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LIFE on Earth experienced a singular revolution just over 500 million years ago. In a geological blink of an eye, most groups of the animal kingdom appeared in the Earth鈥檚 oceans and then diversified. The acquisition of skeletons, the advent of predation and the rise of complex ecosystems all occurred in what鈥檚 known as the .

Life took such a giant leap forward in abundance and complexity during the Cambrian that the rock record itself was indelibly changed. Long before geologists knew the precise age of the Earth, they could divide its history into two parts: the first 4 billion years, known simply as the Precambrian, followed by the Phanerozoic, meaning 鈥渧isible life鈥�, which includes the Cambrian right up to today.

Evolutionary change isn鈥檛 supposed to happen so abruptly, at least not according to Charles Darwin. 鈥�Darwin鈥檚 dilemma鈥� over the Cambrian explosion was explained away in his On the Origin of Species as an artefact of an incomplete geological record, one that failed to preserve fossils of a long Precambrian history of slow-paced animal evolution. A century and a half of study has shown that Darwin was right 鈥� animals do indeed have a Precambrian origin. A fossil record of that long history of simple bodies and behaviours has now been uncovered, proving that the Cambrian explosion was a real evolutionary phenomenon that needed to be explained.

As is the case with any great mystery, many different hypotheses have been proposed for the long lag between the first appearance of simple animals and their eventual diversification during the Cambrian. Perhaps the Cambrian explosion鈥檚 long fuse reflects the time it took to evolve the genes needed to control the formation of complex, differentiated bodies? Animals also require oxygen to power their large bodies and active lifestyles, but concentrations of oxygen in Earth鈥檚 atmosphere are thought to have been rather low during much of the Precambrian, so perhaps the Cambrian explosion couldn鈥檛 happen until atmospheric oxygen topped some critical threshold? Or maybe predation was an innovation that required a long time to evolve, but once in place, escalated via natural selection to the fast-paced, eat-or-be-eaten Phanerozoic world?

We have a new hypothesis to add to the mix. We propose that a (Nature, vol 484, p 363). Although our hypothesis is just another in a long list of non-mutually exclusive potential explanations it does, literally, stand out: the geological record of the transition from the Precambrian to the Cambrian is obvious, even from a mile away.

If you stand on the rim of the Grand Canyon and look down to the bottom you can easily distinguish the 鈥淕reat Unconformity鈥�, so named by the explorer and geologist John Wesley Powell in 1869 during his epic trek down the Colorado river. As implied by the name, the Great Unconformity is a juxtaposition of two different types of rock of very different geologic ages along a prominent surface of erosion. This surface represents mind-boggling amounts of 鈥渕issing鈥� time in the rock record. In many places more than 1 billion years is missing, which is twice the duration of the entire history of animal life on Earth. In the Grand Canyon, younger Cambrian sedimentary rocks deposited on the Earth鈥檚 surface sit directly on top of much older Precambrian rocks, many of which formed deep in the Earth鈥檚 crust.

鈥淭he Great Unconformity represents mind-boggling amounts of missing time鈥�

The ultimate cause of this gap remains a mystery. What is clear, though, is that over tens or even hundreds of millions of years before the Cambrian, the uppermost rocks of the continents were stripped away, exposing underlying crystalline basement rocks. This process of continental denudation occurred over a vast area. Much of North America, from California to New York State has a Great Unconformity similar to that in the Grand Canyon, and geologists have reported similar features on other continents as well.

How might the formation of the Great Unconformity be responsible for the Cambrian explosion? We think it comes down to changes in seawater chemistry. These were brought about by the large-scale erosion and chemical weathering of exposed crystalline rock over an area so large there has been nothing like it in the past 1 billion years of Earth history. When carbon dioxide-bearing, slightly acidic rainwater fell on these basement rocks, calcium, magnesium, phosphate and bicarbonate ions, silicon dioxide and more were liberated and carried into the oceans.

One hallmark of the Cambrian explosion is the widespread acquisition of mineral skeletons by a number of different animal groups involving several types of minerals, such as the silica shells of microscopic , and the calcium carbonate shells of now-extinct arthropods called trilobites. We propose that the origin of biomineralisation 鈥� the ability of organisms to make these minerals 鈥� was a biological response to changes in seawater chemistry. These changes were caused by the last stages in the formation of the Great Unconformity, when the sea spread over the continents. The action of waves over the weathered basement rocks further eroded the surface, exposing fresh mineral surfaces and causing an increased flux of the ions used in biomineralisation to the ocean.

The weathering products of crystalline rocks don鈥檛 just end up in biominerals, they also end up in sediments such as limestone. It has long been known that precipitated from seawater, which provides evidence for our hypothesis.

We suspect that the elevated concentration of ions in seawater effectively lowered the evolutionary barrier for biomineralisation. Today, most organisms invest energy in creating biominerals because hard body parts are so ecologically and evolutionarily advantageous. But evolution couldn鈥檛 鈥渇oresee鈥� how useful biominerals would be when shaped into the teeth, claws and shells we know today. Instead, we think the ion influx promoted by the last stages in the formation of the Great Unconformity may have lowered the energy barrier to biomineralisation or caused biominerals to appear as metabolic by-products. The usefulness of these new raw materials meant that natural selection could quickly take over.

Was the formation of the Great Unconformity directly responsible for the Cambrian explosion? More work needs to be done, particularly with respect to the detailed timing of biomineralisation and the transgression of the sea over vast areas of dry land, and also on the actual biological consequences of our proposed chemical weathering flux. However, one thing is clear: the physical geological record of the Precambrian-Cambrian transition is just as remarkable as the Cambrian explosion, and we now think the two are causally linked.

This article appeared in print under the headline 鈥淭he great leap forward鈥