Earth’s early life may be a model of alien life

Scientists have reconstructed early life on Earth using light-capturing proteins found in living microbes. Early bacteria and single-celled archaea evolved these proteins called rhodopsins to capture light in a way similar to the chloroplasts of plants. 

In a new study published in the journal Molecular Biology & Evolution, experts have studied modern rhodopsins with an aim at understanding ancient life on Earth and possibly extraterrestrial life.

“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 recognize life elsewhere,” explained study co-author Edward Schwieterman, an astrobiologist at UC Riverside.

Much like photosynthesis in plants, microbes used their rhodopsins to create energy from sunlight and create the basis of all life on earth, even before there was an oxygen rich atmosphere. 

“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,” said Schwieterman. 

Using machine learning, the scientists reconstructed rhodopsin protein sequences from organisms all over the world. Doing this, they created a family tree spanning from four billion years ago to 2.5 billion years ago. They also reconstructed the environments in which the different rhodopsins evolved. 

“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 University of Wisconsin-Madison astrobiologist and lead researcher Betul Kacar.

“It’s like taking the DNA of many grandchildren to reproduce the DNA of their grandparents. Only, it’s not grandparents, but tiny things that lived billions of years ago, all over the world,” Schwieterman said.

Modern rhodopsins absorb blue, green, yellow and orange light, appearing to be one of the other colors that they reflect. Ancient rhodopsins, it seems, absorbed blue and green light. Scientists speculate that this is because the atmosphere lacked a UV protective ozone layer. Without this layer, microbes may have lived relatively deep in the water to protect themselves from solar radiation. Blue and green light penetrates deeper than other lights too, allowing deep sea organisms access to these colors of light.  

To learn more, the researchers hope to genetically engineer organisms with ancient rhodopsin DNA to reconstruct ancient life forms. These could also be a model for what life on other planets, especially those lacking an Earth-like atmosphere, might look like. 

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By Zach Fitzner, Earth.com Staff Writer