I have omitted discussion of the Drake equation here because I'd imagine that anyone reading this is likely familiar with it to the point of eye-rolling. If you aren't familiar with it, this is it:
You can read about it to your heart's content here: Drake Equation Wiki Entry
What is interesting is fi, the fraction of planets that harbor intelligent life, and that's where I want to launch. We live on a planet that belongs to that subset (no cynicism about whether humans are 'intelligent' here, we are, our toddling youth can outsmart the adults of the next most intelligent creatures, chimps/bonobos and dolphins). Lets look at a cladistic diagram that is cascaded from the primates:
It's important to note that the creatures at the top of the cladogram are fully modern. That means that at any point where the lines meet indicating a shared ancestor, a seperate evolutionary chain errupted that ultimately led to hagfish or birds. You could just as easily make a cladogram that was Salamander oriented, with humans a distant cousin of the Salamander.
Lets focus in on humans though and look at our full taxonomic classification. Bear with me; We are Homo Sapiens Sapiens. We belong to the group Homo which contains (Homo) Ergaster, Habilis, Rudolfensis, and Erectus. Genus Homo is contained within Hominidae (aka 'hominids'), which includes our ancestors Australopithecus and Ardipithecus as well as chimpanzee, bonobo, orangutan, and gorilla. Hominids are a sub-group of the Chatarrhini, which also include creatures like gibbons and old world monkeys (macacs, mandrills collobus monkeys, etc). Finally we get to primates, which include all Chatarrhini, tarsiers, lemurs, and the adorable marmoset, which is adorable. Marmosets are adorable.
Now we get to Eutheria! Now, Eutheria is a big group, it represents all placental mammals, and it's important to keep this in mind: the variety you see in Eutheria represents the modern variety. At one point, all animals in this group shared a common ancestor. Who's in Eutheria? Anteaters, elephants, dogs, cats, giraffes, orcas, manatees, hedgehogs, humans, orangutans, nearly all of the familiar mammals.
Not all mammals, we have to move out to the bigger class of Mammals to include the marsupials and monotremes (echidnas and platypuses).
If you aren't sure what an echidna looks like, gaze to your left. I'll move quickly through the remaining orders and families that humans are embedded within. Mammals are part of the group Therapsida, which belongs to the group Synapsida, which belongs to the group Amniota, which are part of the Terrestrial Vertebrates, which belong to Sarcopterygii, which are contained within Gnathostomata, which belongs to all the critters with spines collectively known as Vertebrata, and those are among critters with skulls that enclose their brains known as Craniata, which have spinal chords making them Chordata, and belong to creatures which form asshole-first in the womb, making them Deuterostomia, symmetrical on one axis and thus are Bilateria, all of which can be classified as Animals.
Not done. They belong to a group of creatures that are composed of entirely Eukaryotic cells, making them the Eukaryotes, which finally is contained within all life on Earth, which shares a single common ancestor, who's origins I discussed in part 1.
So what's the point? Each containing group represents a deflection point, a speciation event, somewhere in the distant past. We intelligent humans happen to be hairless upright walking primate placental mammals, and it was some unique set of circumstances that chanced upon our lineage developing the ability to create tall buildings, religion, and fast food.
Anywhere else in the Universe, you will have enormous variety. Recognizable variety, in the sense that certain solutions for the same sorts of problems will always be fairly similar (wings, paddles, feet, eyes, etc), but the solutions will be happened upon by different lineages for different reasons. It is fun to speculate about what sorts of creatures stumbled upon the solution of "devote more biomass to the brain" and "instinctively teach others tricks you learn" (this is critical, chimps for example only learn by watching, they have no impulse to teach another chimp how to extract termites with a blade of grass).
Given how readily life can form (I forgot to mention in part 1 that literally as soon as the Earth had cooled from the cataclysm that formed the moon, life started), and how plentiful planets seem to be (429 exoplanets and counting), and the sheer variety of life on Earth as a precedent for the sort of variety out there, it seems almost certain that, given the time, having a big brain as a survival strategy ought to crop up with much frequency. The galaxy should be crawling with civilizations.
That said, why haven't we heard from them yet? The answer in part 3...