Final answer:
The increasing thickness of the Earth's atmosphere could have allowed for the arrival of basic organic molecules from space through impacts of meteorites, comets, and asteroids, as well as the formation of the planet and the contribution from interstellar clouds.
Step-by-step explanation:
The increasing thickness of the Earth's atmosphere could have allowed for the arrival of basic organic molecules from space through several mechanisms. One possibility is that meteorites, comets, and asteroids containing organic molecules impacted the Earth. These extraterrestrial objects would have brought the organic molecules with them during their fiery impacts. Another hypothesis suggests that organic molecules could have originated in interstellar clouds and became part of the Earth as the planet formed. Therefore, the thickness of the Earth's atmosphere could have facilitated the delivery of basic organic molecules from space.