More than 570 million years ago, in the Ediacaran period, complex organisms including soft-bodied animals up to a meter long made their appearance in deep ocean waters.
Origin of complex animals in deep oceans explained by stability of ambient temperature - life short science news

How could complex animal life have started in the deep oceans, where food and oxygen are scarce? Scientists think they have found the answer: because of the stable temperature there.

Early complex animals could not regulate their body temperature themselves, but depended on ambient temperature. In a world with low oxygen, they therefore could not have survived the many temperature swings of up to 10 °C in shallow waters.

Stable temperatures were thus necessary for complex animals to evolve, and this was only found in the deep oceans.

Read the full story: Stanford University
Scientific publication: Proceedings of the Royal Society B