Is extraterrestrial organic matter relevant to the origin of life on Earth?

I review the relative importance of internal and external sources of prebiotic molecules on Earth at the time of life's origin approximately 3.7 Gyr ago. The efficiency of synthesis in the Earth's atmosphere was critically dependent on its oxidation state. If the early atmosphere was non-reducing and CO2-dominated, external delivery might have been the dominant source. Interplanetary dust grains and micrometeorites currently deliver carbonaceous matter to the Earth's surface at a rate of approximately 3 x 10(5) kg/yr (equivalent to a biomass in approximately 2 Gyr), but this may have been as high as 5 x 10(7) kg/yr (a biomass in only approximately 10 Myr) during the epoch of late bombardment. Much of the incoming material is in the form of chemically inactive kerogens and amorphous carbon; but if the Earth once had a dense (approximately 10-bar) atmosphere, small comets rich in a variety of prebiotic molecules may have been sufficiently air-braked to land non-destructively. Lingering uncertainties regarding the impact history of the Earth and the density and composition of its early atmosphere limit our ability to draw firm conclusions.