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  • The tale of life on Earth has been unfolding for about 4 billion years.

  • And we humans are just the last word on the last page of that story.

  • At least so far.

  • And the vast stretches of time that are covered by the history of life can be hard for us

  • to fathom.

  • We wrack our brains just trying to imagine what a few hundred years looks like, let alone

  • billions of years

  • And, like, speaking for myself, I can't even remember what I had for breakfast this

  • morning.

  • So, to help us comprehend the full expanse of time, scientists have turned to the rocks.

  • By looking at the layers beneath our feet, geologists have been able to identify and

  • describe crucial episodes in life's history -- from bursts of evolutionary diversity to

  • disastrous extinction events.

  • These key events -- of new life and sudden death -- frame the chapters in the story of

  • life on earth.

  • And the system we use to bind all these chapters together is the Geologic Time Scale.

  • First, let's talk about the history of geologic time itself.

  • 'Cause figuring out how to read history in rocks was not easy.

  • For much of human history, of course, we had no idea how old the Earth was, or what actually

  • happened in deep time, or what happened in what order.

  • But in 1669, Danish scientist Nicolas Steno published the first laws of stratigraphy -- the

  • science of interpreting the strata, or layers of rock, in Earth's outer surface.

  • Steno argued that the layers closer to the surface must be younger than the layers below

  • them.

  • So the farther down you dig, he thought, the older the fossils are that you find there.

  • Sounds legit, right?

  • But in Steno's day -- when some people thought that fossils had literally fallen from the

  • sky, for some reason -- this was pretty revolutionary idea.

  • Building on Steno's ideas, Italian geologist Giovanni Arduino went a step further and began

  • naming the layers of rock.

  • In the 1760s, Arduino studied the Italian Alps, organizing their layers based on their

  • depth and composition.

  • The lowest layers of metamorphic and volcanic rocks, he called the Primary layer.

  • Above those were hard sedimentary rocks which he called Secondary.

  • And the top layers of softer alluvial deposits he named Tertiary and Quaternary.

  • But, because rock layers don't appear in this same order all over the world, there

  • was no way for geologists to compare rocks from one location to another.

  • Without a way to compare strata, there could be no universal time scale.

  • Finally, in 1819, English geologist William Smith figured out the solution to this problem:

  • fossils.

  • By comparing the remains of ancient organisms from different rock formations, Smith could

  • match their ages, regardless of how far apart they were.

  • For example, Smith realized that fossils of many early species of trilobites are found

  • below ammonite fossils, which are in turn below certain species of shellfish.

  • So, anyplace in the world where you find these first trilobites, you know that you're looking

  • at rock that's older than when ammonites lived.

  • And even in the most ancient rocks, that have little or no evidence of life, scientists

  • can still look for signs of the very earliest major geologic events, like when

  • continents first formed, and even when the Earth itself cooled and solidified.

  • Thanks to the work of early geologists like Steno, Arduino, and Smith, modern scientists

  • have used these and other clues to create what we now call the Geologic Time Scale,

  • or GTS.

  • The GTS has been reworked many times to reflect the latest knowledge of Earth's history.

  • And today, it's organized into five subgroups: Eons, Eras, Periods, Epochs

  • and Ages.

  • Organizing time in increments like this allows us to ask questions about history on different

  • scales.

  • In the largest increments -- like Eons and Eras -- we can ask the biggest of big-picture

  • questions.

  • Like, was there life on Earth at this time?

  • If there was, what did it look like?

  • Did it live in the water or on land?

  • This is the kind of top-level view we're gonna take today.

  • But the smaller increments of time, like Periods and Epochs, help us take a tighter focus and

  • ask more specific questions.

  • Like, what was the climate like during this window of a few million years?

  • And how did life around the world adapt to it?

  • We'll be talking about those in more detail in future episodes, when we talk about each

  • era, period by period.

  • OK!

  • So, let's get the biggest of Big Picture views of Earth's history right now, by taking

  • a tour of all the Eons and Eras in the GTS.

  • Eons are the largest slices of time, ranging from a half-billion to nearly 2 billion years

  • long.

  • And the earliest Eon is known as the Hadean.

  • It begins with the very formation of the Earth itself, around 4.6 billion years ago and ends

  • 4 billion years ago.

  • And this is the only Eon that doesn't have fossils.

  • Because, back then, the world was justhell.

  • Named after the Greek underworld Hades, the Hadean lived up to its name.

  • The planet was wracked by volcanic activity, cosmic bombardments, raging storms, and temperatures

  • that were at times hot enough to melt rock.

  • But even in this searing wasteland, life may have been able to form.

