Getting to Know Your Solar System (4): Earth (Vol. 1)

In the next few parts of our journey through the solar system, we examine our world through the eyes of a stranger, seeing past the assumptions that blind us to the awesome complexity around us.  Earth is a lot more than the birthplace and currently unique home of humanity, but a world of rich diversity where a number of cyclical processes keep a dynamic and precarious balance.  One of those processes, life, may ultimately catalyze a transformation of the solar system through the continuing evolution of technological intelligence.

The progress of our adventure so far: (current in bold)

1.  The Sun
2.  Mercury
3.  Venus
4.  Earth (Vol. 1)
5.  Earth (Vol. 2)
6.  Earth (Vol. 3)
7.  Earth (Vol. 4)
8.  Luna
9.  Mars
10.  Phobos & Deimos
11.  Asteroids
12.  Ceres
13.  Jupiter
14.  Io
15.  Europa
16.  Ganymede
17.  Callisto
18.  Saturn
19.  Mimas
20.  Enceladus
21.  Tethys, Dione, and Rhea
22.  Titan
23.  Iapetus
24.  Rings & Minor Moons of Saturn
25.  Uranus
26.  Moons of Uranus
27.  Neptune
28.  Triton
29.  The Kuiper Belt & Scattered Disk
30.  Comets
31.  The Stellar Neighborhood

Due to Earth being humanity's original and currently only planet, the amount of information we have about every aspect of this world is exponentially larger than any other solar system body - and growing all the time.  The vast majority of modern sciences revolve around processes as they occur on Earth, so fully plumbing the depths of knowledge about Earth would not only make an extensive diary series unto itself, but dozens of such series - each encompassing one of the sub-categories presented here, and every one meriting dozens of entries to explore sub-topics.  So, we will be making four "orbits" of the Earth, and covering it in consecutive volumes of manageable length rather than testing people's patience with the longest diary ever published on Daily Kos.  Please excuse any errors I make or repeat, as the source material is as vast as it is sometimes contradictory.

As per the structure established in earlier entries in this series, our examination of Earth is organized along these lines, with the portion covered in the current volume in bold:

I.  Context
II.  History
III.  Properties
IV.  Natural & Artificial Satellites
V.  Past Relevance to Humanity
VI.  Modern Relevance to Humanity
VII.  Future Relevance to Humanity
VIII.  Future of Earth
IX.  Catalog of Exploration

I.  Context

Earth is the largest and most massive primarily solid body in the solar system.  Diagrams of Earth's location and path of motion, from a receding solar Northern perspective:

While diagrams are all well and good, the best way to illustrate context is simply to show it.  So, without further ado, Earth from beyond Neptune as imaged by the Voyager 1 probe:

As seen through the backlit rings of Saturn, via the Cassini probe (Earth is the dot right-of-center):

From the surface of Mars, courtesy of the Mars Exploration Rovers:

Zoomed-in from Mars orbit, via the Mars Reconnaissance Orbiter:

From the Galileo probe:

Apollo 13 headed home:

From Apollo 12, with discarded hardware falling away:

Two views from Apollo 11 returning in triumph:

The intensity of sunlight at Earth's region of the solar system is only about half that received by Venus, as a result of the Inverse Square Law for electromagnetic radiation.  In other words, if the distance from the Sun doubles, the intensity of its radiative energy is only 1/4 of what it was; if the distance triples, the intensity is 1/9, and so on - the denominator of the fraction is the square of the factor by which the distance is multiplied.  While the danger of a runaway greenhouse effect still exists, this steep decline in solar energy input from Venus to Earth has historically protected Earth from a fate like that of its "sister planet."

However, there is a common misconception that our planet occupies a unique orbital "sweet spot" where the Sun is powerful enough to maintain liquid water - this is not entirely correct, and its orbital position is not the sole reason why our planet has the conditions it does: Solar input is only one of the parameters that defines a broad range of environmental outcomes.  Gravity, geology, and the presence or absence of a magnetic field can be equally decisive factors by determining atmospheric composition, which in turn regulates the degree to which a planet can trap heat.  The energy Earth receives from the Sun is not sufficient in and of itself to allow liquid water, and also does not exclude higher or lower intensities from being ideal under other planetary conditions: With a different atmosphere, a closer or more distant orbit could produce the same temperatures.

