Thank goodness for granite. If not for the formation and subsequent erosion of large quantities of metal-rich granite on a supercontinent that formed billions of years ago, the evolution of multicellular life—including us—could have been stifled or delayed, according to a new study.
For much of its history, life on Earth existed as only single-celled organisms. Certain proteins critical for multicellular life, and presumed to have been equally critical for its evolution from single-celled ancestors, require heavy-metal elements, especially copper, zinc, and molybdenum, says John Parnell, a geoscientist at the University of Aberdeen in the United Kingdom. Previous studies suggest that multicellular life evolved sometime between 1.6 billion and 1.2 billion years ago. Researchers thought that before that innovation, these vital metals were locked away from environments where life thrived—either sequestered in the oxygen-poor depths of the ocean or held in ancient ore deposits in Earth's crust, waiting to be eroded.
Now, Parnell and his colleagues have proposed another option that fits new geological evidence: The essential metals eroded from a rare type of granite that formed in large amounts soon after Earth's landmasses collided to create the supercontinent Nuna, about 1.9 billion years ago. The team's analyses show that most deposits of this variety of granite—whose chemical composition caused the metals to be concentrated in ore deposits that were readily eroded, rather than distributed throughout the rock—formed between 1.8 billion and 1.3 billion years ago, when molten material from deep below Earth's crust rose to just beneath the surface and crystallized.
The geological record worldwide contains copious evidence that this form of granite began eroding almost immediately, delivering a variety of metals to coastal and lowland environments, the researchers say. For instance, ratios of strontium isotopes in ancient rocks originally deposited as seafloor sediments reveal that erosion from the supercontinent peaked at around 1.9 billion years ago. Also, large amounts of sulfate minerals, particularly ones that formed as a result of evaporation of mineral-rich waters in arid environments, began appearing around 1.7 billion years ago, a sign that metal sulfides found in the ore-rich rocks deposits were eroding, thereby releasing metals. Single-celled organisms incorporated these trace metals into metal-binding proteins that ultimately enabled the diversification of multicellular life, the researchers speculate online this month in Geology.
The team's findings "are interesting and intriguing," says Ariel Anbar, a biogeochemist at Arizona State University, Tempe. Although the rise of oxygen concentrations in the atmosphere about 2.4 billion years ago suggests that increased erosion—and, therefore, an increased flux of metals into the environment—was inevitable, the large amount of freshly formed granite were undoubtedly an important source of metals too, he adds.
Results of the new study add to the increasingly detailed picture of Earth's geochemical evolution at a critical juncture in life's history, says Chris Dupont, a microbial physiologist at the J. Craig Venter Institute in San Diego, California. "It's an attractive hypothesis," he notes. However, he adds, another alternative is that life in the shallow waters surrounding the Nuna supercontinent was able to evolve multicellularity by making do with smaller concentrations of metals than are assumed by Parnell and his colleagues, rendering the increased delivery of metals from granite moot.
Genetic studies of modern-day organisms suggest that the proteins that make multicellularity possible, especially those requiring zinc, evolved rather late, between 1.6 billion and 1.2 billion years ago, Dupont notes. "Before that time, life was constrained, and then it expanded. The problem is, we don't know what the metal concentrations in the ocean were during this period, and we still have to identify the sources of these metals."