Science is still unable to explain why, when and how humans became humans. What changed in our genetic database that created the modern human, Homo sapiens sapiens? Now, research has unearthed a clue that might be part of the solution: our intelligence might be the result of a mutation that resulted in an extra supply of a signalling neurochemical called dopamine in several brain regions that help us think and plan. Our brains produce far more dopamine in these regions than the brains of other primates like apes.
When scientists measured the activity of individual genes in tissue samples from six humans, five chimpanzees and five macaque monkeys, they found elevated activities of two enzymes that make dopamine - tyrosine hydroxylase and DOPA decarboxylase - in two parts of the human brain, both vital for higher-level thought.
The researchers found that 1.5 per cent of the neurons in the human striatum, an important part of the brain, were making dopamine, three times more than in the ape striatum. Likewise, they accounted for 0.2 per cent neurons in the neocortex, versus none at all in apes. What's more, the extra dopamine in these regions was made almost exclusively by brain cells called interneurons. These form local connections, rather than linking distant parts of the brain.
What did cause the mutation or mutations that resulted in elevated levels of dopamine? Nobody is quite sure, but I remember, now totally discarded, visionaries like Immanuel Velikovsky and Alfred di Grazia. Their proposition was that radiation created the mutations that changed humanoids into humans.
 Souza et al: Molecular and cellular reorganization of neural circuits in the human lineage in Science - 2017