First of all any biological life form not from this planet is deemed as alien, even simple microbes that may be on Mars due to the detection of methane in which biological processes is one of 2 known methods to producing methane. The second being geological activity.
And if there's bacterial life, there is of course evolution where random selection allows for different traits to come forth, and those who survive are the ones who best adapt to their environments where we get eyes, noses, ears and of course brains.
And of course you mean aliens, you mean flying saucers. Well let's look at that, shall we?
An expert on evolution theorizes that any Earth-like planets will produce Earth-like lifeforms which includes fish, grass, trees, canines, felines, equestrians, and of course primates.
And through evolution, intelligent animals will come forth that uses tools and learn the secrets of fire. This then of course leads to the wheel, clothing, hunting, cultivation, and so forth where eventually they will become curious about those white dots in the night sky.
There will be religion and wars, pollution and governments, believers and sceptics, and so forth. They made discoveries that both benefit their people or brings their people to the brink of annihilation. They may have their own Alcubierre who discovered warp drive, or someone else who discovered the elusive tachyon -- who really knows.