Property

Land existed long before humans came along and it will probably exist long after humans have gone the way of the dodo. But humans have put some fences and said: this is MINE.

Property is really the right to use something.
Property is an acknowledgement of this right by society. That land is yours only because people acknowledge it, it's a social agreement. If another country would conquer yours, they could take your "property" because they don't acknowledge it.

The existence of property leads to social inequalities, as some people will inevitably gain more. The more you own, the more you can gain, that is why the rich become richer. Since resources are limited, the more one owns, the less others own, so the poor become poorer.

We are at the point where 1% own 40% of the planet's wealth, 10% own 85% of the plant's wealth and 50% own 1% of the planet's wealth. (source)

In an extreme situation, a single human can own the entire planet. While this is extreme, this is theoretically possible. What if one human owns all the arable land in the world and he decides to grow biomass crops on it (for biofuel)? Will we respect his property and starve?

Nobody really owns anything. We don't even own ourselves, we just borrow atoms to build our bodies and we give them back when we die. Our atoms were in other beings before us and they will go on to form other beings.