“Fires couldn’t be started on exoplanets that don’t have much oxygen”
the OXYGEN BOTTLENECK for TECHNOSPHERES
Aliens on low-oxygen worlds may never discover fire
by Alex Wilkins / 15 August 2023
“Alien life on a planet with low levels of oxygen might never be able to develop technology because combustion would be impossible. This bottleneck for creating advanced civilisations may also help explain why we have yet to observe life elsewhere in a near-infinite universe. The development of technology on Earth hinged on fire, also known as combustion, a chemical reaction that uses oxygen to generate large amounts of heat. This is crucial for, among other things, extracting metals from ore, which almost every advanced human technology relies on. Laboratory experiments have found that combustion can’t fully occur in atmospheres with oxygen levels below about 18 per cent. Now, Adam Frank at the University of Rochester in New York and Amedeo Balbi at the University of Rome in Italy have suggested that this might be a universal bottleneck for whether alien species can develop technology. “You may have enough oxygen in the atmosphere of an exoplanet to have complex multicellular life, but you may not have enough oxygen to start combustion,” says Balbi.
We don’t know the exact balance of geological and biological processes that lead to an atmosphere with this level of oxygen, because we currently only have Earth as an example. “Oxygen in the atmosphere is also produced by photosynthetic activity, so that’s why it gets complicated, because it’s not just geology, it’s also biology,” says Balbi. There is also an upper limit to what is likely to be a useful amount of oxygen. At about 30 per cent and above, depending on the moisture content in the air, the chance of widespread fires killing lifeforms would become very high as combustion becomes so easy, says Frank. Astronomers are looking for any type of life in the universe, but the search for intelligent extraterrestrial life might be guided by looking for planets with a level of oxygen in the atmosphere of between 18.5 and 21 per cent, the optimum for useful combustion. For now, though, detecting atmospheric oxygen is beyond the current abilities of our telescopes for almost all known exoplanets. “We’re just getting started looking for the atmospheres of terrestrial planets,” says Frank. “This is the exciting thing – we’re at the frontier now.”
“Future telescopes such as NASA’s James Webb Space Telescope will observe atmospheres of exoplanets for signs of life. Oxygen/ozone reveal life on the modern Earth. For the early Earth (bottom left), the combination of methane and carbon dioxide would provide an alternative biosignature, a new study suggests (J. Krissansen-Totton)”
Combustion as a bottleneck for technology makes sense if life emerges the way it did on Earth, but might not apply if life evolved differently on another planet, says Ingo Waldmann at Imperial College London. “That was the evolutionary path we took, but I’m not necessarily sure whether that can be generalised across the universe.” For instance, it is possible that an alien species in a low-oxygen environment may be able to harness heat from geothermal activity, says Waldmann, such as volcanoes, to do their smelting. Another possibility could be concentrating sunlight using mirrors, says Ian Crawford at the University of Birkbeck, UK, although manufacturing mirrors in the first place may be tricky, he adds. One downside for these alternatives is that they aren’t portable like wood and fossil fuels are, says Balbi. This might mean any civilisations would need to stay close to their heat source. If the oxygen bottleneck does prove to be a limiting factor, then Frank and Balbi suggest it might help solve the Fermi paradox, which questions why there are no signs of advanced intelligent life if it is apparently likely, given the size of the universe. “The Fermi paradox is telling us something about the nature of life in the universe that we don’t understand,” says Crawford. “In that context, all potential solutions to the Fermi Paradox are of interest.”
Reference: arXiv DOI: 10.48550/arXiv.2308.01160
“Banded iron deposits like these contain clues to the Great Oxygenation Event”
GREAT OXYGENATION EVENT (CATASTROPHE, CRISIS, HOLOCAUST)
How ‘great’ was the great oxygenation event?
by Weizmann Institute of Science / March 1, 2021
“Around 2.5 billion years ago, our planet experienced what was possibly the greatest change in its history: According to the geological record, molecular oxygen suddenly went from nonexistent to becoming freely available everywhere. Evidence for the Great Oxygenation Event (GOE) is clearly visible, for example, in banded iron formations containing oxidized iron. The GOE, of course, is what allowed oxygen-using organisms—respirators—and ultimately ourselves, to evolve. But was it indeed a ‘great event’ in the sense that the change was radical and sudden, or were the organisms alive at the time already using free oxygen, just at lower levels?
