Life on earth began around 3.7 billion years ago. The single-celled microscopic organisms of the early earth evolved at a gradual pace over the course of billions of years. Until approximately 543 million years ago, living organisms were really simple. They were composed mostly of individual cells and a few multicellular colonies. While these living organisms were evolving, the pace was quite slow. This was until a dramatic evolutionary event gave rise to an astonishing diversity of animals. This event is popularly referred to as the Cambrian explosion.
What is the Cambrian Explosion?
The term Cambrian explosion refers to a short period in geologic history characterized by the emergence of many new and diverse organisms. Often referred to as the biological big bang or Cambrian diversification, the event saw the appearance of many major phyla of organisms that make up many modern and extinct animal life.
The Cambrian explosion occurred between 541 million and 530 million years ago. This was at the beginning of the Cambrian period. These different life forms appeared over a relatively short period of 25 million years.
Although 25 million years seem like a long time, it was a relatively short time compared to how long evolution would normally take. Because this period of life diversity happened “suddenly,” it was termed an “explosion.”
Interestingly, the Cambrian explosion is not the first time life on earth skipped many years on the expected evolutionary trend. A rapid change in evolutionary pace like this has been observed several other times in the fossil record, usually after major extinction events. However, the Cambrian explosion was the first event like this we have on record and certainly one of the most dramatic ones.
Life after the Cambrian Explosion
Before the Cambrian Explosion, life on earth was rather simple. Life was restricted to the ocean and was mostly in the form of unicellular organisms. The Cambrian explosion gave rise to complex life forms with many of the features seen in present-day animals.
The Cambrian explosion marks the beginning of many of the major phyla that make up animal life. Many of the modern phyla we see today (between 20 to 35) and several extinct phyla evolved during the 25 million year period of the Cambrian explosion. Although many of these groups went extinct in the ensuing years, many of the successful modern phyla still around today also evolved during this period.
Perhaps the most important group of organisms that developed during the Cambrian explosion were the arthropods and sponges. They were the most numerically dominant taxonomy group during the Cambrian. Many members of this group evolved chitinous shells that protected them against predators. The shells also ensured that they were well-preserved in fossil records.
Although life on earth became more diverse after the Cambrian explosion, most of the organisms that developed lived in the shallow margins of the world’s oceans. There was no life on land, except for the cyanobacteria (blue-green algae) which lived in moist sediments.
Cambrian life was scarce in the open sea. However, the organisms living in the shallow part of the seafloor were quite diverse. Some of the most notable organisms that developed during the Cambrian explosion include:
What Caused the Cambrian Explosion?
Scientists are not exactly sure of what gave rise to the rapid evolution of organisms at the start of the Cambrian. However, the most prevalent theory is that a rapid rise in oxygen levels in the earth’s atmosphere might have triggered it.
Another theory links the rapid evolution to the formation of the earth’s ozone layer. This layer of gas shields the planet from direct UV rays, which have damaging effects on DNA molecules. The favorable environment triggered the explosion of many new complex lifeforms.
Some experts also propose a theory that points to the biological evolution of organisms as the reason for the explosion. For instance, a biological arms race between prey and predator species might have necessitated rapid evolution on both sides. Predator and prey species probably evolved new traits and genes to adapt better against each other. This happened within a very short time to ensure the continuation of both groups.
Why does the Cambrian Explosion Matter?
The Cambrian explosion had a tremendous impact on life on earth and our present-day understanding of that period of history. We can examine the explosion’s impact based on its effects on our understanding of the theory of evolution and fossil records.
Cambrian Explosion and the Theory of Evolution
For many years, the rapid appearance of new fauna within a short period was used as an argument against Charles Darwin’s theory of natural selection.
On evolutionary time scales, 25 million years was such a short time that it seems inconsistent with the gradual pace of evolutionary change proposed by Darwin. Although the Cambrian Explosion does raise several vital questions about the theory of evolution, it does not contradict its correctness.
At the very core of the theory of evolution, it proposes that all living species evolved from a common ancestor. While the Cambrian explosion resulted in the evolution of many organisms with diverse body plans, there’s still a link between many animal phyla that developed at the time and their modern offsprings. For instance, even though the chordates (a group of organisms characterized by a nerve cord, a notochord, and gill pouches) did not appear till much later in evolutionary history, the Cambrian fauna that evolved during the biological explosion had soft-bodied creatures with traces of these characteristics.
Cambrian Explosion and the Fossil Records
Although plant and animal life began long before the Cambrian, geologists consider the Cambrian as the beginning of the geologic time scale. This is because the Cambrian explosion marked the evolution of organisms with hard parts that could be preserved in rocks. Before this time, most organisms on earth had soft tissues that hardly got fossilized.
During the biological big bang, many groups of animals evolved hard parts. Many experts believe this was either due to the rise in oxygen levels in the atmosphere, triggering the production of collagen in these organisms. It could also be due to an increase in the concentration of carbonates in the oceans of the Cambrian period.
Another major change during this time includes the development of organisms that burrow into sediments on the seafloor instead of just living on top of it. This made it easier for them or evidence of their life to be preserved in fossil records, making the explosion an important period in geological history.
Most scientists agree that a significant event happened at the start of the Cambrian period, and this event is significant to the geological history and our understanding of evolution. However, we can only speculate on what triggered this event and what really happened during this relatively short period.
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- Britannica, Available here: https://www.britannica.com/science/Cambrian-explosion#:~:text=Cambrian%20explosion%2C%20the%20unparalleled%20emergence,make%20up%20modern%20animal%20life.
- Biologos, Available here: https://biologos.org/common-questions/does-the-cambrian-explosion-pose-a-challenge-to-evolution
- Byjus, Available here: https://byjus.com/biology/cambrian-explosion/