The earliest life on Earth got its energy from sunlight, which penetrated deep into the waters of the ocean. What does that have to do with extraterrestrial life? The light-sensitive proteins present in these ancient microbes may help detect life forms on other planets.

primordial soup
Billions of years ago, the earth consisted mainly of water and here and there a huge volcano. There was no ozone layer yet, so it was not so pleasant for the first life forms. Energy resources were scarce and the sunlight was deadly. It is therefore no coincidence that the first microbes arose a little deeper in the ocean, where the harmful UV light could not penetrate.

Green and blue light penetrate the water furthest and so the ancient single-celled organisms feasted on this with the help of the protein rhodopsin. These proteins can also be found behind the rods and cones of the human eye. “In the early phase of the Earth, there were few energy sources. Bacteria managed to feed on solar energy without the complex biomolecules needed for photosynthesis,” says astrobiologist Edward Schwieterman of the University of California in conversation with Scientias.nl.

Rodopsine shows us the difference between light and dark and ensures that we can distinguish colors. The protein is present in many modern life forms in many different ecosystems.

machine learning
The researchers write in the journal Molecular Biology and Evolution how it using machine learning managed to bring the ancient form of rhodopsin to life and to investigate how it reacts to all kinds of impulses. “It’s like analyzing the DNA of a group of grandchildren and thereby reconstructing the DNA of the grandfather. But then it’s not grandpa, but microbes from 2.5 to 4 billion years ago that you bring to life.”

Lead researcher Betül Kaçar: “We paste the ancient DNA into the genome of a modern variant. In this way we reprogram the bacterium and create something that resembles microscopic primeval life as closely as possible. Rodopsine is ideal for this technique to go back in time.”

An image of how the microbes with the rhodopsins capture the sunlight for energy. Photo: Sohail Wasif/UCR

The Great Oxidation
After the Great Oxidation of about 2.4 billion years ago, the oxygen percentage in the atmosphere increased. An ozone layer also formed around the earth. It was now possible to live closer to or above the surface of the water. The rhodopsin proteins continued to evolve. In addition to green and blue light, it was later also possible to use other types of light as an energy source.

“We can see rhodopsins as ‘paleosensors’ by comparing pieces of these proteins with modern organisms. This makes it possible to calculate backwards and make a good estimate of the first life forms on earth. We can also use this information to estimate how extraterrestrial life develops in exotic conditions,” says Schwieterman.

Surprising
“It is surprising how beautifully the stories fit together. The first life forms mainly used the green light at moderate ocean depths, where the toxic UV light could not penetrate. After the ozone layer was formed, a greater diversity of rhodopsins arose. This coincides nicely with the larger potential habitat of the microbes.”

The team hopes to use this new information in the future to search for extraterrestrial life. The next generation of satellites will allow planets in nearby galaxies to be scanned for similar life forms.

exoplanets
“We are looking for biological ‘footprints’ in the atmosphere or on the surface of interesting planets, among other things. Plant material on Earth gives off a certain change in the reflection from visible light to infrared. We call this the ‘vegetation red edge’ or VRE. We can use this signal as a biological footprint for life outside our solar system,” explains Schwieterman. “Our findings may have revealed a new kind of footprint that we can scan for. This change in light reflectance allows us to ‘rhodopsin green edge’ because a similar effect takes place between green and orange-red.”

Top priority
Schwieterman concludes: “Scanning galaxies for signs of life is a top priority for NASA. It is important to understand the usefulness of a ‘rhodopsin green edge’-scan as soon as possible. For example, NASA equipment for the future space observatory can still be equipped with this technology.”

The Great Oxidation

2.4 billion years ago, oxygen suddenly entered the Earth’s atmosphere. In 100 million years, the amount of oxygen in the air increased very quickly. This will be the Great Oxidation Event (GOE). The oxygen was made by blue-green algae (cyanobacteria) via photosynthesis. Without this oxygen, multicellular aerobic life would never have developed. The new life came, the old life of the anaerobic single-celled organisms died out.