  • While no fossils have been found from this Eon, small amounts of organic carbon have

  • been discovered in Hadean rocks that some experts think is evidence of the earliest

  • life.

  • These first organisms were tiny and single celled, but they were eventually able to shape

  • the future of the entire planet, so their appearance is the one major benchmark of this

  • Eon.

  • The Hadean was brought to an end by the cooling of the Earth's crust, setting the stage

  • for continents to eventually form.

  • And this cooling marked the beginning of the next phase -- the Archean Eon, which ran from

  • 4 billion to 2.5 billion years ago.

  • Named for the Greek word for 'origin', the Archean was once thought to be when the

  • first signs of life appeared.

  • But at the very least, it's fair to say it was the first time that life flourished,

  • forming mats of microbes in the primordial seas.

  • The fossils that these microbes left behind are called stromatolites, or sometimes, stromatoliths,

  • and the very oldest of them -- like those found in western Australia -- date from the

  • Archaean.

  • During this time, the atmosphere was mostly carbon dioxide, but the appearance of cyanobacteria

  • was about to change all that.

  • Then 2.5 billion years ago, the Archean gave way to the Proterozoic Eon, meaning 'earlier

  • life'.

  • And around this time, photosynthetic bacteria, along with some multicellular forms of life,

  • spewed tons of oxygen into the atmosphere.

  • This probably wiped out much of the anaerobic life on Earth.

  • BUT!

  • It cleared the path for crucial, new organisms, including the ancestral Eukaryotes, whose

  • cells each have a nucleus and organelles wrapped up in membranes.

  • Eukaryotes developed into the first really big, complex, and sometimes kinda weird forms

  • of life, like the frond-like Charnia and the plate-shaped Dickinsonia.

  • These new, larger organisms quickly diversified, and by 541 million years ago, we were at the

  • doorstep of the next and current eon, the Phanerozoic.

  • Its name means 'visible life,' and the Phanerozoic was when life really becameobvious.

  • This is the eon that's home to trees, dinosaurs, newts, aardvarks, and humans.

  • Basically, life as we know it.

  • Hoo!

  • How are you holding up? You doing OK?

  • We've covered about three and half billion years already!

  • Just got another half billion to go and then we're home free

  • OK, now, from here, it's best to explore the Phanerozoic Eon through its Eras, the

  • next level down in the divisions of time.

  • This'll let us explore more recent history in greater detail.

  • The first era of our current eon is the Paleozoic Era, which began 541 million years ago.

  • This chapter was defined by the diversification of visible life, and it started with a bang.

  • Actually, an explosion!

  • The Cambrian explosion.

  • This flurorescence of diversity and complexity in the world's oceans is such a huge deal

  • in the history of life that all of the eons that came before it -- the Hadean, Archean,

  • and the Proterozoic -- are collectively known as the Precambrian.

  • At the start of the Paleozoic, over about 25 million years, the fossil record suddenly

  • reveals the appearance of complex animals with mineralized remains.

  • Y'know, hard parts -- shells, exoskeletons, that kind of thing.

  • And the first of these new animals to become truly widespread were the trilobites.

  • They were so common all over the world that they've been used as index fossils for the

  • Palaeozoic Era for centuries, ever since the days of William Smith.

  • But the trilobites soon had competition.

  • Fish developed teeth and jaws, and came to dominate the seas, including the first sharks

  • and armored giants known as placoderms.

  • Meanwhile, the land, which had been barren since the formation of continents back in

  • the Archean, was finally being populated -- first by plants and then by arthropods.

  • By 370 million years ago entire ecosystems had developed on the primeval continents.

  • Soon after, the earliest amphibians evolved and hauled themselves out of the water, leaving

  • the first vertebrate footprints in the mud.

  • 299 million years ago, the supercontinent Pangea had formed, with an enormous desert

  • at its center.

  • This desert was quickly populated by the ancestors of what would eventually become reptiles and

  • mammals, which could thrive in dry conditions, unlike amphibians.

  • But this time of incredible growth couldn't last forever.

  • and instead, the Palaeozoic Era ended in cataclysm.

  • 252 million years ago, 70% of land vertebrates and 96% of marine species disappeared from

  • the fossil record, including survivors of previous extinctions, like our friends the trilobites.

  • I still miss those guys.

  • The event, known as the Great Dying, was the most severe extinction in our planet's history.

  • But its exact cause is still unclear.

  • A possible meteorite impact site off the coast of South AmericaIslands,

  • might be one clue.

  • And in Siberia, layers of basalt show that massive volcanic eruptions covered large swaths

  • of Pangea in lava.

