Remember the arduous tale in the Hollywood blockbuster 2012? The premise was the Mayans somehow knew and predicted in primitive hieroglyphics that there would be an episode of crust-shifting that year. As in the Earth’s thin, semi-solid crust would shift radically over the ocean of molten, pressurized, rocky material called the mantle in just a few days. The consequences were, naturally, apocalyptic. Fortunately, in the film, giant arks were constructed so that the 0.01 percent richest people in the world could float safely away in oceans, awash like water rocking back and forth on a global scale in a teetering bathtub. Like so many disaster premises, this is unlikely to actually happen.
But, it turns out something sort of like that might happen over geologically rapid periods of time, which is more like events playing out on the order of 10 to 25 million years. What’s more, there’s some tantalizing but early evidence now being examined that is consistent with something like that really happening just over half a billion years ago. And that kind of massive shift just might help explain an artifact of the fossil record that has been hotly debated by learned scientists since before your grand-pappy ate his first solid food: the Cambrian Explosion.
Come on down and we’ll talk about it, because that Cambrian is a neat time. It’s full of true enigmatic monsters, almost alien-like in their weirdness, and the so-called bloom of biodiversity that characterizes the explosion part is only one the coolest biological events in all of natural history!
Cast your mind back almost 600 million years and the Earth is a very different place. The day went by in 21 hours, the moon was a tad closer. Land and sea are just recovering from an ice age so extensive the glaciers may have reached the equator, encasing the planet in white for more than 10 million years. But the snowball Earth isn’t thermally stable over long periods of time. Without active storms and processes to remove greenhouse gasses, GHGs build up. The atmospheric mix reaches a tipping point, and in the space of a few centuries the ice melts from equator to poles, and the Earth temperature shoots up from a global average at or below freezing to steamy tropical conditions. The storms would have been momentous!
Other than scouring wind and rain, not a lot was happening on land. There were no trees or bushes or plants yet—beyond a few hardy lichens clinging to jagged rocks poking out of the retreating ice—and even topsoil is far away in the future. But underwater, over eons, super collectives of cells and bacteria have been developing. They started out as colonies with little structure, just a big hunk of slop. But now some of those cells have specialized to serve the colony in different ways. Reproduction of the entire colony has been centralized. Thus, several new types of creatures have developed in the dark, frigid water. They look a lot like plants, but they are the first true animals. And given half a chance they’re ready to evolve beyond the simple sponges and corals that came before them. If only they were suddenly exposed to unlimited sunlight, light no longer filtered through ledges in icy coastal eddies, and moved into a warmer environment … well, both those things may have happened, kickstarting the Cambrian:
The mystery is what is known as a "true polar wander," which takes place when the entire solid Earth slips about its liquid outer core over the course of millions of years. This process is contrary to the standard processes of Earth's continental and oceanic plates that are typically in constant motion relative to one another and can cause the geographic locations of Earth's plates to shift altogether in the same directions. The team suggests that approximately 520 millions years ago, a giant lurch of more than 60 degrees led to the shifting of most continents from polar to tropical latitudes.
Imagine you have a big stone sphere spinning in zero gravity right in front of you. It’s almost perfectly round, and now you add a giant dollop of cold, viscous molasses all over one quadrant near the spin axis. Well, that molasses is being slung by centripetal force. It’s going to try and to spread in a thinning, sticky layer toward the equator of our rocky ball. If the goo were embedded in something thin, like a film of tissue paper, it would pull that paper toward the same imaginary line around the sphere’s middle. Now replace that molasses with a plate, and the tissue paper with a super-continent, and you get the gist of this idea.
Over 10 to 20 million years, a big plate gets dragged from the southern hemisphere toward the equator, carrying oceans and land above for the ride. Sea levels spike as the last of the glaciers melt, and tides are ferocious by today’s standards. The biome—the available living space for our new class of critters—expands by orders of magnitude and becomes far more hospitable. That’s a recipe for evolution to kick in, what paleobiologists called a living radiation. But keep in mind the time-scales here: If you were alive during this explosion, over the course of a human lifetime, there would be no observable change. In fact it would probably be rather dull, after the novelty of cavorting among Cambrian biota wore off.
And take off it did: Over a 25 to 50 million year period, some of the coolest, weirdest, and scariest looking creatures evolved from the simple filter feeders and worms that dotted the gently warming, shallow sea floors. It was in a sense the first “modern” ecology because now, for the first time, there was an active prey vs. predator vs. scavenger cycle among multi-cellular animals. The first recognizable close relatives and possible ancestors of today’s familiar insects, clams, and snails arose in the rich, muddy bottoms. The descendants of the snails and bugs in particular would go on to wage an epic evolutionary war for dominance in the ocean lasting hundreds of millions of years. By the time the next geologic period rolled around, known as the Ordovician, one family of snails had evolved into giant squid-like ammonoids with stinging tentacles surrounding a beak that could lop off softball sized chunks of bone and meat. Some of the bugs went on to produce the Eurytpids, the largest arthropod to exist, a sort of 10-foot long water scorpion complete with equally enlarged pincers to lop off pieces of prey into nice, easily swallowed hunks.
Of course the fossil record is incomplete, and that’s especially true for really ancient times like hundreds of millions of years ago. Most of the action was in the ocean, and sea bearing crust gets extruded, flows across toward the boundary of a continent, and usually subducts beneath it like a like a disappearing conveyor belt, where it is melted and recycled in the furnace of the mantle never to be seen again. Marine sediments are sometimes preserved on land when they collide and uplift in small sections at the subduction zone. These ancient rafts of bygone geology are rare indeed. It’s even rarer for that small preserved fragment to be rich, fossil bearing strata. We only have a handful of really good examples of that, the most notable being the Burgess Shale in Canada. That’s the location of half of what we know about late Cambrian biodiversity and the primary source of the explosion notion.
With all these super-snails and super-bugs fighting it out for dominance, it would be easy to miss some of the more unassuming animals wriggling around. They came in a dazzling array of shapes. One is called the Hallucignia, because it’s so weird-looking it would take a naturalist on an acid trip to dream up such a creature. But there was one, not much bigger than a healthy night-crawler, with a sliver of rigid tissue down it’s back housing what would one day be a spine full of nerves. That little guy (or something quite close to it) called the Pikaia would become the first fish. It would give rise to amphibians that would colonize the land and become reptiles, birds, and mammals.
So much of what we know and love, from people to pets, from stone tools to Mozart, from poorly seared rotting flesh heaped on the embers of an early wildfire, to cookie dough fudge ice cream on the Fourth of July, would spring up from that one little clade of proto-vertebrates.