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 land masses 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 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 rock 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 speculated online this summer in Geology.