Cambrian Earth – An Explosion of Evolution

The Cambrian Period

541 to 485.4 Million Years Ago

Smithsonian Institution

The Cambrian seas were rich in marine organisms, as portrayed in this artist’s impression. The large animal is anomalocaris, the largest known animal of the Cambrian period.

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. In the fourth part of my “A Journey through the History of Earth” series, we’ll be looking at the first of the twelve periods that make up the Phanerozoic aeon – the Cambrian Earth.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

Despite lasting a relatively short (in geological terms) 55-million years, the Cambrian period saw the most incredible diversification of evolution ever known in the history of our planet. It was an evolutionary arms race of epic proportions, during which many of the metazoan (animal) phylums that survive to this day appeared in the rapidly warming oceans. While the land was still desolate, the oceans were teeming with all manner of exotic lifeforms, including some of the very first super-predators.

The Cambrian is named after the Latin name for Wales, where the first fossils were found in incredibly ancient rocks dating from this time. For many years, the Cambrian was assumed to be the period when life first appeared, although it is now known that the ecosystem was already quite well-developed during the Ediacaran period of the preceding Proterozoic aeon. In fact, the oldest definitive evidence of life on Earth appeared a staggering 3.5-billion years ago during the Archean.

Highlights of the Cambrian

• Rapid diversification of evolution
• Extreme global warming
• Evolution of the first fish, molluscs and trilobites
• Formation of the Burgess Shale fossil deposits
• 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

Ellenm1

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

Heinrich 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

scorpion451

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.

Evolution of the First Super-Predator

Pixabay

Growing three feet (1m) in length, anomalocaris canadensis was by far the largest animal that lived in the Cambrian period.

One of the most remarkable fossils found in the Burgess Shale deposit belongs to that of an animal called anomalocaris canadensis, which translates as ‘abnormal shrimp from Canada’. It lived during the early- to mid-Cambrian and, at some 3 feet (1m) in length, it was the largest animal of the period so far discovered. Even more impressively, this gigantic shrimp-like creature was likely one of the first apex predators to prowl in the Earth’s oceans. Like the trilobites that evolved around the same time, anomalocaris propelled itself through the water using the lobes that covered the back of its body. With compound eyes on stalks and barbed spines on its disk-shaped mouth, it would have likely preyed on early fish and other arthropods, including trilobites.

While the monster prawn was busy terrorising the Cambrian oceans, the very first fish appeared, possibly evolving from the sessile tunicates of the early Cambrian coral reefs. Though not true fish themselves, the immediate ancestors to fish were cephalochordates, simple creatures without clearly defined heads, such as modern-day lancelets. The first true fish appeared between 530- and 520-million years ago, but they were extremely small, typically only no more than an inch in length and lacking jaws. One of the earliest true fishes was the haikouichthys, which evolved as much as 535-million years ago. Unlike its ancestors, at had a defined skull and tail, thus it vaguely resembled a modern fish. It was just under an inch (2.5cm) long. Several other fish evolved at around the same time, including the eel-like zhongjianichthys.

Changing Ecosystems Herald a Biotic Turnover

Like many geological periods, 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.

Conclusion

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 biomes were 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.

Part 5: Ordovician Earth – Colonising a Barren Land