Theoretical ecologists have proposed a likely way of probing extraterrestrial life.
BY DEBDUTTA PAUL
Searching for extraterrestrial life is difficult because it may look very different from life on Earth. Recently, scientists have been studying life’s features independent of the specific chemical processes that govern life.
Any life anywhere in the universe multiplies its species. And, no species is ever alone. Assuming these two basic features of life, Akshit Goyal from ICTS-TIFR and his colleague have found a universal signature of life. They have demonstrated that resources get layered in decreasing order of energy, just like in stromatolites, the earliest known fossils on Earth. Their work is presently arXiv-ed.
Recently, astrobiologists have started collecting samples from extraterrestrial objects. For instance, in 2023, OSIRIS-REx by NASA brought back samples from asteroid Bennu, and the mission is off to another asteroid to bring back more samples. But, astrobiologists are yet to learn what to hunt for exactly to find alien life. The present study offers a concrete clue.
On Earth, energy-ordered layers are also found in ‘microbial mats’ — rocks infested with layers of microorganisms. “If you go to a tide marsh and you dig up the soil, you see these… layers,” said Akshit.
Microbes in the top-most layers create signatures of metabolism using oxygen, the most energetic resource. Oxygen runs out one layer down, and the microbes residing in this layer predominantly use the next most energetic resource. As scientists look deep, they find the deepest layers’ microbes using the least energy resource for survival. So, the layers signify various resources running out.
Such layering, Akshit and his colleague have proposed, is universal to all life.
General signatures of life
In the search for life beyond Earth, humans shouldn’t be fixated on details that the species may follow. Instead, all life must follow general principles, which leave behind definite imprints. But first, we must know these imprints.
One such signature is the phenomenon called ‘homochirality’. Biomolecules found on Earth have handedness, or a specific direction in which it curls. It comes in two types — left-handedness and right-handedness. Molecules of the same compound but different handedness behave differently. Researchers think homochirality helps life store and pass on information, so it’s a signature of life.
However, such a phenomenon does not necessarily require many forms of life to exist together. But on Earth, wherever we look, many species co-exist.
“No life form really exists alone,” said Akshit. And the co-existence of many species must have a definite imprint.
Symptoms of life: reproduction and ecology
In an environment where resources are limited, different species compete for resources. The most successful ones dominate the most energetic resource because they produce more of their kin. It’s only when that resource runs out that the species is not found any further. The next dominant species then hog the next most energetic resource.
In this way, the reproduction of individual species and the interaction between different species create energy-ordered layers. The duo’s research shows mathematically exactly how this happens.
When the researchers wrote the simplest mathematical model that incorporates reproduction and ecology, the layering appeared. “It’s a mathematical feature of these equations,” said Akshit.
The equations do not specify which molecules will be found. In fact, they don’t consider this detailed chemistry at all. They only considered the amount of energy available from the different resources. “Whatever chemicals they are — they go down in decreasing order of their energy content,” Akshit added.
The specific layering, the duo has shown, is a signature of the collective existence of different life forms. “If you had just one species to start with, you wouldn’t get any layering,” he said.
Looking for aliens
When astrobiologists look for life beyond Earth, they usually look for specific signatures, like water or oxygen molecules or life’s usage of nitrogen for genetic materials. For example, in 2020, scientists discovered phosphine gas in the Venusian atmosphere. Since the molecule is produced only by living organisms on Earth, they proposed there is life on Venus.
Debates about the observations and whether they imply life’s presence on Venus — rage on. So, thinking beyond such specific signatures is key to predicting how to look for life beyond Earth.
The present study shows a clear path for future astrobiology experiments, especially missions that bring back samples from other planets, moons, or asteroids. “People would like to know how to detect life on these samples, but [they] have no idea what to measure,” said Akshit.
The duo’s work has provided astrobiologists a concrete direction as to what to look for — layers of resources in decreasing order of energy.
Looking for aliens might be much easier than we thought.
The author thanks Akshit Goyal for discussions.