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Until recently, most scientists held that true cognition arrived with the first brains half a billion years ago. Without intricate clusters of neurons, behavior was merely a kind of reflex. But Levin and several other researchers believe otherwise. He doesn’t deny that brains are awesome, paragons of computational speed and power. But he sees the differences between cell clumps and brains as ones of degree, not kind. In fact, Levin suspects that cognition probably evolved as cells started to collaborate to carry out the incredibly difficult task of building complex organisms and then got souped-up into brains to allow animals to move and think faster.
scientificamerican.com
Brains Are Not Required When It Comes to Thinking and Solving Problems--Simple Cells Can Do It
Tiny clumps of cells show basic cognitive abilities, and some animals can remember things after losing their head