In ancient times? The king.

Maybe not the original owner, but good luck arguing that when the soldiers come to evict you.

Dolphins

If we are applying it to the US, I would say Native Americans. Originally, this law was because land could only be leased for at most 50 years. It could not be sold. That feels appropriate since most , if not all, Native American groups didn't consider the land to be something that could be owned.

A mat of single-celled organisms, that floated on a shallow sea over what is now Nebraska, 3.5 billion years ago.

The sea!

The person who took out the mortgage, though that wasn't the term used back then.