The Cambrian explosion, as I discussed in last week’s article (“Cambrian Explosion Becomes More Explosive“), is the sudden, simultaneous appearance of the greatest number of phyla to occur during the 3.8-billion-year history of Earth’s life. Many of these phyla were complex animals with skeletons, digestive tracts, circulatory systems, and specialized internal and external organs. As such, they needed a lot of oxygen.
These animals were predominantly living in Earth’s oceans, especially on the bottom sediment surfaces of continental shelves. For them to have the dissolved oxygen they required, they would need a pulse of bottom seawater oxygen levels on the continental shelves. New research shows how exquisitely fine-tuned geological events provided that oxygen.
Ediacaran Oxygenation The first large animals to appear on Earth were the Ediacaran fauna. They appeared before the Cambrian explosion during a period known as the Avalon explosion, which occurred 575 million years ago. This period was marked by a sudden transition from microscopic animals to animals with body sizes as large as two meters and body shapes that included fronds, discs, and segments. This transition was possible, in part, because of an equally sudden transition of ocean waters from anoxic (oxygen-deprived) to oxic conditions.
Geological Contributors A combination of two geological events and two biological events explains the sudden rise of oxygen in Earth’s atmosphere and dissolved oxygen in Earth’s oceans. One of the geological events was the Gaskiers glaciation. The other was the Great Unconformity (GU).
The Gaskiers glaciation lasted from 582 to 580 million years ago. The melting away of vast continental ice sheets that followed the Gaskiers glaciation delivered huge quantities of phosphorus and other nutrients into the oceans. This infusion ignited a multiplication of photosynthetic marine organisms. These organisms gave rise to the Neoproterozoic Oxygenation Event (NOE), resulting in sufficient dissolved oxygen in the oceans to support the Avalon explosion animals.
The melting of the Gaskiers ice sheets was accompanied by the GU. The GU was a worldwide geological upheaval that caused massive landslides to deposit enormous quantities of continental material into the oceans. This deposition created vast continental shelves off the shores of nearly all the continents that generated relatively shallow ocean environments. It also contributed to the influx of nutrients into these continental shelf environments and, thereby, contributed to the multiplication of photosynthetic marine organisms. Since oxygen is a byproduct of photosynthesis, this influx of organisms resulted in the NOE.
Biological Contributors The first biological event making a contribution to the NOE was the development of fungi-lichen ecosystems on the continental landmasses. These ecosystems formed organic-rich upper soil layers. These layers restricted the consumption of atmospheric oxygen required to weather the subsoil regolith.1
The second biological event that made a major contribution to the NOE was the origin of sponges. Sponges range in size from microscopic to a half meter in length. These creatures greatly enhance the living space over which photosynthetic bacteria and algae can thrive. Therefore, they played an important role in the multiplication of photosynthetic (oxygen-producing) marine organisms.
Some sponges, especially the smallest ones, are able to tolerate low levels of marine dissolved oxygen. For at least a major part of their life cycle, these sponges can survive at atmospheric oxygen levels as low as 0.1–0.8%,2 compared with 21% today. Hence, tiny sponges likely were already contributing to a rise in marine dissolved oxygen even before the melting of the Gaskiers ice sheets and the GU. From 575 million years ago until the extinction of nearly all the Avalon fauna that occurred just a million years before the Cambrian explosion, the proliferation of sponges of increasing body sizes contributed to the rise of marine dissolved oxygen. Would it be enough to support the Cambrian animals?
Cambrian Oxygenation The Cambrian fauna required much higher marine dissolved oxygen levels than the Avalon fauna. Unlike the Avalon fauna, most of the Cambrian fauna inhabited the seafloor bottoms of continental shelves. These seafloor bottoms typically have a much lower level of dissolved oxygen than the ocean surfaces. Only recently have marine scientists solved the problem of accounting for the origin of the greater oxygen quantity and the transfer of dissolved oxygen from sea surfaces to seafloor bottoms.
The mass extinction event that wiped out the Avalon fauna greatly increased continental weathering. This enhanced weathering increased both organic carbon burial and nutrient delivery in the oceans. The outcome was more dissolved oxygen in the oceans. However, increased continental weathering cannot explain the transfer of dissolved oxygen from sea surfaces to seafloor bottoms.
