Interesting point, but which do you think is a more reasonable prior: that intelligence is a deterministic process similar other's we've encountered or that there is something uniquely different about intelligence than other physical phenomena? I think the latter requires more assumptions so I would argue philosophically it is the one that requires the proof or more evidence :)
Regardless, my feeling is that there is a healthy dose of human hubris around intelligence. If I train a dog to go fetch me a beer from the fridge, that seems pretty smart. It learned how to understand a request, execute a complex sequence of actions for motion and planning, reason around occluded objects, understand depth and 3 dimensional space, differentiate between objects, and more without me writing a sequence of rules to follow. I'd be happy to have a robot that intelligent. Plants dont have brains or neurons yet react to sensory stimulus such as light touch or sound, communicate, and even have memory and learn. It's not at a scale to do something interesting within one plant but communities of plants are arguably the most successful organisms on the planet.
Andrew Ng likes to point to a Ferret experiment [0] where experimental neuroscientists rewired the visual inputs to the auditory cortex and the auditory cortex learned to "see" the visual signals! This suggests that there may be some amount of unified "learning" rules to the brain. Biology is never so clean but if humans have whatever this intelligence thing is that lesser organisms do not, there is another angle to look at things. We have a lot of neurons which suggests less per neuron specialization than say a C. elegans; basically large populations of neurons perform tasks that in lesser creatures single or few neurons may perform. While the trees are complex and important to understand for biology and medicine, the forest may have some high level rules.
Looking at something that appeared intelligent 50-100 years ago but seems mechanical now, we have text to speech. NETtalk was a simplified computational neuroscience model from the 80s that could synthesize human speech. Today we have far better quality techniques that came out of R&D focused on things like large high quality labeled datasets for training, better soundcards, more processing power, and algorithmic improvements. Researchers didn't continue trying to model the brain and instead threw an HMM and a couple other tricks at it. Now we're going full circle back to neural networks but they aren't using any advances from biology and certainly arent produced by computational neuroscientists like Terry Sejnowski.
It's funny because at the time of NETtalk they thought that learning to read would be an extremely hard problem because it incorporates so many components of the human brain [1]. While it certainly wasn't a trivial problem, state of the art OCR and object recognition came from similar artificial neural networks a decade later with LeNet and MNIST * . And no, ANNs != biological neuronal networks. The models of computational neuroscientists are different; for example look at [2, 3] for high level models or [4] for a tool.
Now I'm even more convinced than before that understanding the brain is great for humanity but wont be necessary for building intelligent systems that can perform tasks similar to biological ones.
[0] http://www.nature.com/nature/journal/v404/n6780/full/404871a...
[1] https://en.wikipedia.org/wiki/NETtalk_(artificial_neural_net...
[2] http://science.sciencemag.org/content/338/6111/1202
[3] http://ganguli-gang.stanford.edu/pdf/InvModelTheory.pdf
[4] http://neuralensemble.org/docs/PyNN/index.html
* Perhaps you could argue that convolutions are loosely inspired by neuron structure, but that sort of knowledge had existed for quite some time, with inspiration arguably within Camillo Golgi's amazing neuronal physiology diagrams from the 1870s let alone the 1960-80s. It's telling that papers on CNNs have little to no neuroscience and a lot of applied math :)