When creating the geologic timescale, scientists were faced with a dilemma: how to break down time into manageable chunks. Luckily for them, the Earth has faced some pretty “world-changing” events, both good–the evolution of new life forms–and bad–mass extinctions. So, when deciding how to end the 3.5 billion year-long Precambrian, the scientists decided to start a new era, the Paleozoic, at the Cambrian Explosion.

 

You might’ve heard about the Cambrian Explosion before, so here’s a brief summary, aided by a National Center for Science Education article. The Cambrian Explosion was the period of time directly following the Precambrian (~540-530 Ma) in which life diversified, or “exploded”. While Precambrian life forms had only soft parts and no true way to move around, animals that evolved in the Cambrian Explosion began to develop teeth, armor, and eyes. In fact, much of the rapid diversification the Cambrian Explosion is known for occurred in only about 10 million years during the Tommotian Period. If you’d like to learn more about the Cambrian Explosion, I’d recommend this short PBS eons video: From the Cambrian Explosion to the Great Dying.

 

According to the UC Berkely Museum of Paleontology, most of the life that evolved during this time was aquatic with small bodies and unusual body layouts. Despite this, many of these Cambrian animals have evolutionary descendants that are still alive today. For instance, one 3.5-inch-long lobster-like creature called the Sanctacaris is actually part of the same arthropod family as today’s spiders, insects, and crabs. (For more information about early Cambrian animals, visit the Museum’s interactive arthropod story).

 

After the Cambrian Explosion, the Paleozoic was rather uneventful for the next 50 million years or so. According to a USGS website, which also informs any later explanation of the Paleozoic’s periods, the Cambrian Period is recognized by the formation of the supercontinent of Gondwana. During the Cambrian, North America was located near the equator, and the climate was warm and wet without seasons.

 

Around 485 Ma, the Ordovician Period began, and the first land plants and corals appeared. The Taconic Mountains (which were related to the formation of the Appalachian Mountains) formed through the collision of North America and Europe, and part of the Gondwana supercontinent (what is today North Africa) moved over the South Pole and started an ice age. As glaciers spread, the sea level and temperature dropped, setting off the Earth’s first mass extinction.

 

The Ordovician-Silurian Mass Extinction, explained in this World Atlas article, occurred about 443 Ma and killed 80-85% of the animals living on Earth, likely due to climate change. This extinction actually occurred in two major waves. The first started when the climate was cooling in 443 Ma, and the second wave began when the climate began to warm again 1 million years later, and even more species were wiped out.

 

As life began to recover in the Silurian Period (444-419 Ma), fish with jaws evolved and the first small insect-like animals moved onto land. The Silurian Climate was similar to today, with cold weather near the poles and warmer weather near the equator. By the Devonian Period (419-359 Ma), there were three major landmasses: North America/Europe, part of modern Serbia, and the remnants of Gondwana. (If you’d like to view the breakup of Gondwana, check out this short animation!) The Devonian had a very warm climate and much of the world was covered in shallow seas, so sharks and reefs became common while the first trees grew on land. Unfortunately, many of these shallow-water swimmers would not survive the Earth’s first mass extinction.

The late Devonian Extinction, also described by this World Atlas article, started around 383 Ma. 70-80% of the Earth’s sea life died, mostly in shallow waters. The open-water sharks and bony fish survived, but most reef fish were wiped out. Scientists aren’t sure what caused this extinction but believe it to be a combination of low-temperature stress and a lack of oxygen, with a possible meteorite or volcanic eruption trigger. Only a few hundred million years after its first evolution, life had already been almost wiped out twice.

 

The Paleozoic might seem like an alien world with its strange marine organisms, but it serves as an important warning. Rapid climate changes played a role in both of these Paleozoic mass extinctions, as well as others I’ve yet to discuss. Climate scientists study the effect of climate change on mass extinctions and the evolution of new life in order to predict how future changes might affect the Earth. While the Paleozoic is certainly ancient history as its name, “ancient life” suggests, it still plays a key role in our understanding of Earth’s future. But hey, I guess what they say is true. Cockroaches (and other arthropods) will outlive us all if their mass extinction track record is anything to go by.

 

Thanks for reading!