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Notes -
Apart from what other users have brought up, there's also the fact that experiments in multicellularity appear very early on in the fossil record. Our oldest evidence for it consists of macrofossils that were discovered in the Franceville basin in current-day Gabon, in what would have been a shallow oxygenated delta at the time, and which have been dubbed the "Francevillian biota" or "Gabonionta". They are dated to 2.1 Ga, in the early Paleoproterozoic.
The emergence of this biota follows the Great Oxidation Event approx 2.4-2.1 Ga, an event where cyanobacteria caused a mass extinction by producing oxygen, something which is toxic to many anaerobes. The interaction of free oxygen with cellular components produces an oxygen radical called a "superoxide anion" which is capable of triggering a chain of destructive reactions in the cell. Aerobes are only capable of withstanding this because they possess enzymes called superoxide dismutase which essentially "neutralise" the superoxide anion (and if exposed to too much oxygen can still experience hyperoxia).
Before then, Earth had a reducing atmosphere practically free of oxygen, and the GOE changed the environment into an oxidising atmosphere, with oxygen levels being as high as 10% of their present atmospheric level by the end of the GOE. And it also seems that oxygenation is a factor which is a prerequisite for the development of large multicellular organisms. Only aerobic respiration can produce enough energy for a complex metabolism, and although there are some exceptions, few multicellular life forms are anaerobic.
The Francevillian biota are surprisingly complex considering how early they appear. There are a number of forms the fossils take. Some look like elongated pearl-strings that end in a "flower". Others look like really bulbous nipples. They exhibit patterns of growth determined from the fossil morphologies that are suggestive of intercellular signalling and thus of mutually synchronised responses that are the hallmarks of multicellular organisation, and there's also evidence that they were capable of moving around in search of food resources - there are string-like tracks at the site which might represent mucus trails.
A particularly striking feature of the Francevillian biota is that they are isolated in time. No structures similar to them are known from earlier times and the biota are conspicuously absent from the overlying layer of black shale. It is notable that their disappearance also seems to roughly correlate with an occurrence called the Shunga event. What caused it hasn't been conclusively pinned down, but it involves the creation of one of the oldest known petroleum deposits on Earth, indicating the demise of a massive primitive biomass. The Shunga reserves in the Lake Onega region of Russia alone preserve up to 25 × 10^11 tonnes of organic carbon, and deposits of about the same age and having similar carbon isotope chemistry have been found elsewhere in northwest Russia, as well as North America, Greenland and West Africa, indicating that this was a global event. The organic blooms associated with the Great Oxidation Event abruptly cease, and oxygen levels drop back down to pre-GOE levels.
In short, these fossils seem to represent a first experiment in megascopic multicellularity that arose during a period of oxygenation and subsequently died off when the environment shifted against them. This seems to indicate that multicellularity can start developing relatively quickly, and part of the reason why there was a delay is because the first experiments in multicellularity were abruptly stopped in their tracks.
Which raises the question as to what would've happened had the extinction not occurred. This was a very crucial point in the evolution of life and small changes in the initial state of a system can lead to huge downstream ramifications, so how different would life be today if they had been able to develop?
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