But it would not be an exaggeration to say that Earth is unique in the solar system, and will likely prove unusual in the broader census of planets around other stars.  This is not only because water exists as a liquid, but because the liquid is stably exposed to the air and sunlight without either forming a global ice shell on the one hand or shrouding the atmosphere in uniform, featureless white cloud cover on the other.  There is insufficient data to say with any certainty, but it is likely that most water oceans on other worlds will occur beneath ice shells or under thick, perpetual storm clouds that hide the surface from view.  The fact that Earth's oceans are visible from space is something to cherish.

II.  History

1.  Formation & The Hadean Eon

About 4.5 billion years ago, the Earth accreted out of droplets of metallic liquid when the Sun's protoplanetary disk cooled enough for metals to precipitate out of the prestellar gas cloud - in other words, it "rained" metal along an orbital region that swept inward toward the Sun as the early solar system developed.  Originally these droplets would stick together due to electrical forces, but as they gained in mass and became more viscous (stickier) with lower temperatures, the gravitational forces between them became predominant, forming increasingly larger and more stable bodies.  A simulation at the microscopic level:

Another simulation, this time illustrating the earliest phase of planet formation on a systemwide scale:

During the process of accretion, Earth had a hot atmosphere of silicate compounds that would soon freeze onto the surface as a rocky mantle, and also a thick envelope of hydrogen (H₂) and helium (He) from the gases of the protoplanetary disk.  However, the planet was too hot, had too little gravity, and was too close to the Sun to hold on to hydrogen and helium over the long-term, so the bulk of these gases escaped - although some hydrogen remained in the form of water (H₂O).

According to the giant impact hypothesis - the currently favored theory for the formation of Earth's moon - a body the size of Mars was perturbed from its regular orbit into a collision course with proto-Earth, blew off a significant portion of the early mantle into orbit, and the material then re-accreted to form Luna.  There are a number of theories as to the origin and formation of the hypothesized impacting body, Theia (also called Orpheus in some cases), but the key point of the theory is that most of Theia's metallic core combined with that of proto-Earth while blasting off a large amount of silicate from the mantle, and thus the planet's density and the size of its core were increased without greatly increasing its gravity.

We may very well owe the strength of our planet's protective magnetic field to this impact, as well as the length of our day and the long-term stability of its rotational axis.  Without the additional iron from Theia, our core would not be capable of producing the same field strength, and the planet would not have the same angular momentum to drive the dynamo effect responsible for Earth's magnetosphere.  

In addition, it is thought that this impact obliterated the water that had been present on the surface and in the atmosphere of proto-Earth - water in much higher abundances than the Earth as we know it, that would otherwise today be a global ocean submerging everything but the highest mountain peaks, and likely causing the atmosphere to be opaque, white, and have much higher wind speeds.  The oceans as they have existed since then have largely come from volcanic outgassing and comet impacts.

Had this impact not occurred and we failed to have a large moon, the planet's axis would also migrate radically, with some eras where Earth is completely on its side - a configuration with devastating consequences for climate.  Such is what happens to Mars periodically due to its lack of a large, stabilizing moon.  In this kind of rotation, only at two parts of the year does the Sun rise and set normally, while at two locations 90° apart from those, the pole would be pointed straight at the Sun.  Even this extreme condition could produce complex life if it were stable, but without the stabilizing force of a large moon, the axis continues to wander aimlessly, radically altering conditions  without leaving much chance for life to adapt and diversify.

In other words, the planet that existed before encountering Theia would have gone on to a future more like that described above, where its oceans were in some eras beneath a global ice shell, and in others were invisible beneath perpetual global storms.  Instead, in the fury of planets colliding, our Earth was born.  

Soon after the orbiting remnant of Theia formed Luna, Earth was rotating rapidly (4-5 hours per day), and the Moon was 15 times larger in the terrestrial sky than it is today.  Even after the heat of the impact had completely dissipated, the terrestrial surface was a maelstrom of molten rock and tumultuous seas due to the intense gravitational influence of the Moon, which flexed and deformed its crust while driving enormous ocean tides.  However, the power of these tides was also continuously reducing the length of Earth's day and sending the Moon into increasingly distant orbits - a process that has continued to do this day, and will continue into the future.  

Although the primordial atmosphere was obliterated in the impact, a brief reenactment of Earth's accretion history occurred when silicates from vaporized rock precipitated on to the surface over thousands of years, leaving behind a thick atmosphere of carbon dioxide (CO₂), nitrogen (N₂), hydrogen, and water vapor.  Atmospheric pressure at the surface would have been much greater than it is today, and temperatures were also well above the boiling point of water as we experience it - but because the pressure was higher, water vapor in the atmosphere was able to precipitate and form oceans.  This process was sped along by the fact that the Sun was only 80% of its current size at the time, so the input from solar energy was lower.

