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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.

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,  dry and rocky due to a lack of vegetation. 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,  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. 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.

With the greenhouse effect in full swing. At the beginning of the Cambrian, sea levels were already higher than they are today, but they continued to steadily increase.. The rapidly advancing oceans were 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. The Earth became a tropical ocean world, the perfect environment for the radiation of complex forms of life.

The First Reefs Form

The first 10-million years of the Cambrian saw the evolution of reef-building organisms. These primitive marine organisms were similar to modern corals but on average, much smaller. 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. Molluscs, jellyfish, tunicates and various segmented animals similar to  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 since nothing other than microorganisms had colonised the land.

Trilobites Take Over

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.

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,  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 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 higher than they are today and still rising steadily. Despite countless species evolving and disappearing, the period was a time of incredible diversification 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.

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