Geology in the Theories and Evidences of Early Life on Earth
By Layla Sevilla
The search for evidence of extraterrestrial life is a famous research topic for both astrobiologists and the general public. The possibility of life outside Earth is commonly questioned but a sufficient number of conclusive proof is still yet to be discovered. Similarly, knowledge on early life within the planet itself and its origin are also as indefinite and complex. There are many theories on the origin of life and several of these take into consideration the geological aspects within and/or outside of Earth.
In discussing the first traces of life on this planet, one of the most well-known discoveries is the presence of stromatolites. These rocks are generally defined as layered structures of sediments and microbial deposits, typically with convex-up laminations and with some having preserved microfossils (Stanley & Luczaj, 2015). They are mainly formed by cyanobacteria wherein surrounding sediments are trapped and bound with precipitated minerals and microbial filaments to produce microbial mats. These then accumulate into the characteristic layers of the sedimentary rocks. As their source can be from simple prokaryotic organisms, stromatolites have been found to be present for billions of years on Earth. The approximate oldest age for this type of rock has been dated to 3.5 billion years ago with cyanobacteria associated with the last universal common ancestor (LUCA) of life (Lal, 2008).
Aside from stromatolites, other geological evidence of early forms of life have also been considered such as deep-sea hydrothermal vents where living organisms known as hyperthermophiles have been known to survive in (Lal, 2008). The presence of these unicellular microbes in harsh conditions of extremely high temperatures suggest links to life that lived in ancient Earth which was significantly hot due to meteorites, residual and radioactive elements, and contraction of the planet. The closest existing environment exhibiting these conditions are those around hydrothermal vents that are usually found along tectonically active areas like mid-ocean ridges (Tarbuck et al., 2012). These vents are formed from hot magma released or exposed in fissures of ocean crusts that interact with seawater to form chemical products like sulfur and hydrogen. It has been theorized that life, or organic components of it, may have originated from this type of area with the nutrients formed from the geothermal energy released in these vents, as supported by the presence of hyperthermophiles (Lal, 2008). The iron-sulfur world hypothesis by Günther Wächtershäuser also proposed this idea where organic molecules are formed in areas of hydrothermal vents and used iron sulfide in surrounding rocks as bases for life (Starr et al., 2015).
While there are several theories revolving around the idea that primitive life started on Earth, this topic has also been approached with the possibility of extraterrestrial origins. This involves Panspermia which hypothesizes that interstellar spores or dust with biological components were transported by radiation in space and landed and acted as the seeds of life on Earth (Lal, 2008). Specifically, different versions of the hypothesis called Lithopanspermia and Ballistic Panspermia explain that asteroids and meteorites were used as covers and vehicles for propagation of these seeds (de Vera, 2011; “Panspermia and the Origin of Life on Earth,” n.d.). This hypothesis is supported by observations of organic materials and bacteria found in interstellar samples like meteorites, as well as spectral analysis of gas clouds in space.
Although there have been several ways to approach the topic of early life and its origins, from terrestrial to extraterrestrial settings, a universally accepted comprehensive explanation remains a mystery. The amount of certain evidence and studies are lacking in comparison to the extremely large question about life which in itself is complex. Still, the many ways that geology has been applied, along with other sciences, are each a step further to achieving more pieces in the endless puzzle of life.
References
de Vera, J. P. (2011). Panspermia. Encyclopedia of Astrobiology, 1213–1215. https://doi.org/10.1007/978-3-642-11274-4_1151
Lal, A. K. (2008). Origin of Life. Astrophysics and Space Science, 317(3–4), 267–278. https://doi.org/10.1007/s10509-008-9876-6
Panspermia and the Origin of Life on Earth. (2020). Panspermia-Theory.Com. https://www.panspermia-theory.com/featured/panspermia-and-the-origin-of-life-on-earth
Stanley, S. M., & Luczaj, J. A. (2015). Earth System History (4th ed.). W. H. Freeman.
Starr, C., Taggart, R., Evers, C., & Starr, L. (2015). Biology: The Unity and Diversity of Life (14th ed.). Brooks Cole
Tarbuck, E. J., Lutgens, F. K., & Tasa, D. G. (2012). Earth Science (13th ed.). Pearson.