The period from earliest formation of the planet up to about 3.8 billion years ago (an interval of about 700 million years) is known as the Hadean Eon, and the events described so far are the extent of its highlights.  It was geologically violent throughout, although the intensity of activity varied, and was also heavily bombarded by asteroids, comets, and a few larger bodies.  Once Earth was calm enough to avoid periodic resurfacing, the first stable crust formed and created rocks that are now the oldest terrestrial rocks on the planet.  Below is a sample from the Acasta Gneiss rock outcrop in Northwestern Canada - the rock is believed to be up to 4 billion years old:

2.  Archean Eon

Following the Hadean, the Archean Eon (3.8 to 2.5 billion years ago) thinned out the atmosphere by trapping most of the carbon dioxide then present in rocks through the carbonate-silicate cycle.  This became possible because the rate of large impacts sharply declined at the beginning of the eon, marking the end of the Late Heavy Bombardment - the period responsible for a large number of craters on the Moon.  As a result, the planet was able to cool and formed stable tectonic plates, which allowed the carbonate-silicate cycle to begin moving toward equilibrium at a much lower level of atmospheric CO₂.  As a result, air pressure declined and nitrogen became the dominant gas, although carbon dioxide was still far more abundant than today.

It isn't known when life began on Earth, but the oldest fossils hail from the Archean Eon.  In fact, the question of life origins may not even be meaningful: It seems entirely likely that under the right conditions, life would be constantly beginning and ending, with the simplest processes occurring the most regularly and ceasing when conditions are no longer supportive.  However, by the sheer power of statistical volume, eventually one of these processes would not end - it would continue, elaborate, and evolve up to the present time.  

To the best of our limited knowledge, Archean life consisted exclusively of prokaryotes - single-celled organisms lacking a nucleus.  The most important of these were cyanobacteria - photosynthetic organisms that began to change the atmospheric chemistry yet again in the late part of the eon, as their production of oxygen (O₂) began to outstrip volcanic replenishment of  CO₂.  To get an idea of what these organisms could have looked like (in aggregate), here are some modern examples of sedimentary structures created by cyanobacteria, called stromatolites (click for attribution):

3.  Proterozoic Eon

From 2.5 billion to 542 million years ago - an interval of about 2 billion years - was the Proterozoic Eon.  Early in this period, about 2.4 billion years ago, minerals that had been capturing oxygen produced by cyanobacteria and preventing its buildup in the atmosphere became saturated.  As a result, the oxygen output from photosynthesis accumulated rapidly - quickly enough that the simple organisms existing at the time were unable to adapt, creating Earth's first great mass-extinction, the oxygen catastrophe.  We're predisposed to view oxygen in a positive light because we breathe it, but objectively it is a highly reactive, corrosive gas, and to most of the anaerobic organisms of the early Proterozoic it was utterly toxic.

Life on Earth was almost completely annihilated by this event, and the few strains that survived either had quirks making them chemically resistant to oxygen or lived in environments isolated from it.  Secondly, a large amount of methane (CH₄) - the most potent greenhouse gas in the atmosphere - was destroyed by the oxygen, triggering a 100-million-year ice age known as the Huronian glaciation: The first of several Snowball Earth periods where polar glaciers extended to tropical latitudes, and at times may have formed a continuous global ice shell.

But the freeze didn't last - volcanoes were still far more abundant at the time than today, and continued to vent carbon dioxide.  Meanwhile, terrestrial life was able - at great Darwinian cost - to reach an equilibrium where the amount of oxygen being produced was in balance with the chemical processes that remove it, and was at a level the survivors of the oxygen catastrophe could tolerate.  We don't know when the first aerobic (oxygen-breathing) life evolved, but once it did both photosynthesis and aerobic respiration were able to mutually reinforce each other and allow both types of organism to grow.  

Prior to this development, an increase in photosynthesis would mean an increase in atmospheric oxygen: The rate at which O₂ can be removed by non-biological processes is a hard physical limit.  But with oxygen-respirating organisms available, any long-term increase in atmospheric oxygen would simply be met by an increase in the number and diversity of organisms that breathe it, and they would in turn produce an increase in CO₂ - a boon to further growth of photosynthetic life.  However, it should be understood that extinctions are occurring every time either gas approaches a limit due to unequal evolution in the organisms that breathe them: The virtuous cycle I've described is a very long-term effect that averages out many disastrous climate extremes caused by respiratory gases becoming highly unbalanced.

