system
Given the following post, respond as if you're commenting on Reddit:
human
How did sea mammals evolve to have a blow hole in the top of their heads?

I understand the loss of legs or the development of fins in place of feet, but I don’t understand how a new physical feature can manifest itself without an example to evolve out of.
gpt
The other posts are correct, and blow holes are absolutely nostrils moved to a new location that aids survival in the sea.

But I'd like to address your other statement - how can a new feature develop if it never existed before?

Eyes are a prime example of this. 

Cells react to light because light causes chemical reactions. They can't NOT react to light. Following or avoiding light is very useful - to find food or to avoid predators, to get to where things are better or away from things that are dangerous.

Over time, some cells do better than others, or are in the right place, and if that is useful for survival, mutations favor creatures with more useful light-reacting cells. This is the beginning of a retina.

Funny trick of physics: a pit is a great way to gather light, it is also a great way to determine the direction of light - like a light compass. Very useful. Mutations that make a pit around such  light-sensitive, proto-retinal cells give a survival advantage.

Deeper pits are even better, and if they get so deep that they leave just a tiny pinhole, well, you have what amounts to a 'pinhole camera' going on, and that allows imaging. Very, very useful, and even more survival advantage.

But pits can get stuff trapped in them. The animals who have transparent membranes over the pits - bits of skin that let light through - do better, so now you have a ball with a lens. The beginning of an eye. 

Keep following this basic concept and, given hundreds of millions of years, you have eyes. From nothing more than skin cells.

That is how new features can develop 'from nothing'.