This episode, we look at one of the most influential and mysterious events in the history of animal evolution, the Cambrian Explosion.
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What is the Cambrian Explosion?
The Cambrian Explosion is an event in the fossil record where nearly all modern animal body plans and phyla appear within a period of 20 million years at the beginning of Cambrian Period.
This event is fascinating both for the fossils found during this time and for the multitude of questions that remain unanswered about it.
Fossils before the Cambrian are extremely rare and often very simple, in contrast to the complex life of the Cambrian and since. This raises question such as: Where did these early animals come from? Who were their ancestors? Why do they appear so abruptly in the fossil record?
The fossil record of early life before the Cambrian is almost all single-celled lifeforms. The first well-known multicellular fossils are those of the Ediacaran Biota. These strange creatures – many of which may not have been animals at all – achieved fascinating diversity, but went extinct at the beginning of the Cambrian.
As the Cambrian begins we see the first hard body parts in the form of Small Shelly Fossils. Eventually, more complex animals appear, including the ancestors of nearly all modern-day animal groups from corals to chordates, the famous trilobites, and some forms so strange we don’t actually know where they fit in the tree of life.
This diversification lasted about 20 million years and established all of the recognizable animal body plans we see today, as well as the modern structure of ocean ecosystems.
Historical Views of the Event
The scientific view of this event has changed significantly over the years as we’ve learned more about it.
The first Cambrian fossils discovered were trilobites in the 1700-1800s. The Cambrian Period was established as the earliest period with animal fossils since, at that time, none had been found earlier in the fossil record. For a long time, trilobites were used to biologically date the rock layers of the Cambrian.
The lack of earlier fossils posed a confusing question for scientists. Darwin commented on the issue, saying in On The Origin of Species that the sudden appearance of complex life and lack of ancestral fossils “may be truly urged as a valid argument against the views here entertained [that is, his theory of evolution]” (Darwin 1859, p308).
As time went on and the field of paleontology matured, more early animal fossils came to be discovered, and the details of the Cambrian Explosion better understood. Extremely well-preserved fossil sites such as the Burgess Shale have helped show that this was not a sudden appearance of life forms, but a case of surprisingly rapid diversification and evolution.
There are a number of hypotheses as to what exactly caused this sudden “explosion” of species and body designs. So far the exact answer is still unclear. A number of studies give an excellent overview of the details and potential causes of this event, such as Marshall 2006 and Levinton 2008.
Changes in the global environment may have set things up to allow for this increase in diversity, making conditions more ideal for life to develop and adapt. Events around this time included increases in oxygen levels, the ending of a global glaciation, and possible environmentally-linked mass extinction. Exactly how these events tie into the Cambrian Explosion remains unclear.
Another possible cause is that the genetic complexity of life reached a level that allowed for an increased rate of adaptation. One genetic tool that many scientist think may have developed around this time is the developmental Hox genes. These are genes that inform the layout of the body, and can be found in many animals today across distantly related groups, suggesting that this feature evolved very early on in animal evolution.
It’s also possible that the rapid appearance of new body plans was forced by changing interactions between animals and with their environment. For example, complex eyes developed first at this time, allowing animals to visually explore their environments. This is also the time (perhaps not coincidentally) that sees the appearance of the first major animal predators.
Now able to look for one another, predators and their prey could chase and flee respectively. These new pressures may have been a driving force behind the increase in diversity of the Cambrian Explosion, kicking off an evolutionary arms race that continues to this day.
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