According to scientists’ best estimates, about 2.9 billion years then passed between the first life on earth and the first multicellular life.[33] Another 500 million years passed before animals walked on land, and 200 million more before the first mammals appeared.[34] Focusing on the brain, the length of time between the first development of primitive nerve nets and the emergence of the earliest centralized, tripartite brain was somewhere over 100 million years.[35] The first basic neocortex didn’t appear for another 350 million to 400 million years, and it took another 200 million years or
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