The title of this post is a bit misleading. Ranma is human (born of a human mother and father) but he is still alien. How so? He is so different from regular humans (even in his own universe not just real life) that he may as well be an alien. Heck aliens on other shows like Star Trek and Sailor moon are closer to being human than he is.
One thing that sets Ranma apart from normal baseline humans is his super human abilities. Some examples of his super human abilities are: super strength (lifts multiple tons, bend steel pipe), toughness (hits wall hard enough to leave massive creator gets up like nothing happened), speed (moves faster than the human eye can track when he wants to), abilities (can scratch his own mid back with his foot, cling to ceilings, can cut things with his bare hands - cut perfect hole in Ryoga's house bare handed), stamina (swam the sea of Japan), energy manipulation (ki blasts), danger sense (senses danger before it occurs), incredibly fast healing rate, etc. We are shown that these abilities are very uncommon in the Ranmaverse several times. Some examples of people being surprised by the martial artists displays of super human abilities are: School kids surprised girl form Ranma can lift and jump with Ryoga's extremely heavy umbrella , school kids thinking that a mere three story fall would kill Ranma, the martial arts ice skaters surprised that Ranma survived the attack much less was able to continue fighting, people running in fear when Happosai fights Ranma or when Happosai and Genma made themselves huge, Kodachi, Akane, and Asuka surprised at the speed Ranma heals, etc., I can list more.
Another thing that sets Ranma apart from the baseline humans of his universe is that Ranma has developed, because of his upbringing, quite a few abilities/habits that could be considered primitive or sub-human. When his fear of cats becomes too strong he acts like a cat (a super cat that can cut through basically anything with its claws and used a multi-ton shark as a chew toy). He has been shown to howl at the moon (did this in battle dogi story - arc surprisingly he wasn't in neko-ken - Kasumi said that at least it was scaring the cats away), yells into the night, sleeps in trees, eating while crouching on the table, using his feet to grip/hold things (we've seen him hanging upside down from his toes, write with his feet, scratch the back of his head/mid-back with his feet while reading, and feed himself with his feet in manga), jumping up and clinging to the ceiling when surprised, lack of modesty (such as him not caring where and who sees him nude), etc.
There are other things as well that set Ranma apart from normal humans. Ranma often sits in weird positions (meditating upside down, sitting on someone's head, hanging from the ceiling). Strange, at least to most people, thought patterns or seeing things that aren't normal as being normal. Ranma is likely to consider a lot of things as being normal that aren't, such as people trying to kill him, people kidnapping people to get stuff, and having a large number of nightmares. Inability to understand regular peoples view points because it is so different from his own (for instance he has a very different view on females than guys his age), etc.
If the Ranma series had continued, Ranma would have become further and further away from normal humans. My reasoning is fairly simple. As Ranma's knowledge of martial arts, ki, and magic grows he would grow further and further from baseline humans. Before the manga started he didn't even know about magic by the end of the manga Ranma has used magical items more than a dozen times and has had magic used on him even more times than that. It's only a matter of time until he actively starts learning how to use and control magic if for no other reason than self defense. He also has been seeking magical items to cure his curse and for power (Dogi). Ranma's knowledge and ki abilities have grown massively as well, at the start of the manga Ranma did not know how to project his ki at all. As the series progressed he learned new techniques and then improved every single one. We never see him do techniques the same way as he did them before every time we saw a technique it was a new variation (Shi Shi Hadoken became Moko Takabisha became Moko Takabisha double). In all probability Ranma would continue to do so.