I guess if you really wanted to start from scratch, you could figure out how to evolve the whole system from a single cell or something like that. In some ways neural networks have kind of evolved in that way, assisted by humans. They started with a single perceptron, and have gone all the way to deep learning and convolutional networks
I also remember a long time ago studying genetic and evolutionary algorithms, but they were pretty basic in terms of what they could learn and do, compared to modern LLMs
Although recently I saw some research in which they were applying essentially genetic algorithms to merge model weights and produce models with new/evolved capabilities