  • Both of these incidents coincided with the end of the Palaeozoic, and it seems more than

  • likely that the extinction had many causes.

  • In any case, the Palaeozoic may have begun as a chapter defined by an explosion of life,

  • but it ended in nearly absolute death.

  • It took millions of years for life to recover, but when it did, a new world, The Mesozoic

  • Era, had arrived.

  • This is often called the Age of Reptiles, and with good reason.

  • Right from the start of the Mesozoic, reptiles were incredibly successful.

  • This is when they took some of their most famous forms, including dinosaurs, pterosaurs,

  • and a variety of marine species.

  • In fact, all of the non-avian dinosaurs lived only in the Mesozoic, so they remain one of

  • the best index fossils of this era.

  • And many modern groups of organisms also evolved in the shadow of the reptiles, like

  • mammals frogs, bees, and flowering plants.

  • But the Mesozoic Era came to an end 66 million years ago, with yet another episode of devastation,

  • known as the Cretaceous-Paleogene, or K-Pg, Extinction Event.

  • Like all mass die-offs, the K-Pg had many causes, but probably the biggest of them was

  • a gigantic asteroid that struck the earth, sending out enormous amounts of ash into the

  • atmosphere, blocking out sunlight, and creating a vicious cold snap across the planet.

  • Without the sun's energy, entire plant communities died, and the animals that relied on those

  • plants perished with them.

  • Evidence of this impact can be found in a layer of iridium, in rocks dating to the end

  • of the Mesozoic.

  • Iridium is an element that's rare on Earth, but very common in asteroids and comets.

  • And a giant impact crater in the Gulf of Mexico, whose age matches the date of this extinction

  • has become the smoking gun for the asteroid hypothesis.

  • The victims of the K-Pg Extinction were some of the biggest reptiles of the land, sea and

  • sky, including all of what we NOW call the non-avian dinosaurs.

  • Birds survived the cataclysm, of course, making them the last surviving lineage of the dinosaurs.

  • Ok we have 66 million years to go and

  • that's the last major extinction event that we have to

  • talk about. I thought you might want to

  • freshen up so I bought these

  • pre-moistened toilettes

  • just going to

  • you have

  • some Iridium

  • Here. On this side.

  • On your forehead. Other side.

  • With all of the great reptiles gone, the smaller animals that remained were able to eke out

  • a living in the next era, the Cenozoic.

  • This is our era, in more ways than one.

  • It's the era that we're in today, and it also marks the rise of the mammals.

  • Soon after the K-Pg extinction, the climate warmed, and jungles stretched across the planet.

  • Mammals quickly recovered in this hothouse world, and by 40 million years ago, most of

  • the mammal groups that we recognize had come about, like whales, bats, rodents and primates.

  • But, starting 34 million years ago, the climate began to shift again.

  • This time Ice caps started to grow at the poles, taking up much of the planet's water.

  • And these drier conditions created a new habitat, the grassland, where ancestral horses and

  • antelope were first hunted by the earliest cats and dogs.

  • It was also on these grassy plains 7 million years ago that a species of ape known as Sahelanthropus

  • became the first known primate to walk upright.

  • 2.6 million years ago, the ice caps expanded even more, and the Earth entered a glacial period.

  • This is the one you hear referred to as The Ice Age.

  • Over the course of these last several million years, most modern lifeforms that we know

  • about developed and thrived, alongside giants like mammoths, ground sloths and saber-toothed

  • cats.

  • Once again, though, this era of lush diversity came to a morbid end: Starting around 15,000

  • years ago, the climate began to warm up.

  • And over the next few thousand years, many of the giant fauna went extinct.

  • By 11,700 years ago, the last major glaciation was over, and modern humans inhabited nearly

  • all corners of the globe.

  • But how big a role we played in the extinction of the so-called Ice Age megafauna is hotly

  • debated.

  • Regardless, there's no escaping the fact that our species has shaped the Earth to its

  • will since then.

  • Like cyanobacteria, and the dinosaurs before us, we've had a huge impact on habitats,

  • other organisms, and the biosphere itself.

  • And as we've learned today, it's the most dominant forms of life that define each phase

  • of deep time.

  • So, even though our time on this planet amounts to the last word on the last page of the story

  • of life, we are the authors of the next chapter.

  • One day, the epoch of humans may be detected by the marks we made on the land, the traces

  • of our cities and farms.

  • And our very bodies will be the index fossils of this time.

  • No matter how our chapter ends up, we get to be characters in a truly amazing story.

  • Thanks for joining me for this epic -- or ee pok -- journey through geologic time.

  • Now, what do you want to know about the story