A trilobite from the Burgess Shale fossil beds of British Columbia. Photo: Mary Caperton Morton
Burgess Shale Fauna, 1989, Carel Brest van Kempen
Half a billion years ago, dramatic changes swept across the planet. In an evolutionary surge, the ancestor of virtually every living animal group emerged. It was the most extraordinary evolutionary event since life had first begun; in a flurry of diversification, complex multicellular life burst into existence, ending the 3-billion-year reign of bacteria and archaea as Earth's dominant life forms.
Fossil records show that during a comparatively short time, every major basal body plan appeared, including arthropods, mollusks and chordates, the lineage which eventually led to humans.
No one knows what triggered this remarkable event, but in 2013, researchers began to fit major pieces of the puzzle together for the first time. We now know that 600 million years ago, violent tectonic upheavals thrust mineral-rich rocks up from deep within the Earth's mantle, forming a colossal mountain range some 2,500 kilometers long across the supercontinent Gondwana, over most of what was to become west Africa and northeast Brazil.
Just like today's Himalayas, this massive pre-Cambrian mountain range was subject to constant, intense erosion, which fed Earth's ancient oceans with tremendous rivers of mineral sediments, radically altering oceanic chemistry.
This was during the Edicarian period, in which the genetic toolkit - body-patterning genes necessary for the eventual emergence of complex animals - seems to have first evolved, some 60 million years prior to the Cambrian. These genes were the legacy of rather... odd... life forms, mysterious creatures which vanished from the planet at the close of the Edicarian, well before the Cambrian began.
Thus, the creatures of the Cambrian did not suddenly appear from nowhere, contrary to the ill-informed claims of religious fundamentalists; there had been tens of millions of years of evolution before the Cambrian. So while it has been cast as a mysterious "explosion" of life, the Cambrian radiation it isn't out of line with simple evolutionary principles. It's also important to remember that "sudden" is an extremely relative term: when one is referring to deep time - epochs of hundreds of millions to billions of years - 20 million years may be comparatively short; but that is still at least one hundred times as long as humans have existed in their present form.
But immediately prior to the Cambrian explosion, there's a prominent, planet-wide gap of "missing time" in the rock record, referred to as the Great Unconformity. Here, a prominent rock boundary separates ancient igneous (volcanic) and metamorphic (altered by heat and pressure) rocks from significantly younger sedimentary ones - those formed by settling materials.
It was long believed that the Great Unconformity was just a gap in the evolutionary record, before multicellular life evolved with shells or bones capable of hardening into fossils, but now researchers say the period's chemical influence upon Earth's ancient oceans promoted biological calcification - the development of the first shelled and bony animals. Thus, the processes which led to that gap may actually have triggered biomineralization, lending impetus to the Cambrian period's evolutionary explosion.
We know for certain that, during the Cambrian, the rapid increase in animal diversity included a global increase in biomineralization, as animals began to evolve hard shells and skeletons. This not only allowed early animals to become preserved as rock fossils, but, coupled with the emergence of Earth's first carnivores, these hard structures seem to have helped trigger an "evolutionary arms race" between predators and prey, resulting in the Cambrian radiation.
Geological evidence gathered from across five continents shows that by 540 million years ago, North America had ripped away in tectonic shifts, carving a deep oceanic gateway between the ancestral Pacific and Atlantic oceans, and isolating Laurentia - North America's ancient core - from the supercontinent Gondwanaland. The event also caused a global rise in sea levels, spreading shallow ocean water across the planet. This redistributed deeply buried minerals, further altering Earth's ocean chemistry, and increasing available atmospheric oxygen - all conditions which helped foster the divergence of complex, multicellular life.
Continental weathering left a gap in the geological record all around the world as sea levels climbed, scrubbing the continents clean. The shoreline gradually crept to ever higher elevations, and wave-base razors washed away soil and sediment which had built up from the breakdown of ancient rock. This cleared away materials which would otherwise have settled into sedimentary rocks, exposed huge ranges of bare bedrock, and rapidly swept ionized minerals into the ancient seas.
Because excess calcium interferes with vital cellular signalling, it's thought that the first marine shells came from marine organisms excreting excess cellular calcium. But these shells offered a distinct evolutionary advantage, sheltering their hosts from predation.
According to Harvard University's Erik Sperling, hypoxia (low-oxygen states) had long held evolution in check, limiting the abundance and diversity of animals, particularly carnivores. But the increase in available oxygen at the start of the Cambrian allowed for a much more efficient processing of food - the carnivorous diet. Higher oxygen levels also allowed multicellular species and body structures to emerge and rapidly diversify for the first time.
The combination of factors triggered adaptive radiations - the rapid diversification of one lineage into several new ones, each with different adaptations to the environment. Such adaptive radiations are all responses to opportunities, as creatures evolve physical or behavioral traits which enable them to exploit newly-available niches or resources.
A single key adaptation has the potential to open up several new niches for an organism, and act as a catalyst for an adaptive radiation. One example is the gradual transformation of reptilian jaw bones into the ossicles - tiny, ultra-sensitive ear-bones which enable land animals to amplify fluctuating air pressure (sound waves). While inefficient within an aquatic environment, they provide an excellent means for creatures on land to locate prey - or escape hungry predators.
While some animals specialize to take advantage of specific food resources, new, unpopulated environmental niches or an escape from competition can also lead to adaptive radiations. Such a process allowed mammals to rise to dominance, when a meteorite the size of Mt. Everest slammed into the ancient Gulf of Mexico 66 million years ago, dooming the dinosaurs.
Sources: "Dawn of Carnivores Explains Animal Boom in Distant Past", press release, Mario Aguilera, July 30, 2013, University of California, San Diego
"Massive Geographic Change May Have Triggered Explosion of Animal Life", press release, Anton Caputo, November 3, 2014, University of Texas at Austin,
"Missing Rocks May Explain Why Life Started Playing Shell Games", Scott K. Johnson, April 25, 2012, Nature