"The very satisfying answer is that the gene did exactly everything…

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https://interestingengineering.com/scientists-grow-bigger-monkey-brains-using-human-genes-replicating-evolution
"The very satisfying answer is that the gene did exactly everything that you could have hoped for."

The results can be broken down into four sections. By inserting the gene:

The size of the monkeys' neocortex increased
Folding of the brain, similar to how a human brain is folded was induced
The relevant pro-generative cell type, which produces neurons, was increased
Specifically, it increased upper-layer neurons, which are the neurons that increase during evolution
"This shows, basically, that the gene — and its expression in humans — is sufficient to expand and to fold the primate, or the monkey, brain," co-author of the study, Michael Heide said.
...
After about 100 days after the fetus had been growing, the international team unanimously agreed to remove the fetus through a C-section. Bringing a "new human-gene-influenced monkey into this world would step over the ethical line," said Huttner. "