A Brief History of Geologic Time

Summary

Highlights

• Nicolas Steno's 17th-century laws of stratigraphy revolutionized geology. 🏔️
• Giovanni Arduino's layer naming in the 1760s was foundational but limited in global comparison. 🌍
• William Smith's fossil-based stratigraphy unified geologic history across the world. 🦴
• Major geologic eons like the Hadean and Archean highlight early Earth eras without fossil records. 🌑
• The Phanerozoic Eon, our current era, showcases abundant visible life, including humans. 🌳

Key Takeaways

• Understanding Earth's history is a mind-bending journey through billions of years! ⏳
• Thanks to fossils and rock layers, we can decode the past epochs and eras of Earth. 🪨
• Mass extinctions reshaped life on Earth, leading to incredible booms of new species. 🌱

Overview

Chapters

• 00:00 - 00:30: Introduction to Earth's History Earth's history spans 4 billion years, with humans appearing only recently. The immensity of time involved in Earth's history is difficult to grasp, as even understanding a few hundred years can be challenging. Scientists use rocks to help us comprehend this vast timeline.
• 00:30 - 01:00: Geologic Time and Stratigraphy This chapter discusses how geologists utilize the layers of the Earth to understand significant events in the history of life, including periods of evolutionary diversity and extinction events. These events are organized within the Geologic Time Scale, which geologists use to structure the narrative of Earth's history. The chapter also touches upon the historical development of understanding geologic time, highlighting the challenges involved in interpreting history through rock strata.
• 01:00 - 01:30: Nicolas Steno and Stratigraphy In 1669, the Danish scientist Nicolas Steno laid the groundwork for the science of stratigraphy with his publication on interpreting Earth's strata. He proposed that the layers of rock closer to the surface are younger than those below, suggesting that deeper layers contain older fossils. At the time, this was a revolutionary concept, as some people believed fossils fell from the sky. Steno's ideas were further developed by other scientists, including Italian geologist Giovanni Arduino.
• 01:30 - 02:00: Naming Rock Layers In the 1760s, Arduino studied the Italian Alps to name rock layers based on depth and composition: Primary (metamorphic and volcanic), Secondary (hard sedimentary), Tertiary, and Quaternary (softer alluvial). These layers varied globally, complicating universal geological comparisons. In 1819, English geologist William Smith found a solution to this issue.
• 02:00 - 02:30: William Smith and Fossils William Smith developed a method to determine the age of rock formations by comparing fossils found within them, such as trilobites and ammonites. His work allowed for the identification of the relative ages of rocks worldwide based on fossil evidence. Even the oldest rocks, with little evidence of life, can provide insights about early geologic events like continent formation and Earth's cooling.
• 02:30 - 03:00: Creation of the Geologic Time Scale The Geologic Time Scale (GTS) was created thanks to early geologists like Steno, Arduino, and Smith. The GTS is frequently updated to integrate the latest knowledge of Earth’s history. Today, it is divided into five subgroups: Eons, Eras, Periods, Epochs, and Ages. This organization helps in exploring historical questions at various scales, with larger increments focusing on broader questions such as the existence of life on Earth.
• 03:00 - 03:30: Eons and Eras Overview The chapter 'Eons and Eras Overview' provides a top-level view of Earth's history by exploring the different Eons and Eras within the Geologic Time Scale (GTS). It highlights how smaller increments of time, such as periods and epochs, allow for a more focused analysis of specific historical questions, such as climate conditions and adaptation of life forms. Future episodes will delve deeper into each era on a period-by-period basis. The introduction emphasizes the significance of Eons as the largest time segments, spanning from half a billion to nearly two billion years.
• 03:30 - 04:00: The Hadean Eon The Hadean Eon marks the beginning of Earth's history, starting from the planet's formation 4.6 billion years ago and ending 4 billion years ago. It is characterized by extreme conditions such as volcanic activity, cosmic bombardments, and high temperatures. Despite these harsh conditions, it is believed that life might have begun to form during this period. The eon has no fossil records, aligning with its name derived from the Greek underworld, symbolizing a hellish environment.
• 04:00 - 05:00: The Archean Eon The Archean Eon marks a significant period in Earth's history, running from 4 billion to 2.5 billion years ago. It succeeded the Hadean Eon, which ended with the cooling of the Earth's crust, setting the stage for the formation of continents. Although no fossils have been discovered from this time, traces of organic carbon found in Hadean rocks suggest the presence of early life forms. These initial organisms were simple, single-celled creatures, yet they played a crucial role in shaping the planet's future. The Archean Eon is named after the Greek word for ‘origin,’ reflecting its significance as a formative period in Earth's geological timeline.
• 05:00 - 06:30: The Proterozoic Eon The chapter titled 'The Proterozoic Eon' discusses the emergence and flourishing of early life, particularly microbial life, in the Earth's primordial seas. The fossils of these ancient microbes, known as stromatolites, date back to the Archaean era. During the Proterozoic Eon, roughly 2.5 billion years ago, there was a significant shift in the Earth's atmosphere - transitioning largely from carbon dioxide to becoming oxygenated due to the emergence of cyanobacteria and other photosynthetic organisms. This change paved the way for the development of multicellular life.
• 06:30 - 08:00: The Phanerozoic Eon and Paleozoic Era The chapter discusses the transition from anaerobic life to the emergence of eukaryotes during Earth's history. The development of eukaryotes marked the appearance of large, complex organisms such as Charnia and Dickinsonia. This diversification led to the Phanerozoic Eon, beginning roughly 541 million years ago.
