Dennis Bray 2009. Wetware. A computer in every living cell New Haven: Yale University Press, 267 pp.
An easily read and convincing case that AI will not be easily achieved, because the hardware of our own conciousness (cells - “wetware”) have many functions that are not present in the electronic (digital) circuits of our computers. Cells are each a computer, and they learn (reprogram) themselves. Proteins and especially enzymes function as transistors, and the “circuitry” is not comprised of fixed inflexible wires but are the compounds in the cell cytoplasm that become functional depending on the state of the other components (proteins). Architecture massively in parallel. And evolution has equipped DNA with the instructions.
Plenty of fascinating history, especially Grey and Vivian Walter who collaborated to create very simple robots with valve circuitry that, despite their simplicity, exhibited complex behaviour and were even able to learn. Also plenty of fascinating natural history about Amoeba, Stentor etc.