Cartesianism is not equal to determinism, so your first sentence is a non sequitur.
The brain is a physical object. It behaves according to the laws of physics. Quantum physics is not deterministic, so the brain is not either, but that non-determinism is in the form of randomness and so indistinguishable from a deterministic system with really good pseudo-random number generators. Physical limitations such as the forced quantization of information in the real universe resulting from the Bekenstein bound mean that the brain is equivalent to a discrete computational device (i.e. it can be simulated on a powerful enough computer), and not super-Turing (Penrose was wrong, and his arguments to the contrary have been exhaustively refuted). We are just working out details, but the needed raw computational power is less than a couple of decades away.
The brain is not highly optimized by evolution for the exact reason of its complexity. Evolution is an optimization algorithm and something as complex as the brain could have only been partially optimized in the relatively short time its been around. Only the simplest processes in living organisms, interacellular chemical processes, are highly optimized (and a few are completely optimal, in the sense that no other setup of the process can be more effective in whatever the particular function), since short life cycles and length of existence of single-celled organisms has allowed exploration of essentially all possible configurations. Not so for anything multicellular.
Humans have been to the moon and use bacteria, but when humanity no longer exists bacteria will still be around. We are extremely fragile and not very good at inhospitable environments or very drastic environmental changes. We are also completely reliant, as is the rest of the biosphere, on bacteria to complete the nitrogen cycle, and algae for the majority of the photosynthetic oxygen recovery. Considering how poorly we are doing in attempts to control the environment, and how little attention space gets (and is likely to get in the future, with the exception of weaponization issues), it's extremely, unrealistically optimistic to think humanity will survive another thousand years, whereas the bacteria will remain until the sun runs out of fusion fuel.
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