Cambrian explosion- The big bang of life ⋆ TheScientificRevelation

9 min readMar 30, 2021

Since Cambrian times earth had seen a diversity like never before but what about before Cambrian? If we go back in time, earth formed around 4.5 billion years ago. First Eon, Hadean has known for magma floor, constant volcanic eruption, and regular comet bombardment. Then comes the archaeon eon marked with first slid basalt floor and water. Around 4 billion years ago first life formed in the carbon-rich greenhouse planet. For most of the part of earth’s history, our planet was dominated by microscopic life. The first fossil evidence of life came from fossil rocks dating back to 3.5 billion years ago. Cyanobacteria also appeared around the same time and they have changed the face of the planet.

The Precambrian times

The oxygen catastrophe

For a long time, I used to think that oxygen is so important for life and everything is possible because of it so, it must have always been part of the earth. It was super shocking to know that it was a by-product. Life itself had created oxygen and vice versa. When cyanobacteria came into the picture they had produced oxygen due to photosynthesis. This free oxygen had made the planet breathable and liveable for us.

The Great oxygen event had flipped over the composition of seas and atmosphere making it oxygen-rich. For a long time, things happened slowly and billion years passed without much on the outside. However, it was a revolutionary time because during this tectonic plates transited into their modern forms.

Read my blog post to know more about The great oxidation event.

The global ice age and appearance of life

In the Cryogenian period, earth faced a global spell of severe ice ages where every corner was covered under the ice. This period ended and immediately came the Ediacaran period. The enigmatic life forms of Ediacaran didn’t appear until the middle when altogether there was a burst of multicellular life in fossils record. We have been finding fossils of macroscopic life for a long time. However, in Ediacaran, they appear in so many numbers that scientists realized that it was a whole ecosystem of weird organisms.

All in all Precambrian life was the transformation of the planet. Geosphere, atmosphere, hydrosphere evolve together into more recognizable forms. Cambrian had joined another element into this picture, the element of life. After the Cambrian whenever we talk about planet transformation we also include life as it had evolved in drastically different forms.

Read more on Cryogenian glaciation from my blog post.

What is so special about the Cambrian period?

The Cambrian period ran from 541 million years ago to 485.2 million years ago. It was filled with so much diversity that it strikingly contrasts with the times before. Cambrian is very important that’s why all eras and periods before it are collectively called Precambrian. So, what was so different about Cambrian? There are some points to discuss

The development of modern phyla

Yes, in just the span of 55.6 million years all the modern phylum and body plans developed. We had primitive arthropods like Morella, trilobites, and Anomalocarids. There in Cambrian waters swam Pikaia, the earliest vertebrate. Primitive mollusks and Cnidarians dominated the Cambrian seas. For surety, invertebrates were in higher numbers and groups in the Cambrian period but vertebrates started their journey from here only.

Read The 12 strangest animals of the Cambrian period to know more about fauna.

Biomineralization

A more understandable term for this is shells and exoskeleton. The Ediacaran biota had simple body structures and soft tissues. These soft parts don’t fossilize except in some exceptional conditions. Not all Cambrian animals had exoskeleton but their percentage was more. Some prominent Cambrian animals were soft-bodied which are preserved in burgess shale, Canada, and Chengjiang China. However, Cambrian had iconic hard-bodied animals with a full-bodied or partial exoskeleton. We have found enough hard-shelled animals, so it’s deduced that they were thriving in the Cambrian period. The question arises that how suddenly animals started to move for hard coverings even when their formation is quite costly. There are two reasons for it.

Firstly, when the supercontinent Rodinia broke apart new coastlines appeared. The supercontinent divided into smaller chunks of land. During this process, huge amounts of sediments released into the water. These sediments were rich in mineral content. This mineral surge had not only promoted photosynthetic bacterial growth but also provided raw minerals for the exoskeleton.

Secondly, Cambrian is iconic for the appearance of predators. Before that earth has not seen an active predator and prey relation. When predators came into pictures preys had to come up with a defense to survive and it had pushed them for the development of the exoskeleton. In starting few parts became hard and then the success of the exoskeleton had evolved full mineralized covering where the tender animal is safe inside.

A post on Cambrian explosion by National Geography.

An active ecosystem with predator and prey

The Cambrian ecosystem was filled with predators that were actively hunting. There was a whole food web of primary, secondary and tertiary consumers. One animal was eating other animals and then it became the food of another animal. Many of you must have seen documentaries on marine ecosystems like reefs. You see how fast and alert animals are on reefs so they can protect themselves. Predators and prey alike use so many sophisticated ways to survive in their ecosystem that it’s mind-blowing. The more or less same thing was happening in Cambrian ecosystems. The rise of predators had made a dynamic relation between animals and ecosystems.

Before Cambrian animals were surviving without much interaction with their environment much but in Cambrian their was a dynamo. Animals were affecting their environment and modifying it according to their need. Primitive coral reef-like structures were formed. You must know what is coral leaf, a colony of coral polyps that are minuscule but with calcium carbonate, they make large hard structures. Without these modified environments we wouldn’t have the diversity as we know it. The rise of predation had pushed animals to develop hard armor for protection. Again here animals had used minerals dissolved in water for their survival and modified the environment. These Cambrian animals had started a race of natural selection. Better suited species would survive and thrive and others will perish and so evolution came with various body plans. It was an active ecosystem and after Cambrian evolution became more experimental and species became diversified.

Was it actually an explosion?

