Will we discover the simple life somewhere? Perhaps on Enceladus or Europa in our Solar System, or further afield on an exoplanet?
As we become more proficient in exploring our Solar System and studying exoplanets, the prospect of finding some simple life moves away from the creative realm of science fiction and into concrete mission planning.
As the hopeful day of discovery approaches, it’s a good time to ask yourself: What might this potential life look like?
A team of researchers from the University of California, Riverside looked at the ancient Earth and some of its earliest inhabitants to shed some light on what simple life might be like on other worlds and what the atmospheres might be like.
The Earth is very different now than when it supported only simple life. The Great Oxygenation Event (GOE) changed Earth forever and set it on the path to becoming the planet it is today, with an oxygen-rich atmosphere and complex life. Before GOE, Earth’s atmosphere was very different, and life drove the change. That short story illustrates an important fact: life and its environment are intertwined.
The earliest life forms on Earth lived in a comparatively energy-poor environment, in an oxygen-poor atmosphere.
Sunlight was the only readily available energy, and long before photosynthesis evolved, life forms used sunlight differently.
They used proteins called rhodopsins to capture solar energy, and these proteins were a simpler way of using the Sun’s energy than the more complicated photosynthesis.
“On the early Earth, energy may have been very scarce. Bacteria and archaea figured out how to use the Sun’s abundant energy without the complex biomolecules needed for photosynthesis,” UC Riverside astrobiologist Edward Schwieterman said in a news release. .
Schwieterman is co-author of a new study published in Molecular Biology and Evolution. The study is “Earliest Photic Zone Niches Proven by Ancestral Microbial Rhodopsins,” and the study leader is Betul Kacar, an astrobiologist at the University of Wisconsin-Madison.
As proof of their usefulness, rhodopsins did not disappear with the first forms of life that gave rise to them. They are widespread in organisms today, including us. They are present in the retinal rods of our eyes, where they are responsible for vision in low light conditions. They are also found in simple modern life in places like salt ponds.
Their presence in modern life provides a link to the evolutionary history of rhodopsins. The researchers are exploring that link using machine learning and protein sequencing. Using those tools, the researchers were able to trace protein evolution over geological time scales.
Observing life and the atmosphere on Earth now is not a good indication of how to look for life on other worlds. Our current atmosphere is rich in oxygen, but early Earth’s atmosphere might have been more like that of Venus, according to some research.
By tracing how rhodopsins evolved, the authors of the new paper built a family tree for the proteins. They were able to reconstruct rhodopsins from between 2.5 and 4 billion years ago.
Much of our search for life focuses on planetary atmospheres. Specific atmospheric molecules can be biomarkers, but to know which ones might signal the presence of simple, early life, we need to know in detail what Earth’s early atmosphere was like once the planet supported simple life.
“Deciphering the complex relationships between life and the environments it inhabits is critical to reconstructing the factors that determine planetary habitability on geological time scales,” the authors write at the beginning of their article, and that sets the stage for the results they present. .
“Life as we know it is as much an expression of the conditions on our planet as it is of life itself. We resurrected ancient sequences of DNA from one molecule and it allowed us to link with the biology and environment of the past,” he said. the leader of the Kacar study.
The team’s research parallels the genealogical evidence available today. We can send our DNA and learn a lot about where we come from. The intense teamwork goes much deeper than that, but the comparison is useful.
“It’s like taking the DNA of many grandchildren to replicate the DNA of their grandparents. Only it’s not the grandparents, but tiny things that lived billions of years ago, all over the world,” Schwieterman said.
The researchers discovered differences between ancient and modern rhodopsins in the light they absorbed. According to genetic reconstructions, ancient rhodopsins absorbed mainly blue and green light, while modern rhodopsins absorb blue, green, yellow, and orange light. This is a clue to the environmental differences between ancient and modern Earth.
We know that the ancient Earth had no ozone layer before the GOE, which occurred between 2 and 2.4 billion years ago.
