The fundamental force of nature responsible for keeping you on Earth's surface is gravity.

Here's why:

* Gravity is a force of attraction between any two objects with mass. The Earth has a large mass, and so does your body. This means there is a gravitational attraction between you and the Earth.

* This force pulls you towards the center of the Earth. This is why you don't float off into space and why objects fall when you drop them.

While other forces like electromagnetism play a role in holding matter together, gravity is the primary force responsible for keeping you grounded on Earth.