Over half a billion years ago, during the Cambrian geological period, life on Earth started to get a lot more interesting. Thanks to the rise in free oxygen generated mostly by photosynthesizing algae, lifeforms could draw much more energy out of the environment. That meant the rise of multicellularity and the beginnings of a world full of the macro-sized plants and animals we know and love. That moment, full of weird-ass animals like Anomalocaris, is called the Cambrian Explosion.

The Cambrian Explosion gets a lot of play because it was the first time multicellular creatures ruled the planet. What few people (other than geologists and paleontologists) realize is that there was an even crazier time for early life. It came during the Ordovician period, right after the Cambrian came to a close 485 million years ago. The Ordovician Radiation, also called the Great Ordovician Biodiversification Event (GOBE), saw a quadrupling of diversity at the genus level (that's the category one step above species). Life also started occupying new ecological niches, clinging to plants floating in the ocean's water column and burrowing deep into the seabed.

Like the Cambrian, the Ordovician was a period when all of life still existed underwater. Most of the continents had formed a supercontinent called Gondwana over the south pole, creating the largest tropical coastline in our planet's history. (There were no polar ice caps during this period.) The warm coastal waters surrounding Gondwana were perfect for new kinds of animals, like brachiopods, crinoids, ostracodes, cephalopods, corals, and bryozoans. Plus, everybody's favorite Cambrian animal, the trilobite, diversified like crazy and moved into many new habitats during this time.

One of the most emblematic animals of the Ordovician Radiation is the largely extinct graptolite. Graptolites spread successfully throughout the world's seas. Most lived in floating colonies made from tubes of collagen or chitin that they extruded from their bodies, much like bees making wax. To get food, they poked their tentacles out of apertures in these tubes.

Environmental scientist Cole Edwards of Appalachian State University in Boone, North Carolina, worked with a team to analyze chemical signatures in ancient rocks that tell us about gases in the atmosphere millions of years ago. In a new paper for Nature Geoscience, Edwards and his fellow researchers offer a possible explanation for the Ordovician Radiation: an even greater dose of oxygen in the atmosphere, which also meant more oxygenated waters in the then-shallow global oceans.

The researchers write:

A global increase in atmospheric oxygen and oxygenation of shallow marine environments may have also eased stressful conditions for benthic animal life and expanded the range of habitable ecospace for infaunal burrowers deeper into the sediment. A more oxygenated ocean could also have supported more predators in the food chain (fish and cephalopods), setting into motion an evolutionary 'arms race.'

Essentially, the rise in oxygen opened up new habitats, thus sparking more evolutionary adaptations to these novel environments. At the same time, there was enough energy to support more predators like cephalopods, the shell-wearing ancestors of today's squid and octopuses. Nothing like an arms race between predator and prey to cause rapid evolution as well. So there was basically a perfect storm for evolution.

Unfortunately, the evolutionary free-for-all came to a terrible end during the world's first mass extinction, which closed out the Ordovician about 440 million years ago. For reasons that are still poorly understood, the planet's temperatures plummeted, ushering in two ice ages in rapid succession. All those warm coastal areas dried up and froze. As a result, more than 75 percent of all life on Earth died out. The researchers speculate in their paper that the rise in oxygen, accompanied by a lowering in carbon dioxide, might have been one factor that led to these catastrophic ice ages.

Nature Geoscience, 2017. DOI: 10.1038/s41561-017-0006-3 (About DOIs).