Fridge Brilliance: The two main humans (in Series 1-4) are Thomas (Crimp) and Richard (Hope). That's Tom, Dick and (old) Harry.
In the last episode of Season 1, the Professor asks God if humans are the only intelligent species, and God laughs at him, saying "YOU REALLY THINK YOU WERE THE BEST THAT I COULD DO?". While we never actually see any aliens in the series, we do meet at least 4 sentient nonhuman species: angels, demons, dolphins, and a sentient computer.
In season 7, it is confirmed that there is no aging in either Heaven or Hell, and Satan mentions that many of the damned are old, senile and have no real understanding of where they are. A Story Arc in that series concerns a baby wrongly assigned to Hell (which should never happen because Children Are Innocent). Given how prevalent child mortality used to be, even within living memory, it seems likely that babies and young children make up a large proportion of the population of Heaven.