Miller and Urey chose the specific gases for their experiment based on what was thought to be present in the early Earth's atmosphere. Their goal was to simulate the conditions of the early Earth and see if organic molecules, the building blocks of life, could spontaneously form.

Here's why they used those specific gases:

* Hydrogen (H2): Hydrogen was thought to be a major component of the early Earth's atmosphere, and it's a fundamental building block for many organic molecules.

* Methane (CH4): Methane was included as a potential source of carbon, a key element for organic molecules.

* Ammonia (NH3): Ammonia is a source of nitrogen, another essential component of life.

* Water vapor (H2O): Water was known to be present on the early Earth and plays a crucial role in many chemical reactions.

It's important to note that this "primordial soup" theory of the early Earth's atmosphere has been challenged in recent years. Scientists now believe that the early atmosphere may have been significantly different, with less methane and ammonia than originally thought. However, Miller and Urey's experiment remains a landmark achievement in our understanding of the origins of life, demonstrating that organic molecules can be formed from inorganic materials under specific conditions.