Exactly how life on Earth began remains a burning scientific question. Many plausible hypotheses have been put forward, but at the crux of them all is a simple truth: There were rocks and minerals on Earth before there was life. Therefore, a thorough understanding of precisely what minerals existed on Earth at the time of life’s emergence, around four billion years ago, is crucial.
In a paper published in the November 2013 edition of the American Journal of Science, DCO’s Executive Director Robert Hazen, Senior Staff Scientist at the Carnegie Institution of Washington, DC, USA, catalogues the 420 minerals that likely characterized Earth’s Hadean Eon (the planet’s first 550 million years) . These minerals comprise a mere 8% of the diversity of minerals we see on Earth today, a complexity that is a direct result of the interplay between rock and life over geological time.
“This is a consequence of the limited ways that minerals might have formed prior to 4 billion years ago,” Hazen explained. “Most of the 420 minerals of the Hadean Eon formed from magma—molten rock that slowly crystallized at or near Earth’s surface—as well as the alteration of those minerals when exposed to hot water.”
To compile a list of minerals present on Hadean Earth, Hazen took two complementary approaches. First, he considered minerals observed in meteorites (effectively leftover pieces of the type of rock accreted by a nascent Earth) and known geological processes of the time, and asked what further minerals would have formed. As a counterpoint he took a reductive approach and, beginning with the approximately 4,800 known minerals on Earth today, used a process of elimination to rule out those that could not have formed under the conditions of a young Earth.
“Fortunately for most origin-of-life models, the most commonly invoked minerals were present on early Earth,” Hazen said.
Indeed, Hazen’s list includes the clay and sulfide minerals often cited in origins-of-life models. Borate and molybdate minerals, on the other hand, are not, which calls into question models that rely on these minerals. Several other areas of inquiry are also raised: Were hydrothermal systems generated by large impacts on the Hadean Earth? When was the tectonic cycle initiated? What aspects of mineral preservation bias our interpretation of the past?
This study also suggests that a similar roster of minerals evolved in the same way on other, perhaps all other, rocky planets and moons. Hazen has suggested that our nearest neighbor, Mars, may have a similar mineralogical composition to Hadean Earth. If so, this points to the importance of other factors, such as atmospheric composition and solar proximity, in any origins-of-life model. Nonetheless, knowing the path of planetary mineral evolution is an important tool for understanding how life began here on Earth, and potentially other planets.
This work was supported by the NSF, NASA Astrobiology Institute, the Deep Carbon Observatory and the Carnegie Institution for Science.
Photo: Fayalite, an olivine mineral and one of the 420 Hadean species, from the Coso Hot Springs, Inyo County, California, USA. Courtesy of Robert Downs and the RRuff Project at the University of Arizona.