Humans are derived from arboreal shrew-like animals that survived the cataclysm which wiped out the larger dinosaurs. As far as I am aware this series of events is entirely the result of chance. Would being marsupials have adversely affected the development of heuristics, tool use, civilization, etc?
Most likely, it's completely random chance. Marsupials today have most things required to develop into the dominant life form, had they just had more pressure for intelligence development. I haven't read that much on how many of them that passes advanced intelligence tests; either we haven't tested them extensively or they just never developed higher cognitive abilities, which would be the requirement for starting to use tools and, later on, become the dominant life force.
And, as John pointed out as I wrote this answer, they deliver their young very early on and then carries them in a pouch, which would be highly advantageous for brain development (provided that the pouch is not getting hit or shaken too hard).
Some things which have been helping us develop our intelligence, which marsupials largely lack:
Pack hunting and spatial awareness - our language development due to hunting/gathering have greatly enhanced our intelligence. We have the ability to remember and to describe for other individuals a location with resources, something we share with Corvidae, bees and ants but not that many other animals.
Very high hand-to-eye coordination - our ability to toss things with high precision have greatly helped us to hunt and have been a great way to defend ourselves with. Many other animals can toss items fairly well, but not as well as humans.
A sense of music - I can't find the source at the moment, but I read an interesting thesis about the development of our singing, which theorized that we might have a lot to thank for our sense of harmonies. Some other primates have a similar, but far less developed, sense for harmonies and often also "sing". This has, according to the thesis I read, been used as a weapon, where we have managed to create fear and chaos among other animals. We apparently used it both for hunting and defense against predators. The difference between just screaming and "singing" (it really wasn't what we consider singing by modern standards) is that when done in harmonies, it creates constructive interference which greatly enhances the effect.
Extreme stamina in hot weather - our lack of fur and our sweat-glands have been of great help in our hunting as we could simply tire animals out by jogging after them until they collapse. No other animal comes close in long distance running capabilities (although many migratory birds show similar stamina while flying, but they do not fly in as hot temperatures as our ancestors have been running).
These points have likely been among the reason why it was us and not marsupials that developed into the dominant intelligence - although it is mainly chance that gave it to us.
If anything marsupials would have a slight advantage, a large brained baby does not risk tearing a marsupial mother a new one.
Marsupials have evolved their own version of a large number of placental mammals so their own primates is not that much of a stretch.
It's my belief that hands are a prerequisite for developing civilisation. For us, it started when monkeys took to the trees, which required grasping limbs (and good coordination). Much later something forced our immediate ancestors out of the trees and out onto the plains, and we developed the ability to walk and run on two feet, and to hunt using sticks and stones instead of claws and teeth.
Certain marsupials demonstrate what may be the only other path to evolving hands. Kangaroos and wallabies have evolved effective three-limbed locomotion (two legs, a powerfully muscular tail, a unique hopping gait). Unfortunately they aren't bright enough to do much with their freed-up forelimbs, but who knows what another ten million years of evolution might accomplish?
If there is a disadvantage to a built-in child carrier I can't see it. As has already been pointed out, marsupial mothers won't ever pay the terrible price for a bigger-brained child that many human mothers did in the past, and some still do today even with modern medical help.
Edit -I should also have mentioned tree kangaroos. These seem to be following the same evolutionary path as monkeys. Up into the trees and developing grasping hands and enormously long tails. Most are now threatened or endangered, because we are cutting their trees down.