Science & Technology

The Cambrian Explosion Was Earthly Life's Awkward Phase

Somewhere out there, there's a picture of you in your high school years that you'd probably rather not show to anyone else. Maybe you had a gnarly set of braces, maybe you were way too into ska, or maybe it was just a painfully emo haircut. Well, you're not alone. The whole planet went through its own awkward phase once — and it lasted 55 million years.

A Biological Explosion

The creatures of the Cambrian Period weren't the first life forms on the planet, but they're some of the first that we have a meaningful record of. And what we've discovered is downright bizarre. This was the period in Earth's history that saw the evolution of all of the animal phyla that survive today. Literally all of our ancestors first evolved in this period — but so did a ton of other critters who left no descendants behind.

Trilobites are probably the best-known creatures of the Cambrian Period. They were certainly the most successful. They're examples of arthropods, just like insects, arachnids, and crustaceans, and for millions of years, they sat comfortably atop the world's food chain. They even survived for about 200 million years after the Cambrian, but ultimately went extinct around 270 million years ago, just before the first dinosaurs evolved.

Trilobites might not have any direct descendants, but they were still arthropods, and we still have giant bug-looking critters that live underwater (looking at you, lobsters). But a lot of the creatures of the Cambrian don't even have a modern cousin that strikes a passing resemblance. Take the spiky tube of legs known as Hallucigenia sparsa, for example. Even its name suggests that scientists thought they might be seeing things. Then there's Opabinia regalis, a truly alien looking creature with a long, flexible snout and a flat, broad, fin-like body. We could go on, but you should really just check out the lineup. For a long time, prehistoric Earth looked a lot more like "Alien" than like "Jurassic Park."

Oh Shale Yeah

Like we said, there was life on the planet before the Cambrian, but we just don't know much about it. There are a few major reasons for that. The years leading up to the Cambrian were the first time that hard materials such as shells turned up, and hard materials fossilize much better than soft ones. There's also the (somewhat controversial) theory that the Cambrian truly marked a period of unprecedented diversification, as opposed to just being the first time that this kind of diversity was able to make a mark on the fossil record. But when you get down to the (metaphorical) bones of it, the answer lies in the Burgess Shale.

Discovered in 1909, the Burgess Shale represents an underwater mudslide that happened about half a billion years ago, trapping an ecosystem of never-before-seen animals in incredible detail. While hard shells fossilize better, this particular set of fossils also features soft-tissue creatures and even organs such as livers, hearts, and nervous systems. Without this discovery, we'd still probably know hardly anything about the Cambrian except for the small shelly fauna, or SSFs, whose name is pretty much all we know about them.

But there's a new piece of the puzzle that could shine an even brighter light on the Cambrian — this time, under a microscope. A revolutionary acid extraction procedure of Greenland rocks has revealed a new wealth of fossils from the era. But unlike those of the Burgess Shale, these are a millimeter in length or smaller. We could soon develop a much clearer picture of Cambrian evolution, and with it, the origin of life as we know it.

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Written by Reuben Westmaas January 17, 2018

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