In the Cambrian period, there was indeed a sudden burst in animal diversity. Although, saying the Cambrian explosion would be an overstatement. Scientists have found many fossils from the Cambrian period. Among them many are hard-shelled. On another note, burgess shale and other assemblages like it are exceptional cases. Here due to sudden events like underwater avalanches sediments fell on animals. The layers of sediments settled one above another and animals sandwiched in between them. Over time their body fluids were replaced with minerals. After millions of years, we have brilliant fossils of soft as well as hard shell animals.

If we take these two points into account so we can say that if it wouldn’t for exceptional fossilizing conditions and exoskeleton we couldn’t have found these many fossils. This is what some scientists argue that before Cambrian there was only soft macroscopic life which couldn’t be fossilized in normal condition. In Cambrian due to a rise in hard-shelled animals, we suddenly have more fossils so, maybe a more gradual diversification looks like an explosion. An explosion can be a very misnomer term as fossilization can very much depend on luck. Perhaps, those Cambrian animals were well thriving before the but they fossilized only after few million years when conditions were suitable.

What caused the Cambrian explosion?

There are many factors that have contributed to the rise of complicated hard-shelled animals and active ecosystems.

Increase in oxygen

This is by far the most important factor for the Cambrian explosion. The Cryogenian period ended almost 635 million years ago with its 2 global glaciations. In late Ediacaran also we have a third important Gaskiers glaciation which was regional but with global impacts. When the ice retreated to the poles landmasses became exposed to the sun. Firstly, rocks had started to absorb carbon dioxide for erosion. In this way, huge piles of sediments released into the oceans. It has contributed to the nutrient increase and subsequent algal bloom.

Secondly, This nutrient increase had not only contributed to algal bloom but also provided the raw material for the exoskeleton. Through erosion, lots of carbon dioxide was removed from the atmosphere. In the same manner, photosynthetic microbes had released oxygen as a by-product. Oxygen was being released into the atmosphere and carbon dioxide was being removed from it. This rise in oxygen pushed animals to evolve and diversified. The melting of glaciers had also increased sea levels. It had helped in the process of coastline erosion.

Breakup of Rodinia

The supercontinent broke apart between 750–633 million years ago at the start of the Ediacaran period. More coastlines formed when landmasses moved from each other. Water interacted with their newly formed coastal region sculpturing it accordingly and releasing loads of sediments. The breakup of Rodinia had formed new continental margins. We know that continental margins are always filled with life. They had provided suitable conditions for life to thrive and new innovations to happen. Along with it, exposed rocks had absorbed carbon dioxide for erosion naturally making oxygen dominant. Sediments had provided nutrients for the coastal areas where photosynthetic microbes thrived releasing oxygen after photosynthesis. These events kept on happening during the whole Ediacaran and Cambrian. Their effects were visible in Ediacaran biota and Cambrian diversity.

Ecological factor

I have mentioned dynamic predator and prey relations in the Cambrian period. Some scientists think that this ecological dynamo had pushed animals to evolve in new ways. In Ediacaran oxygen percentage in the ocean was very less. The Ediacaran animals were well thriving on top of microbial mat breathing less oxygen. During Ediacaran- Cambrian boundary oxygen percentage increased a little bit in the ocean and it crossed a threshold. This tiny bit of oxygen increase had introduced predators into ecosystems. Predators had forced soft-bodied animals to developed such defense mechanisms like an exoskeleton to survive. This desire to survive had make animals go diverse with varied body plans and boom the Cambrian explosion. This rise of Cambrian predators was a partial reason for the demise of the Ediacaran biota.

Read what sparked the Cambrian explosion? by Nature.com

The Cambrian substrate revolution

Cambrian substrate revolution is such a hidden factor that has huge implications for the rise of Cambrian animals. Let’s go back to the Ediacaran period, swimming in shallow waters what do you see? You would see a green slimy mat covering the ocean floor like a sheet of plastic and Ediacaran animals either standing on a solid surface or grazing on it. Although, there were borrowers in the Ediacaran period but they only borrowed shallower depth. In late Ediacaran, we see the rise of vertical borrowers that were going deep for nutrients and food. These worms-like creatures were attacking by surprise. After the other animals had also started using borrowers for different reasons like a refuge from predators above or tapping the rich source of nutrients previously untouched.

This constant borrowing had changed the substrate from solid sediments to churned mixture. The constant bacterial mat that was growing on solid layers of sediments was destroyed. It turned into patched portions. Ediacaran animals were either filter feeders or grazing on mats. All of them depended on the microbial mat for the base as well as food. The ecosystem was no more suitable for Ediacaran animals. The Cambrian animals had filled those empty niches. When worms like animals had churned and mixed the ocean floor nutrient cycle became more widespread. Some experts think that this change in the ecosystem had made conditions unsuitable for Ediacaran animals.

The very informative documentaries on this topic are the Cambrian explosion and what triggered the Cambrian explosion? In the former, lecturer had talked about Sirius passet assemblage, one of the Cambrian biota of Greenland.
Another great lecture on the Cambrian explosion including how we came from the snow enclave planet to here in the boom of diversity.
If you are interested in knowing more about localities and fossils refer to this post The Cambrian explosion by the Burgess shale.
Another post on the Cambrian explosion by Berkely education.

Cambrian was a great start for life. Animals had diversified and evolved into more familiar as we moved towards the present. With every chapter from here on, we will get a flavor of life into our meal. Stay tuned and do Revelation.

Originally published at https://www.thescientificrevelation.com on March 30, 2021.