In fact, while it isn't clear exactly how they were caused or ended, another three global ice ages would occur during this eon, all in a 215-million-year interval called the Cryogenian period: The Kaigas, Sturtian, and Marinoan glaciations.  The Marinoan is believed to have been the worst ice age Earth has ever experienced, then or since, and is thought to have encased the entire planet in ice 1-2 km thick.  This would have been the scene just about everywhere:

In more auspicious periods of the Proterozoic, the first eukaryotes evolve - hardier organisms whose DNA is protected within a cell nucleus.  This allows more elaborate genomes to become viable, increasing the diversity and complexity of lifeforms.  Among the new complexities that appeared: Multicellular life, sexual reproduction, protists, sponges, and eventually simple, relatively fragile animals that would have looked like cnidarian polyps or worms.  These last were the most advanced lifeforms known to have evolved during the Proterozoic, and they only showed up in the last period (the Ediacaran) of the last era (Neoproterozoic) of the eon, between 645 and 543 million years ago.  Some Ediacaran fossils:

The now-stabilized buildup of oxygen in the atmosphere allowed the steady production and accumulation of ozone (O₃), causing formation of the Ozone Layer - a part of the upper atmosphere whose chemistry protects the terrestrial surface from solar ultraviolet (UV) radiation.  Until this point, the land surface of Earth is a sterile waste - all life exists underwater, at depths where it can benefit from the Sun without being harmed.  But now life spreads to increasingly shallow waters, and sets the stage for later colonization of land.  

4.  Phanerozoic Eon & Evolutionary Acceleration

In crossing into the Phanerozoic Eon (which continues today) at about 543 million years ago, we come to the sharpest boundary in the history of life, the Cambrian explosion - a radical diversification of species and acceleration of evolution over a mere 55 million years.  From this point on, major evolutionary developments will accelerate and occur over increasingly shorter intervals of time, so we must zoom in from the Eon-level timescale (about a billion years or more) to the Era timescale - hundreds of millions of years.  A large assortment of bizarre fish, mollusks, and arthropods appear during the earliest stage of the Paleozoic Era, the Cambrian period.  Examples of Cambrian life:

In the next period, the Ordovician (488 to 444 million years ago), atmospheric CO₂ skyrocketed to levels not seen then or since - about 11 times what they are today due to human activity.  As a result, all glaciers disappeared from the planet and sea levels rose up to 700 feet above where they are today.  Ocean temperatures near the surface at the equator may at times have been over 40 °C  (104 °F), and were often temperate at the poles.  Given the much lower surface area of the land, and the (likely) much greater rainfall, it isn't surprising that the first evidence of land plants is found in this period - largely mosses.  However, at the end of the period, the migration of the supercontinent Gondwana toward the South pole allowed glaciers to re-accumulate (land ice can get much colder, much faster than sea ice) and restored the climate to more moderate (but still warm) temperatures.  

Much of the Cambrian biota continued to evolve in the Ordovician, but new forms arose as well - particularly, coral reefs, vertebrates, and cephalopods.   Examples:

In the middle of the Paleozoic Era, in the Devonian period (416 to 359 million years ago), the diversity of fish species radically increases, and one of them gives rise to the first quadrupeds as amphibians whose fish-inherited fins have hardened into little nubs capable of pushing against a solid surface.  Insects appear, and land plants, meanwhile, have become complex, diverse, and relatively large, covering the land in strange, thick, primordial forests that lack large herbivores to reduce their size and density.  At first these forests were largely comprised of plants similar to ferns and horsetails, but wood appeared in the late Devonian and gave rise to the first trees.

In the Carboniferous period that followed (360 to 300 million years ago), atmospheric oxygen skyrocketed to levels 150-160% of what they are today, allowing arthropods and insects to grow to enormous sizes - e.g., 2-foot-long scorpions and dragonflies with 2-foot wingspans.  Another consequence of high oxygen levels was that fires were far more common.  Surface temperatures, meanwhile, declined to levels that would today be considered quite comfortable, but which were comparatively frigid to Paleozoic life.  As the poles re-glaciated, the atmosphere became significantly drier and amphibians were hard-pressed.  

Reptiles appear at about this time, with relatively thick skin and hard-shelled eggs making them resistant to arid conditions.  In the course of this period, reptiles will evolve from small, lizard-like creatures into the ancestors of the dinosaurs.  Fossils of land fauna:

Trees evolve bark in response to the dryness, and it occurs in much greater thicknesses than are observed today.  The first conifers appear late in the period, but ferns are still ubiquitous.  On the geological front, Pangaea - the most famous supercontinent - is largely formed by the end of the Carboniferous.  Also, due to retreated sea levels caused by re-glaciation at the poles, areas that were previously underwater became bogs that would lay the foundation for many rich coal deposits, and hence the name "Carboniferous."  