Prof. Dan Tawfik of the Weizmann Institute of Science’s Biomolecular Sciences Department explains that the dating of the GOE is indisputable, as is the fact that the molecular oxygen was produced by photosynthetic microorganisms. Chemically speaking, energy taken from light split water into protons (hydrogen ions) and oxygen. The electrons produced in this process were used to form energy-storing compounds (sugars), and the oxygen, a by-product, was initially released into the surroundings. The question that has not been resolved, however, is: Did the production of oxygen coincide with the GOE, or did living organisms have access to oxygen even before that event? One side of this debate states that molecular oxygen would not have been available before the GOE, as the chemistry of the atmosphere and oceans prior to that time would have ensured that any oxygen released by photosynthesis would have immediately reacted chemically. A second side of the debate, however, suggests that some of the oxygen produced by the photosynthetic microorganisms may have remained free long enough for non-photosynthetic organisms to snap it up for their own use, even before the GOE. Several conjectures in between these two have proposed ‘oases,’ or short-lived ‘waves,’ of atmospheric oxygenation.
Research student Jagoda Jabłońska in Tawfik’s group thought that the group’s focus—protein evolution—could help resolve the issue. That is, using methods of tracing how and when various proteins have evolved, she and Tawfik might find out when living organisms began to process oxygen. Such phylogenetic trees are widely used to unravel the history of species, or human families, but also of protein families, and Jabłońska decided to use a similar approach to unearth the evolution of oxygen-based enzymes. To begin the study, Jabłońska sorted through around 130 known families of enzymes that either make or use oxygen in bacteria and archaea—the sorts of life forms that would have been around in the Archean Eon (the period between the emergence of life, ca. 4 billion years ago, and the GOE). From these she selected around half, in which oxygen-using or -emitting activity was found in most or all of the family members and seemed to be the founding function. That is, the very first family member would have emerged as an oxygen enzyme.
From these, she selected 36 whose evolutionary history could be traced conclusively. “Of course, it was far from simple,” says Tawfik. “Genes can be lost in some organisms, giving the impression they evolved later in members in which they held on. And microorganisms share genes horizontally, messing up the phylogenetic trees and leading to an overestimation of the enzyme’s age. We had to correct for the latter, especially.” The phylogenetic trees the researchers ultimately obtained showed a burst of oxygen-based enzyme evolution about 3 billion years ago—something like half a billion years before the GOE. Examining this time frame further, the scientists found that rather than coinciding with the takeover of atmospheric oxygen, this burst dated to the time that bacteria left the oceans and began to colonize the land. A few oxygen-using enzymes could be traced back even farther. If oxygen use had coincided with the GOE, the enzymes that use it would have evolved later, so the findings supported the scenario in which oxygen was already known to many life forms by the time the GOE took place.
“The first oxygen-dependent life on land may have been bacteria that “eat” pyrite, also known as fool’s gold. Similar bacteria still exist at mining waste sites, where pyrite has been discarded creating highly acidic conditions. Above, a modern-day acidic drainage at an abandoned copper mine.(credit: Kurt Konhauser)”
The scenario that Jabłońska and Tawfik propose looks something like this: Oxygen is one of the most chemically reactive elements around. Like one end of a battery, it readily accepts electrons, thus providing extra metabolic power. That makes it extremely useful to many life forms, but also potentially damaging. So photosynthetic organisms as well as other organisms living in their vicinity had to quickly develop ways to efficiently dispose of oxygen. This would account for the emergence of oxygen-utilizing enzymes that would remove molecular oxygen from cells. One microorganism’s waste, however, is another’s potential source of life. Oxygen’s unique reactivity enabled organisms to break down and use “resilient” molecules such as aromatics and lipids, so enzymes that take up and use oxygen likely began evolving soon after. Tawfik says, “This confirms the hypothesis that oxygen appeared and persisted in the biosphere well before the GOE. It took time to achieve the higher GOE level, but by then oxygen was widely known in the biosphere.” Jabłońska concludes, “Our research presents a completely new means of dating oxygen emergence, and one that helps us understand how life as we know it now evolved.”