• 08:00 - 09:30: The Great Dying and End of Paleozoic The chapter titled 'The Great Dying and End of Paleozoic' discusses the Phanerozoic Eon, which is marked by the emergence of visible life. This eon includes all major life forms, like trees, dinosaurs, and humans, amounting to life as we know it today. The Phanerozoic Eon is divided into three eras, starting with the Paleozoic Era, which began 541 million years ago.
• 09:30 - 11:00: Mesozoic Era and Dinosaurs The chapter 'Mesozoic Era and Dinosaurs' discusses the diversification of visible life beginning with the Cambrian explosion, a significant event in evolutionary history. This explosion marked the emergence of complex animal life with hard parts like shells and exoskeletons, as evidenced by the fossil record spanning over 25 million years. The chapter places this event in context with previous eons, referencing the Precambrian as a collective term for the Hadean, Archean, and Proterozoic eons.
• 11:00 - 13:00: The K-Pg Extinction The chapter titled 'The K-Pg Extinction' discusses the early dominance of trilobites as widespread animals and their role as index fossils for the Palaeozoic Era, established since William Smith's era. However, trilobites soon faced competition from fish that developed teeth and jaws, leading to early marine dominance by species such as sharks and placoderms. On land, which was previously barren since its formation in the Archean, plants and arthropods began to populate and by 370 million years ago, complex ecosystems were established on the continents.
• 13:00 - 15:00: Cenozoic Era and Rise of Mammals The chapter discusses the evolutionary history of early amphibians that transitioned from water to land, marking the advent of vertebrates. It mentions the formation of the supercontinent Pangea 299 million years ago, characterized by a large desert that became a habitat for the ancestors of reptiles and mammals. These species adapted to dry conditions better than amphibians. However, the growth of this era was interrupted by a massive extinction event that ended the Paleozoic Era 252 million years ago, wiping out 70% of land vertebrates and 96% of marine species.
• 15:00 - 17:00: The Ice Age and Human Era The chapter explores a significant extinction event known as the Great Dying, which represents the most severe extinction period in Earth's history. The exact cause of this extinction remains unclear, with possible factors being a meteorite impact off the coast of the South America Islands and massive volcanic eruptions in Siberia. These events coincided with the end of the Palaeozoic era, suggesting that the extinction had multiple causes. The Palaeozoic is noted for its initial explosion of life, which eventually led to this mass extinction.
• 17:00 - 18:00: Human Impact and Future The chapter begins by detailing a significant extinction event that led to nearly absolute death on Earth, requiring millions of years for recovery. Following this period, the Mesozoic Era, known as the Age of Reptiles, emerged. During this era, reptiles thrived and evolved into well-known forms such as dinosaurs, pterosaurs, and various marine species. Notably, non-avian dinosaurs existed solely during the Mesozoic, thus serving as important index fossils for the era. Additionally, the chapter notes that many modern organism groups also evolved during this time alongside the dominance of reptiles.
• 18:00 - 20:00: Conclusion and Call to Action This chapter delves into the mass extinction event known as the Cretaceous-Paleogene (K-Pg) event, which occurred 66 million years ago, marking the end of the Mesozoic Era. This extinction was primarily caused by a massive asteroid impact that launched significant ash and debris into the atmosphere, blocking sunlight and triggering a catastrophic cold snap. The absence of sunlight led to the collapse of plant communities, followed by the extinction of plant-dependent animals. The chapter highlights the evidence for this event, such as the iridium layer in rocks from this period.

A Brief History of Geologic Time Transcription

• 00:00 - 00:30 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.
• 00:30 - 01:00 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
• 01:00 - 01:30 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
• 01:30 - 02:00 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:
• 02:00 - 02:30 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.
• 02:30 - 03:00 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?
• 03:00 - 03:30 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
• 03:30 - 04:00 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 just … hell. 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.
• 04:00 - 04:30 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
• 04:30 - 05:00 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.
• 05:00 - 05:30 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.
• 05:30 - 06:00 Its name means ‘visible life,’ and the Phanerozoic was when life really became … obvious. 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.
• 06:00 - 06:30 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.
• 06:30 - 07:00 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.
• 07:00 - 07:30 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
• 07:30 - 08:00 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,
• 08:00 - 08:30 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
• 08:30 - 09:00 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
• 09:00 - 09:30 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
• 09:30 - 10:00 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
• 10:00 - 10:30 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.
• 10:30 - 11:00 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
• 11:00 - 11:30 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.
• 11:30 - 12:00 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 of life on Earth? Let us know in the comments. And don’t forget to go to youtube.com/eons and subscribe! And the fun doesn’t end here! Do yourself a favor and check out some of our sister channels from PBS Digital Studios.