A potential problem for any intelligent aliens that could be out there that may prevent them from developing technology has been highlighted by a recent paper: the oxygen bottleneck.

Free atmospheric oxygen is, of course, one of the key signs of potential life to look out for. The only planet where we know there is intelligent life (well, kinda) has free atmospheric oxygen in abundance. Naturally, we have looked for it on other planets, given the important role it has played on Earth in the evolution of animal life.

As the new paper highlights, the benefits of breathing isn't the only thing oxygen has done for humans.

"On Earth, fire certainly played a crucial role in the establishment of human civilization and the onset of technology," the paper explains. Cooking food was of course important in our development, giving us more energy and possibly helping our brain size to increase. Fire has also been utilized by humans to control our environment, and help us in hunting.

"However, by far the most important role for fire in the rise of human civilization is its use as an energy source," the paper continues. "This probably started very early on as a way to keep warm, thereby increasing the range of environmental conditions suitable for settlements; then it evolved into more sophisticated uses, such as metal smelting, melding and tool fabrication; finally, it provided both the energy source and the fuel (for example, charcoal) that initiated the Industrial Revolution and led to the ‘Great Acceleration’ and the Anthropocene."

Without fire, there would be no Industrial Revolution. And here's where it might be bad news for anyone who wants to meet other alien life: For there to be fire, you don't need just a bit of it, but a lot of it. In terms of fire, there is a sweet spot. Not enough oxygen, and plant matter will not combust. Any higher than 35 percent it will combust so well that forests would not be able to grow and sustain themselves. According to this paper, below 18.5 percent oxygen and the chance of combustion is limited, but just above 20 percent (on Earth it is around 21 percent at the moment) seems to be the sweet spot where fires can burn and vegetation can grow.

Any lower than 18.5 percent, and species could become intelligent, but find themselves unable to make complicated technology (or even smelt metals, as we have done for thousands of years. 

"Imagine a young and intelligent species on an alien world with an atmosphere that’s just 1 percent oxygen," co-author on the paper, Adam Frank, wrote for Big Think. "Those clever tool-using creatures would never get the chance to watch a tree burn after being hit by lightning and get the idea of using fire for their own purposes. They would never have the chance to learn how fire could be used to cook food, clear land, or, most importantly, melt metals. The poverty of oxygen in their air would likely box these creatures in forever, limiting their development."

“The presence of high degrees of oxygen in the atmosphere is like a bottleneck you have to get through in order to have a technological species,” Frank added in a statement. “You can have everything else work out, but if you don’t have oxygen in the atmosphere, you’re not going to have a technological species.”

The oxygen bottleneck, as the team termed it, could prevent intelligent, tool-using, and possibly communicative species from becoming technologically advanced ones capable of making contact with us, perhaps explaining the lack of contact. The team suggests other possible ways primitive species could produce heat, such as focusing the rays of their stars or geothermic energy, but none are as easy or freely available as combustion.

On the bright side, the team hopes that the oxygen bottleneck may assist the search for extraterrestrial intelligence. As well as letting us know that in the search for technological civilizations it may be better to look for oxygen-rich planets, should we receive potential technosignatures, oxygen levels on planets in the region could help us know whether technology is likely present on them. If it's too low, we can guess that it is probably unlikely a species would have developed the technology needed for contact.

“The implications of discovering intelligent, technological life on another planet would be huge,” co-author Amedeo Balbi added. “Therefore, we need to be extremely cautious in interpreting possible detections. Our study suggests that we should be skeptical of potential technosignatures from a planet with insufficient atmospheric oxygen.”

The paper is published in the journal Nature Astronomy.