Early Earth’s Violent Changes Powered Evolution of Our Last Universal Common Ancestor

 

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“Encountering such environmental variability early on may be necessary to build up the level of complexity needed for LUCA to have the evolutionary potential to continue to diversify and colonize nearly every habitat on Earth over four billion years,” Greg Fournier, an evolutionary biologist at MIT.


All life on Earth today originates from two distinct developments in our planet’s biological history. These are the emergence of the first life forms billions of years ago, and the subsequent evolution of the last universal common ancestor (LUCA) of all extant organisms.

 

Whatever they were at the time, writes Charles Q. Choi in Astrobiology, these two extinct species – the first life and LUCA – likely occupied radically different environments, suggesting that early life had to undergo a series of evolutionary changes of which traces may still be detectable in organisms alive today.

Fournier and his colleague Marjorie Cantine detailed their findings in the journal Origins of Life and Evolution of Biospheres.

Although nowadays DNA reigns supreme as the blueprint of life, one theory shared by many evolutionary biologists is that the first living things on Earth possibly used the simpler RNA molecule, which is able to both encode genetic information like DNA and trigger vital chemical reactions like many proteins.

 

 

The researchers analyzed records of gene sequences that are found in all organisms currently alive on Earth, including the ones likely similar to Earth’s most ancient organisms, in order to figure out which sequences early life probably possessed. They next examined previous research exploring how well these RNA sequences perform in a variety of conditions, such as temperature, acidity and radiation to deduce what the environment of the earliest life on Earth might have been like.

Ultraviolet light can damage RNA, but it may have also triggered chemical reactions that helped create key building blocks of life. At the time of life’s origins, approximately 4.4 billion years ago, the Sun gave off more ultraviolet rays than it does now. The scientists suggested that life first emerged near Earth’s surface under some form of radiation shield, such as water, ice cover, sediment or other barriers, and had access to unshielded environments that could generate key biomolecules.

Temperatures on Earth may have also been relatively cold at that time, given the Sun’s cooler youth, enough for significant ocean-ice to form. It is easier for amino acids (the building blocks of proteins) and long RNA molecules to assemble in cooler temperatures. Moreover, icy surfaces and slush could have concentrated biomolecules together to assist the emergence of life.

In contrast, the last universal common ancestor – the microbial species from which all life that exists today came from – may have lived in moderate temperatures, perhaps at least four billion years ago. Scientists can surmise what LUCA was like by looking at what genes organisms on Earth today have in common, analyzing how these genes changed over the course of evolution, and deducing what the ancestral versions of these genes might have been like. The DNA sequences making LUCA’s 600 or so genes and the amino acids making up its proteins are usually most stable in moderate temperatures, the researchers said.

“Building on the work of many others, we suggest that life dispersed into and adapted to new environments very early in its history,” says study co-author Marjorie Cantine, a geobiologist at MIT.

The NASA/JPL–Caltech image at top of page shows the Late Heavy Bombardment, which could have instigated a dramatic shift in climate and environment that helped spur the formation of life on Earth.

The Daily Galaxy via Astrobiology

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