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I think adding lanugage produced something like a quantum leap for the mind and that there's no similar quantum leap for other organ's like the human heart. The quantum leap means that other parts have to adapt and optimize for now language being a major factor.
You could look at IQ.
The mental difference between a human at IQ 70 and a human at IQ 130 is vast. Intelligence is also highly heritable. With a few hundred thousand years and a decent amount of evolutionary pressure on stronger intelligence you wouldn't have many low IQ people anymore.