Chorionic gonadotropin (CG) first evolved in Aotus and Callicebus, Anthropoid primates. This generated hemochorial placentation, which replaced epitheliochorial placentation. The first CG was close to neutral, pI=6.3, with a circulating ½-life of 2.4 hours. Hemochorial placentation was more efficient leading to brain growth genes promoting brain size in Aotus and Callicebus from 0.07% (in Lemur) to 0.17% of body mass. Then with evolution, the baboon developed a more acidic hCG, this had a longer circulating ½-life, which led to a more efficient hemochorial placentation, which led to brain growth genes promoting the brain size, from 0.17% of body mass to 0.49% of body mass. Then with the evolution of the orangutan, it developed a more acidic CG, pI=4.9, which had a circulating ½-life of 6 hours, which led to more efficient hemochorial placentation, which led through brain growth genes to a brain size of 0.74% of body mass. Then Homo habilis had an even more acidic form of CG, which led to even more efficient hemochorial placentation, which led through brain growth genes to an even bigger brain size of 1.2% of body mass. Finally, humans made a super-acidic form hCG, pI=3.5, which led to super-efficient hemochorial placentation, which led to brain growth genes making a massive brain of 2.4% of body mass. Other factors like bipedalism, fingers and tools all developed with advancing brain size. This is the exact evolutionary pathway of humans.
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