driven through the metabolic processes.

“Worked absolutely in lack of oxygen”

“Arsenic-based life has been an issue in terms of, does it have a biological role or is it just a toxic compound?” says Visscher. “I have been working with microbial carpets for about 35 years or so. This is the only system on earth where I could find a microbial mat that worked completely in the absence of oxygen. ”

Light-driven photosynthetic organisms appear in the fossil record as stratified carbonate rocks called stromatolites from about 3.7 billion years ago, says Visscher. Stromatolite mats are deposited over the eons of microbial ecosystems, where each layer contains clues about life at that time. There are contemporary examples of microbes that photosynthesize in the absence of oxygen using a variety of elements to complete the process, but it is unclear how this happened in the earliest life forms.

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Theories about how life’s processes worked in the absence of oxygen have mostly relied on hydrogen, sulfur or iron as the elements that traveled electrons around to meet the organisms’ metabolic needs.

Other disputed theories

As Visscher explains, these theories are questioned; for example, photosynthesis is possible with iron, but scientists do not find evidence of this in the fossil record before oxygen appeared about 2.4 billion years ago. Hydrogen is mentioned, but the energy and competition for hydrogen between different microbes shows that it is very impossible.

Arsenic is another theoretical possibility, and evidence for this was found in 2008. Visscher says that the link to arsenic was strengthened in 2014 when he and colleagues found evidence for arsenic-based photosynthesis in depth. To further support their theory, the researchers needed to find a modern analogue to study biogeochemistry and element cycling.

Challenges of working in the fossil record

Finding an analogue to the conditions of early soil is a challenge for several reasons, apart from the fact that oxygen is now abundant. For example, evidence shows that early microbes captured atmospheric carbon and produced organic matter at a time when volcanic eruptions were frequent, UV light was intense in the absence of the ozone layer, and oceans were basically a toxic soup.

Another challenging aspect of working in the fossil record, especially those as old as some stromatolites, is that there are few left due to rock cycling as continents move. However, a breakthrough occurred when the team discovered an active microbial mat, which currently exists under the harsh conditions of Laguna La Brava in the Atacama Desert in Chile.

Otherworldly Microbial Mats

The rugs have not been studied before but present a different worldly set of conditions, such as those from early Earth. The carpets are in a unique environment that leaves them in a permanent oxygen-free state at high altitude where they are exposed to wild, daily temperature fluctuations and high UV conditions. The rugs serve as powerful and informative tools for truly understanding life in early Earth conditions.

A blood-red river

“We started working in Chile, where I found a blood-red river,” said Visscher. The red sediments consist of anoxogenic photosynthetic bacteria. The water also contains a lot of arsenic. The water that flows over the carpets contains hydrogen sulphide which has a volcanic origin and it flows very quickly over these carpets. There is absolutely no oxygen. ”

Microbes that metabolize active arsenic

The team also showed that the carpets made carbonate deposits and created a new generation of stromatolites. The carbonate materials also showed evidence of arsenic cycling – that arsenic acts as a means of electrons – proving that microbes actively metabolize arsenic, much like oxygen in modern systems. Visscher says that these results, together with the fossil evidence, give a strong sense of the Earth’s early conditions.

“When looking for evidence of life on Mars, they will look at iron and probably they should also look at arsenic,” says Visscher, noting that an important tool they used to conduct this research is similar to one aboard the Mars Perseverance Rover, for currently on its way to Mars.

Source: Pieter T. Visscher et al. Modern arsenotrophic microbial mats provide an analogue for life in the anoxic Archean, Communications Earth & Environment (2020). DOI: 10.1038 / s43247-020-00025-2

The Daily Galaxy, Jake Burba, via University of Connecticut

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