Scientists have reconstructed the biological processes in some of the Earth’s earliest life forms, an advance that could help find alien life on other planets with atmospheres similar to those on early Earth.
Researchers, including those from the University of California (UC) – Riverside in the US, say the earliest life forms on Earth, including bacteria and single-celled organisms called archaea, inhabited a mainly oceanic planet without an ozone layer to protect them from the sun’s radiation.
These microbes had proteins called rhodopsins that have the ability to turn sunlight into energy, using them to power cellular processes.
Rhodopsins are related to the rods and cone cells in human eyes that enable people to differentiate between light and dark as well as see colors, and are also distributed among modern organisms and environments like saltern ponds that present a rainbow of vibrant colors.
In the new study, published recently in the journal Molecular Biology and Evolution, scientists used artificial intelligence to analyse the building block sequences of the rhodopsin protein from across the world, and tracked how they evolved over time.
They then created a type of family tree that allowed them to reconstruct the protein from 2.5 billion to 4 billion years ago, and the conditions that they likely faced when life first originated on Earth.
“On early Earth, energy may have been very scarce. Bacteria and archaea figured out how to use the plentiful energy from the sun without the complex biomolecules required for photosynthesis,” study co-author and UC Riverside astrobiologist Edward Schwieterman said in a statement.
While modern-day versions of the protein absorb blue, green, yellow, and orange light, scientists say that ancient rhodopsins were tuned to absorb mainly blue and green light.
“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 DNA sequences of one molecule, and it allowed us to link to the biology and environment of the past,” said study lead Betul Kacar from the University of Wisconsin-Madison.
In ancient Earth, without the benefit of an ozone layer, scientists say billions-of-years-old microbes lived several meters down in the water column to shield themselves from intense UVB radiation at the surface.
They say the earliest rhodopsins primarily absorbed blue and green light that best penetrates water.
“This could be the best combination of being shielded and still being able to absorb light for energy,” Dr Schwieterman said.
As the Earth’s atmosphere began to experience a rise in the amount of oxygen over 2 billion years ago, researchers say rhodopsins evolved to absorb additional colors of light.
Modern-day versions of the protein absorb colors of light that chlorophyll pigments in plants cannot.
“This suggests co-evolution, in that one group of organisms is exploiting light not absorbed by the other. This could have been because rhodopsins developed first and screened out the green light, so chlorophylls later developed to absorb the rest. Or it could have happened the other way around,” Dr Schwieterman said.
In further studies, scientists hope to resurrect model rhodopsins in a laboratory using synthetic biology techniques.
“We engineer the ancient DNA inside modern genomes and reprogram the bugs to behave how we believe they did millions of years ago. Rhodopsin is a great candidate for laboratory time-travel studies,” Dr Kacar explained.
Researchers also hope to apply their new understanding to search the skies for signs of life on other planets.
They say the features of the rhodopsin protein, including its structural simplicity and functional variability, make them a testbed for assessing life signature molecules on other potential “microbe-dominated planets.”
“Early Earth is an alien environment compared to our world today. Understanding how organisms here have changed with time and in different environments is going to teach us crucial things about how to search for and recognise life elsewhere,” Dr Schwieterman said.