History of Life: The Cambrian Explosion
Life can get tricky sometimes. I mean the history of life, of course… The Cambrian Explosion is a great example. Evidence for the first bacterial forms living on Earth date back to 4 billion years ago. It then took nearly 2 billion years for these unicellular organisms to get it together, as they say, and become multicellular. Then, after only a billion and a half more years (that’s 500 million years ago for those playing at home) nearly every animal phylum that exists today had evolved and diversified to some extent. This was quick jump for a slow and gradual process.
The sudden appearance of diverse life in the fossil record baffled scientists and even caused Darwin to question his theory on evolution by natural selection. They called this the Cambrian Explosion. Today, scientists have several theories to explain the phenomenon. One theory is that life may have been able to evolve quickly because the Earth had changed so much that it created enough new niches that species could rapidly fill them. The second theory is that the explosion was really a dud; that some lineages can be traced farther back than the Cambrian and so there was no real ‘explosion’. Like a lot controversial science topics, the answer probably lies somewhere in the middle.
In the last 542 million years there have been three eras of Earth’s history, the Paleozoic, the Mesozoic, and the Cenozoic. Each of these eras is broken up into two or more periods. The Cambrian period is the first period of the Paleozoic era, the period in Earth’s history when we can see more evidence for diverse life than anytime before that. This period began 542 million years ago with an explosion of life and ended with a mass extinction 52 million years later.
Building a Bomb
Earth was changing drastically in the millions of years before the Cambrian Explosion. Multicellular animals had taken some strange forms, and many of them have not been fully classified by scientists because they are too weird to fit any one category. Some animals, like flatworms, segmented worms and maybe cnidarians and sponges (though this is up for debate) made their appearance just before the Cambrian. These animal-wierdos are known as the Ediacaran Fauna. Unfortunately, these animals were nearly all soft bodied and didn’t fossilize well. This is one of the arguments against the explosion theory; that the fossil record is too incomplete to say that animals diversified as fast as it appears. It is possible that we evolved from stranger beings than we thought.
Life on Earth today would probably have been much different if the Ediacaran Fauna had persisted and lived on. But, fortunately for us, they did not. Geologists agree that, before the Cambrian, in the late Proterozoic era, Earth’s temperature dropped insanely low and glaciers covered almost the entire surface of the planet. Temperatures got so low that even the equatorial areas were a slushy mess. This is known as the Snowball Earth theory. The massive glaciers that covered Earth may explain why Earth gave the cold shoulder (get it?!) to the Ediacaran Fauna and many went extinct. Thankfully, the ice thawed and life was able to move on.
The late Proterozoic was changing shape as well as temperatures. A massive supercontinent called Rodinia began to fracture and split into separate continents. This allowed shallow seas to dominate a significant chunk of Earth’s surface. Today, shallow seas are biodiversity hotspots for a reason. They are warm, they have plenty of sunlight, and plenty of food. Surviving Ediacaran fauna would have naturally migrated to such a place. Because many ecological niches were empty after the last major extinction, these animals would have been able to radiate easily and speedily.
Who came to the big bang?
As we have said, some phyla like flatworms, segmented worms, sponges and cnidarians, hung around from the Proterozoic. Others, like mollusks, arthropods and chordates evolved in the Cambrian.
Mollusks are represented by what scientists call ‘small shelly fauna’. Basically, they are just like they sound: small (a couple of millimeters long), soft bodied organisms encased in a calcium carbonate shell. Later in the Cambrian, more shelled fossils appear.
Arthropods are represented by everybody’s favorite fossil, Trilobites, among other animals that evolved a hard chitin exoskeleton, instead of a calcium carbonate shell.
The earliest Chordates, our own illustrious phylum, even made their appearance in the Cambrian! This animal was called Pikaia, and was one of the earliest true chordates.
The list goes on to include nearly every animal phyla that currently exists! The only phylum that was tardy to the party were the Bryozoans (moss animals), who evolved in the early Ordovician period, right after the Cambrian.
Fat Man and Little Boy
Whether the phyla that appear are a result of long, gradual evolution or fast radiation, we can see that the fossil record becomes much more crowded in the Cambrian. Much of this is due to animals finally evolving hard parts like exoskeletons and shells, which fossilize very well.
Many scientists call the Cambrian Explosion an ‘evolutionary arms-race’ to account for such a fast radiation. The adaptations that arose in the Cambrian were mainly for protection and predation. If one animal evolved a hard exoskeleton, another would have to evolve better predation strategies to survive. Some evolutionary marvels like eyes, brains and ears may have come out of such arms races.