Observe always that everything is the result of a change and get used to thinking that there is nothing Nature loves so well as to change existing forms and make new ones like them. ~ Roman Emperor Marcus Aurelius in the 2nd century
The fossil beds that became known as the Burgess Shale were discovered by Canadian geologist Richard McConnell in 1866. McConnell noticed unusual black shale outcroppings in the Canadian Rockies of British Columbia, studded with tiny fossils.
McConnell’s find came to the attention of American paleontologist Charles Doolittle Walcott, administrator of the Smithsonian Institution. Walcott made his 1st expedition there in 1907. He made several more over the next 17 years.
All told, Walcott accumulated 65,000 specimens of Cambrian life, which he proceeded to catalog into the existing taxa of modern times. Walcott saw no meaningful evidence of evolution in the curiosities he had carved out.
From 1924 into the 1930s, American paleontologist Percy Raymond collected further fossils from the Burgess Shale formation, to no great advance in understanding.
After that, interest dwindled until the early 1960s, when English paleontologist Harry Whittington initiated his own survey. Whittington was prodded into action by a fellow trilobite enthusiast while the two were rummaging in Raymond’s collection.
Whittington had a team to examine his and Walcott’s fossils, which had languished in a Smithsonian storeroom since Walcott’s death in 1927. Papers poured out of the group in the early 1970s, diagnosing many of the specimens as hitherto unknown animal forms; whence the legend of the Cambrian explosion.
Discoveries in the early 21st century pushed back into the Ediacaran period evolutionary developments that were previously thought to have occurred during the Cambrian. For example, complex nervous systems and brains developed during the Ediacaran.
Whatever the jumping off point from the Ediacaran, a relatively sparse number of species encountered conducive conditions at the onset of the Cambrian that put evolution into hyperdrive. Genetic codes changed 5.5 times faster than they do now. New traits arose at 4 times today’s pace.
The fossil record of oceanic animal forms exploded during the early Cambrian. 100 different phyla (body plans) seem to have come into being, including many unique and bizarre creatures. Today there are 30 or so phyla.
Geological events on land and under the seabed created the conditions for the Cambrian. The warm seas became spiced with species. Seawater became more alkaline, affording a profound expansion of shallow marine habitats.
Oceanic oxygen levels rose during the Cambrian. Photosynthetic plankton played a role, but so did the animals that ate them.
Dead organisms sink, decomposing as they go. Because decomposition consumes oxygen, this would have kept ocean waters anoxic; but filter-feeding sponges started a clearing process. Over time, this helped oxygenate the oceans.
The sponges basically march from shallower to deeper water, oxygenating as they go. ~ American evolutionary biologist Douglas Erwin
While overall oceanic oxygenation occurred during the Cambrian, there were pulses when the global marine oxygen level dropped. These short-lived catastrophes may have emptied ecological niches and spurred evolutionary developments. Whereas rising oxygen permitted animals to prosper, it may have been intermittent drops that doled out diversity.
Rising sea levels during the Cambrian increased erosion: spilling nutrients, such as calcium, phosphate, and potassium, into the oceans. Less acidic water meant greater viability and longevity of these marine nutrients.
Calcium was the key ingredient that afforded the building of hard shells and skeletons. These durable structures spelled protection.
With predation came an intense set of evolutionary pressures: spurring the development of better burrowers, faster-moving creatures, stronger predators with sharpened weapons, increasingly sophisticated senses, and so on. The proliferation of life created an evolutionary cascade, spawning predators in prodigious variety. Prey adapted to evade these threats. The predator-prey gyre began in earnest during the Cambrian.