Erased Universe

May 13, 2011

The shadow biosphere is the concept that in between the formation of the planet and the earliest-known undisputed evidence of life on Earth, there could have potentially have been more than one genesis. Stanley Miller of the Urey-Miller experiment (which proffered that amino acids could be made from non-living chemical reactions) suggested that chemical evolution could take on the order of magnitude of around 20-100 million years. But the Earth’s crust cooled around 4.5-4.3 billion years ago and the earliest fossils may date to around 3.5 billion years ago (with some sketchier evidence suggesting it was as early as 3.8 billion years ago) This means that in between the formation of the Earth and the formation life, there is a gap of half a billion to a billion years where life could have potentially arisen. And if life could take even a conservative 100 million years to form, then there could have been up to five genesis events between 4.3 and 3.8 billion years ago, or radically, there may have even been up 50 possible events. Could life from an earlier iteration have survived into the present day? How would we know? All life on Earth shares the same characteristics and we can study molecular clues to determine levels of relatedness between species. Researchers found that all life must have had a last universal common ancestor, most likely a single-celled archean life called a thermohalophile, a creature that could have survived in acidic, boiling water. The adaptations this creature used enabled it to thrive and multiply, creating ever more complex progeny through the slow process of evolution.

All amino acids (the building blocks of proteins) on Earth are right-handed and all nucleic acids (the building blocks of DNA) are left-handed. If we found a life form that differed, it would be completely unlike anything on Earth. Scientists believe that amino acids “flip” their configuration called an enantiomer (from left-handedness to right-handedness) on exposure to cosmic radiation. This suggests that because proteins show a predisposition to “right-handedness”, the amino acids found in life may have been formed out in space, where they were exposed to cosmic radiation, and then were delivered to Earth via meteorite impacts. All life uses the same 20 amino acids; some bacteria and plants can produce all 20 standard amino acids to make proteins, but animals like humans have to acquire at least 8 of them from eating plants, bacteria, or other animals. Any creature that used a different set of amino acids would be unlike anything on Earth. Amino acids are distinguishable by their chemical R-group, and changing an amino acid R-group completely changes the identity (and chemical properties) of that amino acid. While there are regular 20 amino acids used by life on Earth, researchers have discovered 40 other non-natural amino acids that aren’t used in our biology, but theoretically, they could be used in an alien biochemistry.

Our Areiosan life uses some amino acids recognizable to us, like methionine and cysteine, but their cells also utilize other amino acids like fluoro-tryptophan, which is used in labeling chemicals with a radioactive tracer. Fluoro-tryptophan is similar to our amino acid tryptophan, but the hydrogen R-group has been replaced with a fluorine atom. Areiosian amino acids aren’t right-handed like ours; they are left handed. This suggests that amino acids carried from meteorite collsions which were prodominently right-handed did not play a role in the formation of life on Areios. Amino acids would more likely have come from the abiotic chemistry on the planet’s surface. The genetic material for Areoisan life is right-handed, in contrast to our left-handed DNA. This further isolates our biology because we cannot metabolize anything with right-handed DNA or left-handed amino acids. Mechanically, the shape of those molecules will not function with our metabolism. Add this to the fact that Areiosan life utilizes chemicals like arsenic or fluoride that are deadly to humans and we can reach the  conclusion that our two biospheres are all but mutually exclusive. Any life from either biosphere that comes in contact with the other would likely poison each other on contact making any chance for interaction restricted to long-distance communication.

Life from Earth and life from Areios would face a mutual environmental restriction because our metabolisms are so different that we cannot co-mingle.


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