(Inside Science) -- More than 500 million years ago, during a time known as the Cambrian explosion, novel categories of multicellular life forms appeared in a carnival of biological innovation unmatched in our planet's history. New research ties together this burst with an ancient mystery called the Great Unconformity.
The Great Unconformity refers to the huge gaps left in the planet's rock record where relatively young sedimentary rocks -- still a few hundred million years old -- sit atop much older igneous and metamorphic rocks. For example, in the Grand Canyon, 1.7-billion-year-old metamorphic rock is topped by a layer of sandstone that's about 500 million years old. Similar unconformities exist throughout much of the world, leaving a limited record precisely when life was advancing so quickly.
Many scientists, including Charles Darwin, considered gaps left in the geologic record a significant obstacle to understanding the Earth's history. But researchers now say that the process that formed this break is crucial to understanding what happened during the explosion of life.
An article in the April 19 issue of the journal Nature links changes in ancient ocean chemistry to the transformation of life. One significant change was the advent of biomineralization, when organisms first began using minerals such as calcium carbonate to build structures, including shells and skeletons.
"This is a unifying hypothesis that ties everything together into a single mechanism," said Shanan Peters, a geologist at the University of Wisconsin-Madison. "I think a lot of geologists are going to slap their heads and say, 'Of course!'"
The research links the appearance of biomineralization to the Great Unconformity. Peters and his colleague, Robert Gaines from Pomona College in Claremont, Calif., used a massive database of over 20,000 rock samples collected from across North America, to bring together many clues in what Peters called a "detective story."
The researchers hypothesize that when erosion exposed and wore down the ancient rocks that now lay below the Great Unconformity, the rocks reacted with air and water to change seawater chemistry, making key ingredients for the formation of biominerals more available.
The formation of the Great Unconformity "may have been an environmental trigger for the evolution of biomineralization and the 'Cambrian Explosion,'" the researchers wrote.
High concentrations of calcium and other elements might have made it less costly to animals to make the effort to begin incorporating minerals, said Peters.
It's not that animals noticed the availability of calcium and decided to make shells and skeletons out of the mineral, but once they began taking in and transforming minerals, natural selection may have led to the spread of the most useful structures.
"[The Cambrian explosion] was so major that it used to be thought of as the origin of life itself," said Matthew Powell, a paleontologist at Juniata College in Huntingdon, Penn., who did not work on the study. "It looked like the origin of life because that's when things acquired skeletons and could be found as fossils."
Other research has linked the Cambrian explosion to the evolution of certain genes or changes in oxygen concentration. Peters admitted that more than one factor may have enabled the event. He said that before this research, no one had linked together so many pieces in a quantifiable, cohesive fashion, nor suggested that chemistry was such a key issue.
"This is really synthesizing a lot of different ideas into a new hypothesis," said Powell. "I wouldn't characterize this as closing the debate, but it opened it very strongly."