The origin of life is perhaps the most tantalizing question in science. Was it catalyzed by a high-energy lightning strike to the right amalgam of molecules? Did it travel from deep space on an asteroid, only landing on our planet by an improbable collision?
Scientists at the Massachusetts Institute of Technology (MIT) think they found the answer. Life originated in a cold environment near the surface and eventually moved to a temperate environment. The prominent theory for the origin of life is that it started near deep sea thermal vents, where a mixture of chemicals and nutrients could cultivate life.
Marjorie Cantine and Gregory Fournier of MIT are proposing a much different solution, largely based off the theorized genome for the Last Universal Common Ancestor (LUCA).
LUCA is the organism at the base of the tree of all modern life. However, it was not the first organism on Earth. During LUCA’s time, there was a whole different ecology of organisms, unrecognizable to what exists today. LUCA was the only of these organisms to successfully pass down its genes.
To recreate LUCA’s genome, the genomes of modern organisms are analyzed to find genes that are widely shared. Scientists estimate LUCA’s genome to be about 600 genes long.
Armed with this knowledge, and the assumption that early life used RNA to encode biological information, Cantine and Fournier created an argument for life originating in a cold, sheltered environment.
Studies have shown that the regions where ancient Earth life began were more temperate than commonly thought. Also, amino acids, a key proponent of life, have been shown to synthesize more readily in cold environments.
Moreover, cold environments are favorable for blocking a process called hydrolysis. Hydrolysis hinders the accumulation of nucleotides, the base element of RNA and DNA. Hydrolysis occurs much less frequently in cold environments.
Although chemical reactions occur quicker in warm environments, the benefits to the synthesis and stability of RNA may have outweighed the benefits of quick replication.
Radiation also plays a key role in limiting where life may have originated.
RNA is very unstable in high flux radiation environments, be it from radioactive materials or UV radiation from the sun. Although modern organisms have processes that can fix damage from radiation, ancient life forms would not have developed these processes yet. Therefore, early life must have been sheltered from direct radiation.
However, it has been shown that an environment with high flux radiation is necessary for prebiotic synthesis, or the creation of molecules necessary for life. This paradox indicates that the origin of life must be directly sheltered from radiation, but also have access to non-sheltered areas. This could come as environments beneath ice, under water, or even under a layer of sediment.
Further evidence for life starting in a cold environment comes from modern psychrophiles, or organisms that live in the extreme cold.
Unlike thermophiles, organisms that live in extreme heat, there is no singular microbial class that are all psychrophiles. This indicates organisms that live in cold environments are evolutionarily much older than organisms in warm environments.
Curiously, the information from LUCA’s genome points towards LUCA being a mesophile, or an organism that lives in moderate temperatures. This was concluded from the number of guanine-cytosine pairs in LUCA’s genome.
Guanine and cytosine, two of the four bases that make up RNA and DNA, are more stable in hot environments than adenine and uracil (or thymine in DNA). This is because of an extra hydrogen bond between the base pairs. Therefore, organisms that live in hot environments have more guanine-cytosine pairs in their genome than organisms from cooler environments.
Using this biological “thermometer,” the MIT scientists concluded that LUCA was a mesophile. They also hypothesized that LUCA was a surface dweller; LUCA had a protein called reverse gyrase that, in modern organisms, repairs a very specific type of damage caused by UV radiation. This means LUCA must have adapted to an environment with high flux radiation, indicating it adapted to surface conditions.
Long story short, life originated in a cold, sheltered environment, and many years later, evolved into LUCA, who lived in moderate-temperature environments on the surface of Earth.
This adaptation to different environments may have been crucial to the survival of all life on Earth. Early in the history of life, organisms were learning to adapt to different environments, paving the path for modern organisms to adapt and thrive on the multitude of environments found on Earth.
Once again, what we thought we knew about biology is changing. Hot or cold? Bottom of the sea or on the surface of land? Lightning, an asteroid?
The problem is that all the options are rooted in sound science, so how do we choose? I say we don’t have to. As far as we know today, life could have started with any of these events, and likely many more that we haven’t considered. So, we have to wonder: across the entire universe, how many times, and how many ways, has life begun?