The ozone layer cannot exist without free oxygen in the atmosphere, and without an ozone layer, life on Earth was subject to much more ultraviolet radiation than it is now.
Currently, the Earth’s ozone layer absorbs between 97 and 99 percent of the sun’s UV rays.
The researchers believe that the ability of ancient rhodopsins to absorb blue and green light and not yellow and orange light means that the life that depended on it lived several meters deep in the water column. The water column above the organisms protected them from the strong UVB radiation at the water surface.
After the GOE, the ozone layer provided protection from the Sun’s ultraviolet radiation, and life evolved into more modern rhodopsins that can absorb more light. So modern rhodopsins can absorb yellow and orange light along with blue and green light.
Modern rhodopsins can absorb light that photosynthetic chlorophyll pigments cannot. With a note of evolutionary elegance, modern rhodopsins and photosynthesis complement each other by absorbing different light, although they are independent and unrelated mechanisms. This complementary relationship represents a bit of a puzzle in evolution.
“This suggests co-evolution, in the sense that one group of organisms exploits light that the other doesn’t absorb,” Schwieterman said. “This could be because the rhodopsins developed first and filtered out the green light, so the chlorophylls developed later to absorb the rest. Or it could have happened the other way around.”
Many of the clues about the nature of early life on Earth are contained in geology. Scientists routinely study ancient rocks to understand how the first living things survived and evolved.
They also study the behavior of the Sun and how much of its energy reached the planet’s surface as the Earth changed over time. But now they have another tool.
“Information encoded in life itself may provide new insights into how our planet has maintained planetary habitability where geological and stellar inferences fall short,” the authors explain in their paper.
In ancient times, rhodopsins acted as a kind of proton pump. A proton pump creates an energy gradient in a life form. That’s separate from photosynthesis, which produces chemical energy for an organism to survive. A proton pump and energy gradient create a difference in electrochemical potential across the cell membrane. It is like a battery because the gradient presents energy for later use.
But as scientifically curious people, we don’t need to know precisely how they work. We can understand how they can help us identify exoplanet atmospheres similar to those of early Earth and the simple life that thrived there.
The team say they can use information encoded in biomolecules to understand niches where ancient life survived that are present nowhere in our paleontological record. They refer to them as paleosensors.
The researchers say that because the “…functional diversification and spectral fine-tuning of this taxonomically diverse family of proteins…” are coupled, rhodopsins are an excellent laboratory test bed for identifying remotely detectable biosignatures. on exoplanets.
And they’re not done yet.
They intend to use synthetic biology techniques to understand ancient rhodopsins, how they helped shape Earth’s ancient atmosphere, and how they might shape the atmospheres of exoplanets.
“We engineered ancient DNA into modern genomes and reprogrammed insects to behave as we think they did millions of years ago. Rhodopsin is a great candidate for laboratory time travel studies,” Kacar said.
Some evidence of early life on Earth and in the atmosphere is hidden from us. But the team’s method is overcoming some hurdles in our search for that evidence. Who knows where it will take us.
“Our study demonstrates for the first time that enzyme behavioral histories are amenable to evolutionary reconstruction in ways that conventional molecular biosignatures are not,” said Kacar.
The more we learn about early Earth, the more we learn about other worlds. If multiple planets harbor life, each probably took a different path on its way to life. But there will be parallels in chemistry and physics behind this. And just as it has here on Earth, the interaction between life and the environment must shape the history of other worlds.
“The co-evolution of environment and life early in Earth’s history serves as a model to predict detectable universal biosignatures that might be generated on a microbially dominated planet beyond our Solar System,” the authors write in their paper.
“Early Earth is a strange environment compared to our world today. Understanding how organisms here have changed over time and in different environments will teach us crucial things about how to search for and recognize life elsewhere,” Schwieterman said.
This article was originally published by Universe Today. Read the original article.