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> A theory about multicellularity, low temperature that is steady in a smal l range:
> 
> https://cosmosmagazine.com/biology/temperature-the-key-to-multicellular-evolution
> 
> "Researchers at Stanford University in the US have a theory about why multicellular life first developed deep in the ocean: the temperature. 
> 
> "Working with sea anemones, researchers confirm that the organisms, which are similar to the world’s earliest complex lifeforms, are adapted to survival in a low-oxygen environment that changes only within a narrow temperature range. 
> 
> "The finding provides insight into one the most puzzling aspects of life on Earth. After billions of years during which all organisms on the planet were unicellular and microscopic, an explosion of complex marine soft-bodied organisms, up to a metre long, suddenly appears in the fossil record.
> 
> "The organisms are known collectively as the Edicaran biota, and the timing of their emergence is curious. It occurred in a period in which terrestrial temperatures fluctuated wildly, oxygen was in short supply, and even light was restricted. 
> 
> “'The only place where temperatures were consistent was in the deep ocean,” says Erik Sperling, senior author on a new paper, published in the journal Proceedings of the Royal Society B.
>  
> “'That's why animals appeared there.”
> 
> ***
> 
> 
> "Scientists have previously theorised that simple organisms use more oxygen if they are stressed by higher or lower temperatures. Boag and colleagues tested this idea using sea anemones, because in many ways they resemble Ediacaran species. 
> 
> “'We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased,” Boag explained, “because that had been observed in more complex animals like fish and lobsters and crabs.” 
> 
> "They found that, as expected, oxygen use increased when the water temperature dropped below a certain level. 
> 
> "During the Ediacaran period, the deep oceans were oxygen-poor, but their temperatures were very stable – and that, Sperling points out, could have been the most important factor.
> 
> “'Temperatures change much more rapidly on a daily and annual basis in shallow water,” he explains. In deep water, temperatures can vacillate less than one degree Celsius, compared to as much as 10 degrees at the surface. "
> 
> Comment: This is an interesting point about the conditions that  might have allowed the advance to  happen, but really  doesn't tell us why it occurred.