Metals and the Beginnings of Life
Long ago life began on Earth. One of the most intriguing questions is what caused it to start just then. A little less than 2 billion years ago, metals including copper, molybdenum and zinc became available to primitive cells, at the same time that the cells began to become much more complex. Some scientists indicate that they have identified the event that introduced these metals, which made it possible for those primitive cells to develop, evolve, and spread.
The secret, according to these researchers, was granite.
The new research suggests that the large amount of heat within the Earth at this time caused metal-laden magmas to rise from great depths, which cooled into granites near the surface. The scientists conclude that this event caused the substantial change in the Earth's surface and ocean chemistry that began about 2 billion years ago.
This hypothesis challenges the prevailing consensus that changes in ocean chemistry were responsible for enabling life to undertake this transition.
"There's no doubt that probably a lot of metal was locked up in the oceans," said John Parnell, a geologist from the University of Aberdeen, in the U.K. "[We're] suggesting that it's the terrestrial environment where the metal was really being made newly available."
Parnell brought together several pieces of evidence to show that granites formed, came to the surface of continents, then weathered. Weathering freed up metals, he said, which traveled with runoff to fill lakes and shallow seas, places where primitive life could incorporate the metals and become more complex.
"We propose that metals were delivered due to a critical combination of continental growth, near-surface metal concentration, and erosion into the surface environment during the Mesoproterozoic [a time period from roughly 1.6-1.0 billion years ago]," Parnell and his colleagues wrote in their paper, published in Geology.
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma (million years ago). The Mesoproterozoic was the first period of Earth's history with a respectable geological record.
The major events of this era are the formation of the Rodinia supercontinent, the breakup of the Columbia supercontinent, and the evolution of sexual reproduction. The evolution of sexual reproduction is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago.
Life originated more than 3.5 billion years ago, as basic prokaryotic cells, which did not have a nucleus. Advanced cells -- called eukaryotes -- added a nucleus. They evolved around 2 billion years ago. Then, sometime between 2 billion and 1 billion years ago, these cells proliferated, sexual reproduction evolved, and the first multicellular organisms developed.
Parnell said that he brought together two areas of research to form this conclusion.. The first was from scientists who identified that critical advances in life lined up with increased access to metals about 1.5 to 2 billion years ago. A second group had shown that granites bearing many of these same metals formed at about the same time.
Columbia was one of Earth's oldest supercontinents. It is thought to have existed approximately 1.8 to 1.5 billion years ago in the Paleoproterozoic Era. It is proposed when this continent arose it was in the midst of intense volcanic activity that allowed interior metal sources to arise to the surface to be weathered and release their metal for life organisms to use.
For further information see Metals and Geology.
Parnell image via University of Aberdeen.