What is the Cambrian Explosion ?

Question

One of Darwin's most popular icons is the "Tree of Life" that supposedly connects all living things. Implicated in this tree are thousands of supposed "transitional forms" as one species gradually evolved into two separate species.

1. What is the Cambrian Explosion ?
2. How does it undercut Darwin's theory of gradual change?
3. There are other such "explosions" that have been found in the fossil record. Find one other such explosion and explain how long ago it occurred (in standard evolutionary timescales) and what organisms suddenly appeared in it.

Answer 1

Assuming that this question is not for a graded school assignment, test, quiz, or exam, but is rather for the benefit of an inquisitive mind, I would be happy to explain these fascinating topics in evolutionary biology.

What is the Cambrian Explosion?

The Cambrian Explosion is the relatively sudden appearance of most major animal phyla (the primary body plans) in the fossil record. This monumental event in the history of life began around 541 million years ago and unfolded over approximately 20-25 million years—a remarkably short timespan in geological terms.

Before the Cambrian, life consisted mostly of simple, soft-bodied organisms. The explosion marks a dramatic turning point with the emergence of animals possessing entirely new and complex features, such as:

1. Mineralized skeletons, shells, and exoskeletons.
2. The first sophisticated compound eyes.
3. Distinct body parts like legs, antennae, and claws.
4. Evidence of active predation, which created an evolutionary arms race.

Famous animals from this period include the ubiquitous trilobites and the large, apex predator Anomalocaris. The sheer diversity and novelty of life that appeared during this period is why it's considered an "explosion."

How It Challenges Darwin's Theory of Gradual Change

The Cambrian Explosion presented a significant puzzle that Charles Darwin himself acknowledged as a major challenge to his theory of slow, gradual evolution. The "undercut" to his idea of gradualism comes from two main observations:

1. The Sudden Appearance of Complexity: Darwin's "Tree of Life" model predicts that new, complex body plans should emerge slowly, with a long succession of simpler, transitional forms leading up to them. However, the fossil record shows the major phyla appearing fully formed and distinct from one another, seemingly without a long, gradual series of fossilized ancestors in the preceding rock layers.
2. The Missing Ancestors: Darwin expected the Precambrian layers to be teeming with the simpler ancestors of the Cambrian animals. In his time, these fossils were nowhere to be found, a fact he admitted was a "valid argument" against his theory.

Modern Scientific Explanations

Today, scientists have several powerful explanations for this apparent paradox:

1. The "Artifact" Hypothesis: The ancestors of Cambrian animals were likely very small and soft-bodied, which means they had a very poor chance of fossilizing. The "explosion" may be an explosion of hard parts (shells, skeletons) that fossilize easily, making these animals suddenly "visible" to us in the rock record.
2. The Genetic Toolkit: Scientists have discovered a set of master control genes, called Hox genes, that dictate the basic body plan of an animal. It is now believed that this genetic "toolkit" evolved in the Precambrian. The Cambrian Explosion may have been a period of rapid experimentation with these pre-existing genetic tools, triggered by new environmental conditions.
3. Environmental Triggers: A rise in atmospheric oxygen would have supported larger, more energetic animals, while changes in ocean chemistry made it easier to build shells. These new conditions could have been the catalyst for the rapid diversification.

Another Evolutionary "Explosion"

Yes, there are other such events. The most significant one after the Cambrian is the Great Ordovician Biodiversification Event (GOBE).

1. When It Occurred: The GOBE started around 485 million years ago, during the Ordovician Period, and lasted for about 25 million years.
2. What Suddenly Appeared: Unlike the Cambrian Explosion, the GOBE was not about the appearance of new phyla (body plans). Instead, it was a massive diversification within the phyla that already existed. This event saw the number of marine genera quadruple, creating the first truly complex and multi-layered marine ecosystems.

Key organisms that appeared or radiated dramatically during the GOBE include:

1. The first true corals, which began to build large reef systems.
2. A massive increase in the diversity of brachiopods (shelled animals), bryozoans (moss animals), crinoids (sea lilies), and cephalopods.
3. The establishment of vast communities of plankton, which fundamentally changed the marine food web.

The GOBE established the framework for marine life that would dominate the planet's oceans for the next 200 million years.