It may be hard to believe, but our planet started off with very little oxygen. When O2 levels began to rise, species and organisms started to diversify and become more complex. While some experts believe that complex eukaryotic life was able to emerge and evolve on Earth due to the rise of oxygen levels, its direct link to the emergence of metazoan1 life is intensely debated (Stamati et al., 2011).
A recent study by Bellebroid et al. (2018) using marine carbonate cerium anomalies (Ce) took on the challenge of measuring the earth’s oxygen during the Palaeozoic Era, a time when life was beginning on Earth. The amount of Ce found in the rocks relative to rare Earth elements and yttrium levels indicate the levels of oxygen surrounding the rock during the Proterozoic Era. The ratio used by Bellebroid et al. (2018) is proven to be accurate as Ce dissolves in anoxic conditions, leaving the rock, while rare Earth elements and yttrium levels remain constant. The study found that oxygen concentrations in the Paleoproterozoic2 atmosphere were very low. In fact, it was near 0.1% of present atmospheric level, which is crucial evidence implying that the level of oxygen in the atmosphere lowered back down after the Lomagundi Event.3 Extending this evidence across the Palaeozoic, the data suggests that there was: 1) sustained low atmospheric O2 conditions, as well as 2) a thin layer of oxygen on surface of ocean for much of the era. This environment of low oxygen remained for a long time and should have impacted the ecology of Earth’s early complex organisms. Now here comes the interesting part: it didn’t.
The oxygen levels during this era were well below what active aerobic metazoans need to survive. Or at least, the atmospheric oxygen levels were. However, as detailed in a study by Reinhard et al. (2016), organisms were able to survive due to locally produced oxygen at the sea surface. After the evolution of oxygenic photosynthesis4, it was common for there to be changes in the equilibrium of oxygen on marine surfaces due to bacterial photosynthesizers (this is still true for oceans today). Biogeochemical models indicate that the oxygen produced by bacteria at the ocean’s surface can create areas of higher pO2 (dissolved oxygen) concentration, even with exceptionally low atmospheric O2 levels. From this data, it is possible to draw the conclusion that even if there was low atmospheric oxygen levels, certain metazoan organisms could survive. The pO2 produced at the marine surface level saved Earth’s basic mesozoic development, sustaining oxygen levels above their respiratory requirements.
Evidence collected by the novel study indicate that there was very low amounts of oxygen in the atmosphere during the Palaeozoic Era. Even so, complex organisms were able to survive and differentiate, contrasting earlier scientific discoveries (Stamati et al., 2011). The ability of basal life forms to rely on marine surface oxygen for survival provides a new perspective on how life can grow and be sustained. This new viewpoint presents a radical approach to planetary exploration. After all, if we were wrong about the indicators of life on our planet, couldn’t the same be for others?
1multicellular animals with differentiated tissues (Oxford Dictionaries, n.d.).
2 The beginning of the Proterozoic Eon (Encyclopædia Britannica, n.d.).
3 The rise of oxygen during an interval of 130-250 million years in the Paleoproterozoic Era (Bachan, 2015; Kump, 2015).
4 The process of photosynthesis that releases oxygen. It is responsible for the transformation of Earth from it’s primitive atmosphere to one with molecular oxygen (Peretó 2011).
References
Bellefroid, E.J., Hood, A.S., Hoffman, P.F., Thomas, M.D., Reinhard, C.T. and Planavsky, N.J., 2018. Constraints on Paleoproterozoic atmospheric oxygen levels. Proceedings of the National Academy of Sciences, [online] Available at: <http://www.pnas.org/content/115/32/8104> [Accessed 14 Sep. 2018].
Cascales-Miñana, B., Gerrienne, P., Cleal, C.J., 2015. A palaeobotanical perspective on the great end-Permian biotic crisis. Historical Biology, [e-journal]. DOI: 10.1080/08912963.2015.1103237.
Encyclopædia Britannica, n.d. Proterozoic Eon. [online] Available at <https://www.britannica.com/science/Paleoproterozoic-Era/media/175886/133053> [Accessed 22 Sep, 2018].
Oxford University Press, n.d. Oxford Dictionaries. [online] Available at: <https://en.oxforddictionaries.com/definition/metazoa> [Accessed 22 Sep, 2018].
Peretó J., 2011. Oxygenic Photosynthesis. Springer Link,[online] Available at: <https://link.springer.com/referenceworkentry/10.1007%2F978-3-642-11274-4_1721> [Accessed 20 Sep. 2018].
Reinhard, C.T., Planavsky, N.J., Olson, S.L., Lyons, T.W. and Erwin, D.H., 2016. Earth’s oxygen cycle and the evolution of animal life. Proceedings of the National Academy of Sciences, [online] Available at: <http://www.pnas.org/content/113/32/8933> [Accessed 14 Sep. 2018].
Stamati, K., Mudera, V. and Cheema, U., 2011. Evolution of oxygen utilization in multicellular organisms and implications for cell signalling in tissue engineering. Journal of Tissue Engineering, [online] Available at: <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3258841/> [Accessed 18 Sep. 2018].
Windley, B.F., 2014. Proterozoic Eon. Encyclopaedia Britannica, [online] Available at: <https://www.britannica.com/science/Proterozoic-Eon> [Accessed 20 Sep. 2018].