Since we only have one planet that we know of with life, it's a bit difficult to make good estimates on the probability of various events in the history of life. To make a good estimate, you'd want to have thousands of planets very similar to earth to compare. Since we don't have access to that kind of data, one proxy which you can look at is how long did it take for some event to happen in earth's history. In a vacuum you'd expect things that took longer to happen to be rarer. From this point of view, it seems likely that the development of multi-cellular life from simpler life forms is a low probability event because it took 3 billion years of simpler life forms before it happened.
Another way to get information on probabilities is to look at widely separated branches of the tree of life. Since wings evolved separately in birds, bats, insects, and pterosaurs, the probability of wings evolving in a planet with a similar atmosphere and multicellular animals is likely pretty high. Similarly, you can argue that the development of human level intelligence is relatively low probability because it has only developed once in the history of the planet in one lineage that very nearly died out. Dinosaurs never developed human level intelligence, nor did rodents, or mollusks, or arthropods. On the other hand, it seems that a lower level of intelligence at say the dog level is relatively high probability (since it has evolved separately in mollusks, birds, mammals, etc.).
