How Did Life End Up on Earth?

By Ashley Indictor

Is Earth the only planet in the universe that contains life? Scientists and philosophers for millennia have debated how life developed on Earth, coming up with several hypotheses and theories. One possible explanation is the theory of panspermia, in which a comet or other celestial body brought life to Earth (Hardy, 2014).

Figure 2. Microscopic view of a tardigrade (Pickett, 2015)

As far as we know, no other moon or planet in our solar system (i.e., the Sun, its planets, their moons, and all asteroids, comets, and rocks) has life on it. Although moons like Titan and Enceladus could harbor life (simple organic life forms), and Mars is theorized to have once supported life, Earth is the only planet in the Solar System we can call “living” (NASA, 2017). About 4 billion years ago, Earth underwent a period of heavy bombardment where it was barraged with asteroids and comets (Kaufman, 2017). The earliest evidence of life on Earth dates back 3.83 billion years, coinciding with the period of such violent event (2017). One explanation for this eerie overlap is that one or more of these millions of comets and asteroids carried life from somewhere else and brought it to Earth when it crashed on the surface (2017).

It is not too far-fetched to believe that these lifeforms survived space’s harsh climate. After all, some organisms on Earth can survive temperatures as low as -18°C and as high as 113°C, even after being preserved in liquid nitrogen at -196°C (Joshi, 2008). Tardigrades (see Figure 2), for instance, can live in these extreme temperatures, and can even survive many days at low Earth orbit while being exposed to a space vacuum and harmful radiation (Bradford, 2017). Tardigrades are living proof that life can be carried across space and survive until it reaches a celestial body. This further indicates the possibility of life coming from somewhere else.

Figure 3. Bacterial spore (National Academy of Sciences, 2018)

An additional consideration is that bacteria could have entered Earth through bacterial spores [see Figure 3], as they can survive without nutrients (Joshi, 2008). Bacterial spores have protective bodies that allow bacteria to carry DNA while withstanding conditions that would normally kill them (Cornell, n.d.). Furthermore, bacteria are famous for their ability to survive in extreme conditions and even campuses about one-third of Earth’s biomass (total mass of organisms on Earth) (Joshi, 2008). The German Aerospace Centre found that it is possible bacterial spores can travel within comets or meteorites (2008).

Overall, we still do not know the true origin of life. While the theory of panspermia is possible, more research must be conducted to reach an answer. Perhaps it will take another thousand years before we figure it out.



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