Wednesday, August 6, 2014

The Proterozoic Eon, Part 5

Full Linear Timeline of Life on Earth
You may remember that we started our journey through the Proterozoic Eon two and a half billion years ago. After a hell of a lot of Monopoly, we now arrive in the still-Proterozoic of only one billion years ago.

We've come more than half of the way home, watching 1.5 billion years of the Earth's history. The continents have shuffled around a lot, volcanoes have come and gone, and we've suffered a couple devastating meteor impacts.

Life has undergone a few major changes, all on the microscopic level. Cyanobacteria destroyed the environment. Complex cells called eukaryotes evolved, then invented sex. At some point along the way, multicellular organisms evolved—we can't say when with any certainty—but they're not much to look at. In fact, from where we're standing along the Toronto coastline, life still looks pretty much like pond scum.

Cyanobacterial fields forever
"You guys had a billion and a half years to evolve, and this is the best you can do?"
Adapted from an image Copyright and CC Martyn Gorman

And if life in the oceans isn't looking all that evolved, life on dry land has gotten absolutely nowhere. The coastlines, rivers, and lakes of Earth are teeming with life, but go any distance inland and it's the same sandy regolith we saw when we got here, with nothing but the odd desert crust to prove that life exists.

Land in the Proterozoic Eon

Now let’s play a couple more games of Monopoly, which will bring us to an even billion years ago. Something funny is happening now. Remember the supercontinent Columbia, which broke up, just like the Eagles?

Well, it’s getting back together, just like the Eagles. The ocean off the coast of Toronto is narrowing. In fact, you can just about see Montevideo coming over the horizon. Wave! All the major plates are smashing into each other, forming the new supercontinent of Rodinia.

Life has been busy, too. Following up on their recent triumph of sex, they’ve now invented murder.

As with sex, it’s entirely possible that murder existed long before now, but it’s here, at about a billion years ago, that we see the first clear signs of it, in the form of predation. We don’t actually have a fossil of a predator eating its prey this early in history. For that matter, we don’t have a fossil of a predator at all.

But we’re pretty sure predators were around, because we can see what their prey were doing to defend against them. Because this is the era when acritarchs, an umbrella classification encompassing microscopic fossils, started to get spiky1.

Image from PNAS cover July 2005

Other eukaryotes , around this time, also start evolving mineralized coatings that might have been a defense against predators. And stromatolites, those bulbous formations created by bacterial mats, begin to decline, perhaps because they fell victim to newly evolved predators1.

Stromatolite taxa, acritarchs, macrophagous predators, and grazers
From: Bengston. Origins and Early Evolution of Predation. The Paleontological Society Papers 8. pp 289-317

We’re going to play a couple more games of Monopoly to take us to about 800 million years ago. Here, the supercontinent Rodinia is breaking up. Yeah, again. You’re going to have to get used to it, because this shit happens a lot.

In the oceans, more crazy shit is going down. The first armored amoeba have shown up2, and there’ll be no hemming and hawing this time, because we know these little bastards are predators. There is some hemming and hawing about how long amoeba have been around, though. These little armored dudes are the first to leave fossils, but amoebas may have been around for a while. They may, indeed, have been the predators we saw signs of two hundred million years earlier.

And there’ll be no hemming and hawing about animal life this time, either, because something has just shown up off the coast and no one with a lick of sense would dare deny that it’s an animal. And here it is, in all its glory. The first animal:

The First Animals?
From: The first animals: ca. 760-million-year-old sponge-like fossils from Namibia. South African Journal of Science. Vol. 108 no. 1-2 Pretoria 2012.

Yeah, it’s a sponge. They’re animals, you know. It may seem weird, but they are definitely our ancestors, and the ancestors of every worm, fish, lizard, and human being alive today. They’re multicellular, can reproduce sexually, are made of highly specialized and differentiated cell types, have a primitive immune system, and even primitive muscles. They’re animals, and they’re here to stay.

And they’ve shown up just in time for another ice age. Sucks for them.

This ice age won’t last quite as long as the last one—only a couple hundred million years—but it’s going to be even more severe. Again, the glaciers and pack ice will come and go, because an ice age doesn’t mean that it’s always frozen, all the time. But there will be long periods, lasting from 20 to 60 million years, when most of the planet is covered in a kilometer or more of solid ice. Some of these may even have been more “Snowball Earth” events.

I told you to bring a sweater. It’s not my fault you don’t listen.

Two games of Monopoly later, and the planet is finally warming up again. We’re now at about 600 million years ago, the continental plates are coming together yet again, this time to form the supercontinent Pannotia. And, I’m sad to say, we’re only one Monopoly game away from the end of the Proterozoic Eon.

Here at the end, the oceans are warming up and conditions are ripe for life. The cyanobacterial mats are still here, and they still make up most of the biomass on Earth, but eukaryotes aren’t doing too bad.

Larger life (meaning, larger than a microorganism) from this period is hard to classify, because most species are soft-bodied and live on or above the bacterial mats. They don’t leave great fossils, unlike the hard-bodied trilobites and burrowing worms that will start to show up in force a hundred million years from now. But maybe they look something like this:

Ediacaran (Late Proterozoic) Ocean Life
Image CC Ryan Somma

We don’t know what most of these animals evolved from, and we don’t know what they evolved into, assuming they even left ancestors to evolve into anything. We’re not even sure when they died out. We think it was at the end of the Proterozoic, but without any clear sign of an extinction event such as a meteor or extreme volcanic activity, it’s possible they survived right into the Cambrian. In the Cambrian, the paucity of bacterial mats may have made fossilization of soft-bodied creatures less likely, and so they may have disappeared from the fossil record despite living on.

What we do know is that they’re here in the last days of the Proterozoic, and there are a freaking lot of them. In modern times, you’ll find them in the fossil record all across the planet, if you know what to look for. They spread, they evolved, they covered the planet with an explosion of complex life.

And then they disappeared.

Sorry to say, but so must I. The old Monopoly board has gotten a little decrepit over the past two billion years, and you can hardly tell St. Clare’s Place from Baltic Avenue anymore. We’ve had fun, but it’s time to go. The Cambrian is coming, and we don’t want to get eaten by an anomalocaridid.


If you enjoyed this trip through the Proterozoic Eon, check out my other science articles in the Archives!

Citations and References
  1. Bengston. Origins and Early Evolution of Predation. The Paleontological Society Papers 8. pp 289-317.
  2. Porter. Testate amoeba in the Neoproterozoic Era: evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon. Paleobiology 26 (3) pp. 360-385.

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