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2.7 The Oxygen Availability Hypothesis

This theory suggests that the oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life. This hypothesis is based on the correlation between the emergence of multicellular life and the increase of oxygen levels during this time. This would have taken place after the Great Oxidation Event (GOE ) but before the most recent rise in oxygen. Mills concludes that the amount of oxygen present during the Ediacaran is not necessary for complex life and therefore is unlikely to have been the driving factor for the origin of multicellularity.

* Include image here of pO2 over geo time scale?

2.8 Snowball Earth Hypothesis

A snowball Earth is a geological event where the entire surface of the Earth is covered in snow and ice. The most recent snowball Earth took place during the Cryogenian period and consisted of two global glaciation events known as the Sturtian and Marinoan glaciations. Xiao suggests that the between the period of time known as the "Boring Billion" and the Snowball Earth, simple life could have had time to innovate and evolve which could later lead to the evolution of multicellularity. The snowball Earth hypothesis in regards to multicellularity proposes that the Cyrogenian period in Earth history could have been the catalyst for the evolution of complex multicellular life. Brocks suggests that the time between the Sturtian Glacian and the more recent Marinoan Glacian allowed for planktonic algae to dominate the seas making way for rapid diversity of life for both plant and animal lineages. Shortly after the Marinoan, complex life quickly emerged and diversified in what is known as the Cambrian Explosion.

2.9 Predation Hypothesis

The Predation Hypothesis suggests that in order to avoid being eaten by predators, simple single-celled organisms evolved multicellularity to make it harder to be consumed as prey. Herron et al performed artificial evolution experiments on the single-celled green algae, C. reinhardtii, using paramecium as a predator. They found that in the presence of this predator, C. reinhardtii does indeed evolve simple multicellular features.

Refs:

Brocks JJ (2017) The rise of algae in Cryogenian oceans and the emergence of animals. Nature

Herron MD (2019) De novo origins of multicellularity in response to predation. Nature

Knoll AH (1992) The early evolution of eukaryotes: A geological perspective. Science 256(5057):622–627

Mills DB (2014) Oxygen requirements of the earliest animals. PNAS

Nursall JR (1959) Oxygen as a prerequisite to the origin of the Metazoa. Nature 183(4669):1170–1172

Xiao S (2018) After the boring billion and before the freezing millions: evolutionary patterns and innovations in the Tonian Period. Emerging Topics in Life Sciences