Sponges came into play in a completely unexpected way. Sponges are filter feeders, meaning that they strain nutrients from water. Researchers have learned thatthey removed large amounts of dissolved and fine particulate organic carbon from seawater at depths relatively close to the surface. In doing so, sponges shifted respiratory oxygen demand to greater depths.3 The resultant increase in bottom seawater dissolved oxygen led to greater phosphorus burial and greater sequestration of phosphorus by sponge microbial symbionts.4 This phosphorus burial and sequestration lowered oxygen demands in bottom seawater and, hence, led to higher dissolved oxygen levels on seafloor bottoms.
Affirmation of Sponges’ Crucial Role The impact of sponges on seafloor bottom oxygenation depends on their abundance. Sponge spicules (structural elements) do not preserve well. Therefore, sponge abundance cannot be directly obtained from the fossil record. However, geochemists have shown that siliceous sponges manifest distinct silicon isotope ratios.5 Isotopes help provide a chemical signature where fossilization is absent.
A team led by Michael Tatzel employed stable silicon isotope data to establish that there was a large expansion of siliceous sponge abundance over the Ediacaran-Cambrian transition.6 Tatzel’s team affirmed the crucial role played by sponges in seafloor bottom oxygenation through using germanium/silicon and yttrium/holmium isotope ratios to determine the amounts of dissolved organic carbon in various seawater environments. Molybdenum isotope measurements made by a team of seven geochemists led by Yuntao Ye provided additional confirmation.7
Design Implications Thanks to the work of several independent research teams, it is now established that a two-step sudden rise in atmospheric and dissolved marine oxygen coincided with and made possible the Avalon and Cambrian explosions of animal life. Multiple simultaneous events operated together to produce each of these jumps in oxygen abundance. These events included extraordinary and highly fine-tuned geological episodes as well as extraordinary and highly fine-tuned creations of new life-forms.
As I demonstrated in last week’s article8 and in my book Improbable Planet,9 the Avalon and Cambrian explosions of animal life show just how intractable naturalistic explanations for life’s history really are. These remarkable events in Earth’s history also show how crucial their fine-tuned features and timing are to make the later appearance of human beings possible.
Endnotes
Lee R. Kump, “Hypothesized Link between Neoproterozoic Greening of the Land Surface and the Establishment of an Oxygen-Rich Atmosphere,” Proceedings of the National Academy of Sciences USA 111, no. 39 (September 15, 2014): 14062–14065, doi:10.1073/pnas.1321496111.
Daniel B. Mills et al., “Oxygen Requirements of the Earliest Animals,” Proceedings of the National Academy of Sciences USA 111, no. 11 (March 18, 2014): 4168–4172, doi:10.1073/pnas.1400547111.
Timothy M. Lenton et al., “Co-evolution of Eukaryotes and Ocean Oxygenation in the Neoproterozoic Era,” Nature Geoscience 7 (April 2014): 257–265, doi:10.1038/ngeo2108; Douglas H. Erwin and Sarah Tweedt, “Ecological Drivers of the Ediacaran-Cambrian Diversification of Metazoa,” Evolutionary Ecology 24, no. 5 (July 13, 2011): 417–433, doi:10.1007/s10682-011-9505-7.
Fan Zhang et al., “Phosphorus Sequestration in the Form of Polyphosphate by Microbial Symbionts in Marine Sponges,” Proceedings of the National Academy of Sciences USA 112, no. 14 (April 7, 2015): 4381–4386, doi:10.1073/pnas.1423768112.
Katherine R. Hendry et al., “Deep Ocean Nutrients during the Last Glacial Maximum Deduced from Sponge Silicon Isotopic Compositions,” Earth and Planetary Science Letters 292, nos. 3–4 (April 1, 2010): 290–300, doi:10.1016/j.epsl.2010.02.005; Martin Wille et al., “Silicon Isotopic Fractionation in Marine Sponges: A New Model for Understanding Silicon Isotopic Variations in Sponges,” Earth and Planetary Science Letters 292, nos. 3–4 (April 1, 2010): 281–289, doi:10.1016/j.epsl.2010.01.036.
Michael Tatzel et al., “Late Neoproterozoic Seawater Oxygenation by Siliceous Sponges,” Nature Communications 8 (September 20, 2017): id. 621, doi:10.1038/s41467-017-00586-5.
Yuntao Ye et al., “Tracking the Evolution of Seawater Mo Isotopes through the Ediacaran-Cambrian Transition,” Precambrian Research 350 (November 2020): id. 105929, doi:10.1016/j.precamres.2020.105929.
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