That 3rd section, 'Beyond Infinity' (now there's a paradox, if ever I read one), is just a total trip... surreal and mind boggling.
That black slab seems very significant, it's like a transmitter of consciousness, with perhaps some evil purpose... it turned ignorant ape men into vicious killers and wielders of weapons, leading them to evolve into Homo Sapiens and eventually into the modern man that was out to qonquer the universe. HAL 9000 was able to spot it and it tried its damnedest to thwart the mission and so protect itself, even at the cost of the crew.
The very end, shows the guy had been bestowed with so much conscious energy that he was able to actually experience the very end of his life and the very beginning of his next, even though he was (probably) dead after being flung through a wormhole.
In any case, even though the film is showing its age now, it's still a brilliant work of art and totally original and awe inspiring, especially when you consider that it was made 35 years ago and, without any CGI.