LLMs are interest case, but they are nearly flat. Neural system of mammals is structured, as I understand, neocortex consists of about 100 millions structures each ~ 100x400 neurons, something like this. Anyway, just from calculation of human thinking delay appear human NN is just about 500 layers.

Second difference, natural I are feed-forward network, not back propagated as typical AI NN, because natural use some chemical method to "calculate" parameters.

I think, with right structure and some FF method, GAI is already technically achieved.

PS FF NN already exist, but problem that it now using classical method of differential equations which is prohibiting real use, because too much computation need.