Is a "Shadow Biosphere" Hiding on our Planet?
A tardigrade, a microscopic animal. If a shadow biosphere exists, it would be at this miniscule level of life, undetectable to the naked eye. (Source: National Geographic)
For hundreds of years, scientists have pointed their telescopes at the sky, searching for some trace of alien life, but it may be they’ve been looking in the wrong direction entirely. Evidence supporting the existence of alien life may already exist here on Earth, in what’s been called the shadow biosphere. It’s not quite as spooky as it sounds. The idea is that hypothetically there could be a microscopic biome of life that originated independently of the rest of the life on Earth. If such a biosphere existed, it would support the notion of life as a cosmic imperative, not a fluke, and thus lend support to the idea of life existing elsewhere in the universe.
The theory of a shadow biosphere is a marriage of two different scientific fields: astrobiology and microbiology. The former studies the conditions needed for life to originate on other planets, while the latter examines microscopic life here on Earth. Were a shadow biosphere discovered, it would not only support the idea that life isn’t a fluke, but it would also give astrobiologists a roadmap of conditions to look for. Currently, the search for extraterrestrial life is premised on the idea that alien life needs the same basic environment as life on Earth, but it could be alien life functions in an entirely different way.
Jupiter's moon Europa, a candidate for microscopic alien life. Such life could mirror a shadow biosphere here on Earth. (Source: NASA)
While the overwhelming majority of terrestrial species are microbes, we’ve studied a mere 1% of these organisms. Among that remaining 99 could be life that originated separately from LUCA, the last universal common ancestor that all known life is descended from. Perhaps the shadow biosphere exists at an extreme temperature lethal to other life, either at the mouth of a roiling oceanic volcano, or embedded in the freezing cold of an Antarctic ice sheet. The shadow microbes could also use different elements than the basic carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
While the shadow biosphere is a compelling theory, it remains just that. No definitive evidence of such a microbe has yet been found, though there have been some contenders. In 2010, a microbe dubbed GFAJ-1 was reported to be able to briefly substitute its consumption of phosphorus with arsenic, a highly toxic element. Later studies challenged the initial findings, with the bacteria now accepted as merely an extremophile capable of surviving deadly environments. Though GFAJ-1 is not a part of the shadow biosphere, it demonstrates the upper limits of the conditions life can survive in, which itself is useful both in understanding life on Earth and potential life out in space.
Mono Lake, an alkaline lake where GFAJ-1 was discovered. (Source: Mono Lake Committee)
There are a number of theories of how the shadow biosphere could have theoretically originated. One has it that the shadow biosphere began before current life and was ejected into space during a meteorite impact. Entombed within a newly formed asteroid, the shadow microbes crisscrossed the solar system before landing again on Earth, natives returning home as aliens. Such an origin remains a point of speculation. The shadow biosphere is just a theory, but searching for its evidence allows us a greater understanding of life on Earth as we know it now.
Confirmed life on Earth started out as a microscopic biome, preserved today as stromatolites. These rock formations contain within them fossilized microorganisms, dated to 3.5 billion years ago. Head over to the shop if you want to learn more and own a piece of stromatolite for yourself.
Davies PCW. "Searching for a Shadow Biosphere on Earth as a Test of the 'Cosmic Imperative.'" Philosophical Transactions of the Royal Society of London Series A: Mathematical, Physical, and Engineering Sciences. 2011;369(1936):624-632. doi:10.1098/rsta.2010.0235
Wolfe-Simon, Felisa, et al. "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus." Science 332.6034 (2011): 1163-1166.
Marcheselli V. "The Shadow Biosphere Hypothesis: Non-knowledge in Emerging Disciplines." Science, Technology, & Human Values. 2020;45(4):636-658. doi:10.1177/0162243919881207