Complex sentence: The red junglefowl was first domesticated at least five thousand years ago in Asia, since then it has spread around the world, and the domestic form is kept globally as a very productive food source of both meat and eggs.
Split into simpler sentences: The red junglefowl was first domesticated at least five thousand years ago in Asia. Since then it has spread around the world, and the domestic form is kept globally as a very productive food source of both meat and eggs.