FIRE FEEDBACK HYPOTHESIS
Extreme fires caused by ancient humans wiped out Californian megafauna
by Michael Le Page / 17 August 2023
“A series of catastrophic fires was the immediate cause of the extinction of many large mammals in southern California 13,000 years ago, according to a study of fossils from the La Brea tar pits. The findings suggest these extreme fires were probably a result of humans abruptly changing the ecosystem by killing off herbivores – meaning there was more vegetation to burn – and deliberately starting fires. “It’s a synergy of the drying climate and the humans, and the fact that they are killing herbivores and increasing fuel loads, and all of those things go together to make a feedback loop that takes the ecosystem to a chaotic state,” says Robin O’Keefe at Marshall University in West Virginia. “The fire event is really catastrophic.”
The tar pits at La Brea in Los Angeles have trapped numerous animals over the past 50,000 years and preserved their bones, providing an extraordinary window into the past. Many of the bones have never been precisely dated because radiocarbon dating was more expensive in the past and required destroying large chunks of bone, and also because results were skewed by the tar inside the bones. Now, costs have fallen, only tiny quantities of bone are needed and the tar contamination problem can be solved by extracting preserved collagen and dating only this material. As a result, O’Keefe and his colleagues were able to precisely date 172 bones from eight species.
Seven of these species are extinct, including the sabre-toothed cat (Smilodon fatalis), the dire wolf (Aenocyon dirus), the western camel (Camelops hesternus) and the ancient bison (Bison antiquus), which was even larger than surviving bison. The team also dated coyote (Canis latrans) bones as a control. The dating shows that the seven species were all gone from the La Brea area by 13,000 years ago, though some survived elsewhere in North America for another millennium or so. Their disappearance from La Brea coincides with massive spikes in the number of charcoal particles in lake sediments, which are deposited during wildfires. “Some of those spikes for those fires are just enormous, orders of magnitude more than has ever happened before,” says O’Keefe.
Pollen in lake sediments shows that the vegetation had begun changing from woodland to a more open landscape around 16,000 years ago, as the area became drier due to the retreat of the ice sheets. But there was a sudden shift to fire-resistant vegetation around 13,000 years ago. “The results of this study are consistent with humans increasing fire both directly though ignitions and indirectly through hunting of herbivores,” says Allison Karp at Yale University, who wasn’t involved in the study. If the tiny number of people alive at the time could do this, the much greater number of people alive now can have a much bigger impact, says O’Keefe. “It’s super relevant to today,” he says. More extreme wildfires are happening in many parts of the world as it warms, and O’Keefe says his findings show there is a risk this could lead to ecosystems flipping into another state, resulting in many species going extinct. “Hopefully, by learning these things about what happened at La Brea, maybe we can change our trajectory,” he says.
Earlier research had suggested that the development of the Clovis stone tool technology, whose distinctive feature is finely crafted large spear points for tackling big animals, enabled people in North America to wipe out the continent’s megafauna. However, these findings show that some large mammals were going extinct in places before Clovis tools appeared. O’Keefe and his colleagues think Clovis tools were instead a response to the loss of some megafauna. “The things that seem to get hunted out first are the things that are easier to catch, like camels and horses and bison,” says O’Keefe. “It’s only when you start running out of those that we think that the Clovis technology evolves, because you have to do this really dangerous thing and try to take on a mastodon because all the easier to kill animals are gone. Clovis wasn’t a driver of extinction. It evolves because the extinction was already under way,” he says.”
the CASE for LETTING MALIBU BURN
SURVIVING the PYROCENE