When, in 1990, I found diagnostic shock-metamorphic and shock-melting evidence of an impact origin for the buried Chicxulub structure, I also found secondary mineralization and veins produced by an impact-generated hydrothermal system. Soon thereafter similar alteration was described by other investigators, notably Naumov, McCarville, and Crossey, in other impact craters. Hydrothermal activity seemed to be a widespread consequence of impact heating of hydrous planetary crust.

It was also understood from analyses of Apollo samples that the Earth, at the dawn of life, was being pummeled with impacting asteroids and comets. Zahnle and Sleep showed the largest of those impacts should have vaporized seas, making conditions untenable for life at the surface. Based on my observations at Chicxulub, I proposed those same Hadean impact events produced vast subsurface hydrothermal systems that were perfect crucibles for pre-biotic chemistry and habitats for the early evolution of life; i.e., the impact origin of life hypothesis. Analyses of ribosomal RNA made in the same decade by Woese and Pace indicated the earliest organisms on Earth were thermophilic and hyperthermophilic. It seemed plausible that life originated in an impact crater.

Several features of the impact origin of life hypothesis have since been investigated. A period of bombardment is confirmed; impactors were dominated by asteroids; and additional impact-generated hydrothermal systems have been described; all of which imply Hadean impacts resurfaced the Earth and generated geographically-broad and long-lived hydrothermal systems within craters and possibly in the thickest and hottest impact ejecta blankets. Thermophilic to hyperthermophilic microbial ecosystems have also been detected in the Chicxulub hydrothermal system, demonstrating such systems’ capability to host life. In Chicxulub, the detected ecosystem was dominated by sulfur-metabolizing organisms. More research, however, remains to be done. Chicxulub core samples should be probed further to extract additional clues about the thermal, chemical, and biological evolution of large hydrothermal systems. Theoretical or experimental studies of potential energy sources for organisms in Hadean impact craters are needed. Samples from specific basins on the Moon, such as the Schrödinger, South Pole-Aitken, and Orientale basins, should be collected to clarify the cadence of impacts. Finally, surveys of lunar regolith samples for fragments of Hadean Earth should continue to provide a direct record of the early evolution of life on Earth.