The Cambrian Explosion: Biological Diversity Gone Wild

Escaping the Oceans

Slowly, the oceans became saturated with life as all of the niches became filled. It was time for biology to expand into the next frontier: land. However, this was impossible at first. Ever since the formation of Earth, there was a problem with the elements land was exposed to. Similar to Mars today, the planet had no defense against solar radiation, specifically ultraviolet radiation. Essentially, the sun was acting as a laser capable of destroying anything directly exposed to it. The effects of radiation on life range from DNA damage to cellular destruction, something I have briefly covered in the past.

Once again, chemistry solved this problem entirely by chance. It turns out that the oxygen gas produced by the photosynthetic organisms is able to interact with and absorb ultraviolet radiation, converting it into heat as the gas transforms from oxygen (O2) to ozone (O3) and back again. This process blankets the planet, shielding it from the rays.

Colonizing the Continents

Plants were the first to migrate onto land, probably due to sufficient carbon dioxide in the atmosphere as well as pressure to escape their predators (remember that plants are also subject to predation, creating evolutionary pressure for them as well). Over time, organisms living in shallow water slowly evolved to spend some time out of the water entirely. After all, there were plants to eat and no other animals trying to eat them.

However, there are a few problems with animal migration onto land. First, every organism on Earth has evolved from the oceans. This means that every cell in every complex life form has no defenses against the challenges of dry land survival. The only way to overcome this is to create a barrier between an internal aqueous environment and a dry exterior. If you think about it, this is true for every land animal.

Challenges of Land-Based Living

The laws of entropy function in very different ways on land. By this, I mean that life did not develop ways to resist the expansive forces that are present on land. While primitive organisms were easily able to resist currents of water, they would have been dried up and taken apart by the hot, dry wind currents on land. This is simply because wind was never a factor that evolution had to address. This made biological diversity a prerequisite for life on land. 

Dead Cells Make the Best Shells

Take humans for example. We descended from the water dwelling lifeforms millions of years ago. Our barrier against land is our skin. Under our skin, we maintain an aqueous environment that our cells are able to function in (much like the environment in bodies of water). However, there are no living skin cells on the surface of our body.

In fact, the most effective way evolution developed a barrier is incredibly inefficient. Our barrier against the world is a shell of dead cells that fall off at a rate of 40,000 cells per minute! Our body spends lots of its resources making sure our skin cells are dividing rapidly (up to twice a day). The only living cells that ever are exposed to air are the cells in our eye. Because of all the potential things that could affect the DNA in these cells, they never divide and still degrade over time.  This example shows how much needs to be sacrificed to make life on land possible.

Thus, the first challenge for biological diversity in land dwelling life was to create a protective shell around the living matter within. This shell, along with muscles that could resist gravity, enabled animals to spend the majority of their lives roaming the continents as long as plant matter was accessible. However, the next challenge was that of reproduction on land.