Thousands of years?

We've only had the tech to be able to research this in some technical depth for a few decades (both scale of computation and genetics / imaging techniques).

And then we discover that DNA in (not only brain) cells are ideal quantum computers, DNA's reactions generate coherent light (as in lasers) used to communicate between cells and single dendrite of cerebral cortex' neuron can compute at the very least a XOR function which requires at least 9 coefficients and one hidden layer. Neurons have from one-two to dozens of thousands of dendrites.

Even skin cells exchange information in neuron-like manner, including using light, albeit thousands times slower.

This switches complexity of human brain to "86 billions quantum computers operating thousands of small neural networks, exchanging information by lasers-based optical channels."