Timeline 541 to 485.4 Million Years Ago Cambrian Earth – An Explosion of Evolution

Cambrian Seas
541-million years ago, one of the most spectacular events in the history of the Earth took place. The fourth and current geologic aeon begun with the Cambrian Explosion, the sudden and unprecedented diversification of life, the likes of which had never been seen before.

The end of age of slimes marked a profound change in life on Earth. The rapid diversification of lifeforms mixing in the slime, known as the slime explosion, produced the first representatives of all native phyla.

Although diverse life forms prospered in the oceans, the land was barren. dry and rocky due to a lack of vegetation. The seas were relatively warm, and polar ice was absent for much of the period.

The Cambrian period saw the most incredible diversification of evolution ever known in the history in the rapidly warming oceans. While the land was still desolate, the oceans were teeming with all manner of exotic lifeforms, including some super-predators.

Highlights of the Cambrian

  • Rapid diversification of evolution
  • Extreme global warming
  • Evolution of the first fish, molluscs and trilobites
  • Steadily rising sea levels
  • The first super-predators

A Rapidly Changing World

Heralding in the beginning of an entirely new aeon, the Phanerozoic that continues to this day, and a new era, the Palaeozoic, the Cambrian is perhaps the most important of all the geological periods. 541-million years ago, the Ediacaran period ended, and many of the mysterious creatures of the time disappeared with it, likely due to a combination of rapidly evolving predatory behaviour and unprecedented climate change. The dawn of the Cambrian saw the beginning of the evolution of an almost entirely new ecosystem, the so-called Cambrian Explosion.

The early Cambrian Earth was probably a relatively chilly world, as the last of the glaciers from the preceding Phanerozoic aeon receded. The Snowball Earth was well and truly gone by this time, and oxygen levels were also rising, eventually reaching two thirds of what they are today. The amount of carbon dioxide was, as in the late Ediacaran period, a staggering 16 times higher than it was at the beginning of the industrial revolution.

With the greenhouse effect in full swing, the consequences of climate change that we fear today were profound indeed. At the beginning of the Cambrian, sea levels were already around 98 feet (30m) higher than they are today, but they continued to steadily increase, reaching a peak of almost 300 feet (90m). The rapidly advancing oceans were likely the main cause of the breakup of the Precambrian supercontinent of Pannotia, thus greatly expanding the amount of shallow maritime environments. Through the Cambrian, the temperature also rose rapidly, reaching 21°C, compared to today’s 14 °C. The Earth became a tropical ocean world, the perfect environment for the radiation of complex forms of life.

The First Reefs Form

Cambrian diorama

A photograph of a diorama depicting the primordial reef-like environment of the Cambrian period. The coral-like formations are archaeocyatha.

The first 10-million years of the Cambrian saw the evolution of a now long-extinct taxon of reef-building organisms known as archaeocyatha. These primitive marine organisms were similar in many ways to modern corals in that they were a very diverse taxon and were all sessile (unable to move by themselves). They were, on average, much smaller than most of today’s corrals, as evidenced by the small, shelly fossils that we’ve discovered dating from that time. Thanks to the great abundance of shallow waters around early Cambrian landmasses, corals became one of the dominant lifeforms throughout the first epoch of the period.

Many other marine invertebrates, a few of which even predate the Cambrian, were already well-established by the middle of the period. In fact, could you go snorkelling around the Cambrian seas, there would be a lot you’d recognise, including molluscs, jellyfish, tunicates and various segmented animals similar to today’s shrimps, sea snails and burrowing worms. Seaweeds also became widespread during the first half of the Cambrian. In fact, they were still the only forms of plant life found on Earth at the time, especially since nothing other than microorganisms had colonised the land.

Trilobites Take Over

TrilobitesHeinrich Harder

Trilobites crawling amongst seaweed in an artist’s representation of a typical scene in the mid-Cambrian.

The most famous of all fossils dating back before the time of the dinosaurs belong to those of trilobites, one of the most successful taxons of early animals that ever existed. Lasting some 270-million years, trilobites first appeared around 2-million years into the Cambrian period and quickly came to dominate the shallows around the world’s coastlines. In fact, more than 90% of the Cambrian fossils that have ever been discovered belong to these symmetrical, three-lobed arthropods. They were so common that there’s even a website dedicated entirely to the 17,000 species that existed throughout the Palaeozoic era.

By the middle of the Cambrian, the Earth’s oceans had changed dramatically. Most archaeocyatha species rapidly died out during the most significant extinction event of the time, possibly because of dropping magnesium levels in seawater profoundly changing the chemical composition of the sea floor. In other words, the reefs were no longer the dominant biomes, and metazoan species, such as trilobites, found an entirely new niche to colonise and thrive in.

The Burgess Shale Fossil Deposit Reveals All


An artist’s representation of the hallucigenia, a bizarre worm-like creature whose fossils were discovered in the Burgess Shale deposit.

The Burgess Shale formed in the mid-Cambrian around 508-million years ago, and it’s now found in the Canadian Rockies of British Columbia. Discovered in 1909, the Burgess Shale remains one most important fossil-bearing deposits dating from the Cambrian period, and it’s one of the only places in the world sporting a plenitude of well-preserved organisms from this period. It’s almost exclusively thanks to this amazing discovery that we know so much about what life was like in the Earth’s oceans over half a billion years ago. Among the many creatures discovered in the Burgess Shale was the hallucigenia, a thoroughly alien-looking creature that strolled along the seabed on seven or eight pairs of clawed legs.

The very first fish appeared in the early Cambrian coral reefs.

Changing Ecosystems Herald a Biotic Turnover

The last few million years of the Cambrian was marked by a significant extinction event. Whereas the period is best known for its explosion of multicellular marine life, rapid changes to the global environment transformed the world’s ecosystems yet again. After a long period of near global tropical temperatures, the world started to cool significantly towards the end of the Cambrian, although this was not likely the main cause of the Cambrian-Ordovician extinction event. Rather, it was probably caused by what’s known as a biotic turnover event, in which more successful organisms replace their primitive ancestors. In other words, natural selection saw that only the strongest survived.


By the end of the Cambrian, sea levels were 300 feet (90m) higher than they are today and still rising steadily. Despite countless species evolving and disappearing, the period was a time of incredible diversification during which the first arthropods evolved, heralding in the first exploration of the land by multicellular organisms. Still, while the marine life was thriving by the end of the Cambrian, desolate landmasses void of plant or animal life continued to shrink while, in the seas, the first corals formed enormous reefs teeming with ever-increasingly diverse forms of life.