As we become more efficient at exploring our solar system and studying exoplanets, the prospect of finding simple life is moving from the realm of creative science fiction to planning concrete missions.
With the day of the hoped-for discovery approaching, it’s time to ask: What would this potential life look like?
A team of researchers at the University of California, Riverside, has looked at ancient Earth and some of its early inhabitants to shed some light on what simple life might look like on other worlds and what their atmospheres might look like.
The Earth is very different now than it was when it hosted only a simple life. The Great Oxygenation Event (GOE) forever changed Earth and put 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 much different, and life led the change. This brief history illustrates an important fact: life and its environment are intertwined.
Earth’s early life forms lived in a relatively energy-poor environment, in an atmosphere lacking oxygen.
Sunlight was the only readily available energy, and long before the development of photosynthesis, life forms used sunlight differently.
Proteins called rhodopsin were used to capture solar energy, and these proteins were a simpler way to use energy from the sun than the more complex process of photosynthesis.
“Early on Earth, energy was probably very scarce. Bacteria and archaea figured out how to use the abundant energy from the sun without the complex biomolecules needed for photosynthesis,” astrobiologist Edward Schwitterman of the University of California, Riverside, said in a press release.
Schwittermann is known as a co-author of a new study published in Molecular Biology and Evolution. The lead study for “The oldest photonic zone niches examined by microbial ancestral rhodopsin” is Petul Kakkar, an astrobiologist at the University of Wisconsin-Madison.
As evidence of its usefulness, rhodopsin did not disappear with the early life forms from which it arose. It is prevalent in living organisms today, including us. It is also present in the retina of our eyes, where it is responsible for vision in low light. They are also found in modern and simple life in places such as salt pans.
Its presence in modern life provides a link to the evolutionary history of rhodopsin. The researchers are exploring this link using machine learning and protein sequencing. With these tools, researchers can track the evolution of proteins across geological time scales.
Also, looking around at Earth’s life and atmosphere now is not a good indicator of how to look for life on other worlds. The current atmosphere is rich in oxygen, but Earth’s early atmosphere may have been more like Venus, according to some research.
By tracking how rhodopsin evolved, the authors of the new paper built a family tree of proteins. They were able to reconstruct the rhodopsin between 2.5 and 4 billion years ago.
Much of our search for life focuses on planetary atmospheres. Specific atmospheric particles can be biomarkers, but to know which ones could indicate simple and early life, we need to know in detail what the early Earth’s atmosphere was like when the planet hosted simple life.
“Deciphering the complex relationships between life and the environments in which it lives is essential to reconstructing the factors that determine the habitability of planets over geological timescales,” the authors wrote at the beginning of their paper, and this paves the way for their findings.
The researchers discovered the differences between ancient and modern rhodopsin in the light they absorb. According to genetic reconstruction, ancient rhodopsin mainly absorbed blue and green light, while modern rhodopsin absorbed blue, green, yellow and orange light. This is evidence of the ecological differences between ancient and modern Earth.
We know that the ancient Earth had no ozone layer before GOE, which occurred about 2 to 2.4 billion years ago.
The ozone layer could not exist without free oxygen in the atmosphere, and without the ozone layer, life on Earth would have been exposed to much more ultraviolet radiation than it is now.
Currently, the Earth’s ozone layer absorbs between 97 and 99% of the sun’s ultraviolet rays.
The researchers believe that the ancient rhodopsin’s ability to absorb blue and green light, but not yellow and orange, means the life that relied on it lived several meters deep in the water column. The water column above the living organisms protects them from the harsh UVB rays at the water’s surface.
After GOE, the ozone layer provided protection from the sun’s UV rays, and life developed more modern rhodopsin that could absorb more light. So modern rhodopsin can absorb yellow and orange light along with blue and green light.
Modern rhodopsin can absorb light that chlorophyll optical pigments cannot. Adding a touch of evolutionary elegance, modern rhodopsin and photosynthesis complement each other by absorbing different light, although they are independent and unrelated mechanisms. This complementary relationship presents a kind of puzzle in evolution.
In their paper, the researchers explain that “the information encoded in life itself may provide new insights into how our planet maintains its habitability on a planet where geological and stellar inferences fall short.”
In ancient life, rhodopsin functioned as a kind of proton pump, which creates an energy gradient in the life form. This is separate from photosynthesis, which produces chemical energy for an organism to survive. The proton pump and the energy gradient create an electrochemical potential difference across the cell membrane. It’s like a battery because the color gamut delivers energy for later use.
The team says they can use the information encoded in biomolecules to understand areas in which ancient life survived that was not found anywhere in the fossil record. They refer to them as paleosensors.
They plan 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.
“The co-evolution of environment and life early in Earth’s history serves as a model for predicting global and detectable biomarkers that might be generated on a microbial dominated planet outside our solar system,” the authors wrote in their paper.
Source: Science Alert