Changes
====Early earth atmosphere====
There is significant uncertainty to what the primitive atmosphere was composed of.<ref name="Bada2013" /> The Miller-Urey experiment assumed an a primitive atmosphere was composed of ''reducing'' (oxygen poor) gases [e.g. methane (CH<sub>4</sub>), ammonia (NH<sub>3</sub>), hydrogen (H<sub>2</sub>), and water vapor (H<sub>2</sub>O)], but by the 1980s geochemists believed that early earth atmospheres probably had ''neutral'' gases like those emitted by volcanoes [e.g. water vapor (H<sub>2</sub>O), carbon dioxide (CO<sub>2</sub>), and nitrogen (N<sub>2</sub>)] and perhaps some ''reducing'' gases like carbon monoxide (CO). Since hydrogen was a light element, there probably would not have been any or perhaps very little in early earth.<ref name="wells" /><ref name="Bada2013">Bada JL. [https://web.archive.org/web/20190225141103id_/http://pdfs.semanticscholar.org/6f46/3e8a3611fa7f25c143991dfddac49c396b73.pdf New insights into prebiotic chemistry from Stanley Miller's spark discharge experiments]. Chem Soc Rev. 2013 Mar 7;42(5):2186-96. doi: 10.1039/c3cs35433d</ref><ref name="wells">Jonathan Wells. 2017. Zombie Science: More Icons of Evolution. ISBN: 978-1936599448. 3. Survival of the Fakest</ref> A possible source of lightning could have been "volcanic lightning", which occurs sometimes near volcanic eruption plumes, and some volcanic gases emitted have been hydrogen sulfide (H<sub>2</sub>S), ammonia (NH<sub>3</sub>), methane (CH<sub>4</sub>), and carbon dioxide (CO<sub>2</sub>).<ref name="Bada2013" /> Some scientists believe that early earth atmosphere was similar to earth today.<ref>[https://web.archive.org/web/20250902132614/https://astrobiology.nasa.gov/news/earths-early-atmosphere-an-update/ Earth's Early Atmosphere: An Update] - NASA</ref>
====Hydrothermal vents====
