Before ~541 million years ago, animals were relatively simple and similar. Beginning with the Cambrian period (541-485.4 million years ago), animal life suddenly became abundant and varied. This was called the “Cambrian Explosion.” It was during this sudden (“sudden” in evolutionary terms) change that many modern lineages began, including mollusks, arthropods and the chordates. The variety of animals “blew up” during the beginning of the Cambrian.
Why the sudden diversification of animals in the Cambrian?
Like “What killed off the dinosaurs?”, there are plenty of theories to go around:
- Gondwanaland (the original supercontinent) broke up, providing shallow seas
- Erosion increased as Gondwanaland broke up, increasing calcium in the water
- Biochemistry complexity of multicellular organisms reached a point at which they could support more complex animals
- Biochemical processes developed as animals became more diverse that could support more complex animals (1)
- Burrowing behavior developed, destroying the algal mats on the shallow sea floor and changing life forever
- Meteorite bombardment caused accelerated evolution ( ((I don’t quite follow this one, but it’s worth noting the Moon shows increased meteorite bombardment about the time of the Cambrian.
- The development of a new sense, the “eye” spurred evolutionary change
- An evolutionary “arms race” occurred, where being able to eat other animals and avoid being eaten caused life to develop many forms
- Development of bilateral symmetry.
These are just a few of the theories as to why the Cambrian Explosion occurred. Which one’s right?
When we think of explosions, we think of things being blown to pieces. But what if this metaphorical “explosion” was caused by all these pieces? The suggestion is obvious, but this is science. Hypotheses are a dime a dozen; you need data to figure out if your hypothesis is on the right track or just a useless idea.
Professor Paul Smith of Oxford University and Professor David Harper of Durham University published Causes of the Cambrian Explosion in the journal Science:
Earth system, developmental, and ecological processes have been hypothesized as isolated, singular causes of the major diversification of marine taxa early in the Cambrian. Instead, many of these processes sit within a series of cascading and nested feedback loops that together generated the Cambrian explosion. Each box corresponds broadly to a stand-alone hypothesis or suite of related hypotheses (red, geological; blue, geochemical; green, biological). The figure represents a narrow interval of time at the beginning of the Cambrian (541 million to 521 million years ago).
I mentioned the dinosaurs going extinct earlier for a very specific reason: there are a lot of theories as to why the dinosaurs went extinct, and more and more, it’s looking like they all had a hand in the extinction, with one surprising possible exception. The fossil records don’t contain a lot of dead dinosaurs from the time of the Chicxulub meteorite strike. Now, being saved in the fossil “record” is hit-or-miss, but it’s still a little odd. But what if the dinosaurs already died out and the meteorite strike was just “icing on the cake?”
Side note: The featured picture for this post is one of my fossils, a Dalmanites limulurus trilobite from 430 million years ago. Now, the Cambrian ended 485.4 million years ago, so this isn’t from the Cambrian. But it was during the Cambrian that trilobites arose, so I used the picture. Besides, I like trilobites.
- You’re probably thinking “chicken and egg” after these last two, aren’t you? [↩]