The end of the next period, the Permian (300 - 250 million years ago), would witness the most thorough mass-extinction since the oxygen catastrophe - one so profound that it dwarfs the events attending the demise of the dinosaurs much later.  This disaster, the Permian-Triassic extinction event (sometimes called the Great Dying), would exterminate 96% of marine life, 70% of land vertebrates, and is the only known mass-extinction severe enough to have affected insects: The hardiest animal organisms on Earth.  

A number of theories exist to explain this event, but consensus leans toward a chain-reaction of catastrophes perhaps made worse by coincidental disasters with separate causes.  A huge shield volcano called the Siberian Traps erupted over an extended time, which is considered one possible cause of CO₂ levels at the Permian-Triassic boundary 7 times higher than they are today.  The global warming that ensued, resulting in an 8 °C (14.40 °F) temperature increase, may have destroyed the ability of oxygen to circulate in the oceans, making them anoxic and killing off nearly all oxygen-breathing marine life.  Even the venerable trilobites that had populated the seas for 270 million years were finally wiped out.    

But that's just the beginning of the disaster.  As almost all oxygen-breathing marine life were now dead, the population of more primitive anaerobes could have surged, and that in turn would have produced a massive spike in emissions of hydrogen sulfide (H₂S) into the atmosphere - a gas toxic to both plants and animals.  Another consequence of H₂S is that it destroys ozone, so aside from exterminating land species with toxic clouds, emissions of hydrogen sulfide could have badly compromised the Ozone Layer and temporarily exposed the Earth's surface to the Sun's UV rays for the first time since the the Proterozoic Eon.  If this occurred, it would have been made worse by the continuing aridity of the climate, which itself was exacerbated by the vast surface area of Pangaea's continental interior.  With fewer clouds, UV rays are more direct and intense.  This extinction marks the end of both the Permian period and the Paleozoic Era.

The eradication was so complete that recovery took 30 million years.  During the early part of that time, the land would have been a silent, hardscrabble place with diffuse ecosystems dominated by only a few species, and the oceans an ironic desert.  Due to the bulky orientation of Pangaea, there were far fewer shallow waters in which simpler life could flourish, and not far from the coast the arid interior climate dominated the vast majority of land area.  There were not a lot of nurturing environments in which new life could flourish and begin generating diversity again.  

4.  The Mesozoic Era

But recovery did happen eventually, and once it got going it produced the greatest flourishing of life in Earth's history, then or since - the Mesozoic Era (250 - 65 million years ago), or informally the "Age of the Dinosaurs."  During this era, Pangaea was slowly torn apart into what we would recognize as our present-day continents, once again drastically increasing shallow, protected waters where ecosystems could prosper and diversify.  The three periods in which the era is divided - Triassic, Jurassic, and Cretaceous - are almost universally recognized among the general public, and are common knowledge among children due to the enduring popularity of dinosaurs.  

But, ironically, the Triassic (250 to 200 million years ago) was not quite part of the Age of Dinosaurs, so much as the period of their long rise before they had come to dominate the land.  Rather, the recovered environment was more like the Permian due to the enduring presence of Pangaea and a hot, dry climate.  However, there were significant evolutionary developments, and they too were ironic in light of later events: The dominant land fauna were actually the ancestors of mammals, the therapsids.  They had dominated the late Permian as well, but most had died in the extinction, with a few of the survivors - cynodonts and dicynodonts particularly - going on to repopulate the Triassic.  Some examples:

However, by the time of the mid-Triassic the small and mid-sized early dinosaurs had become significant, and would gradually come to dominate by the end of the period.  Why this happened is not exactly clear, but it may have been caused by the initial breakup of Pangaea, which introduced inland seas into what had previously been continental interiors.  This and other factors may have allowed for the introduction of the swampy, humid climate that would characterize the Jurassic.  As the waters intruded, there would have been more plant life for herbivores, allowing them to grow larger, and that in turn would allow predators to grow larger.  Some Triassic dinosaurs:

It is also not clear why conditions that would benefit dinosaur herbivores would fail to equally benefit therapsid herbivores, or why the flourishing of the former would not give the same benefits to predatory therapsids as to their dinosaur competitors.  One possibility is that therapsids were already exhibiting some level of high metabolism - i.e., warm-bloodedness - and became pressured by increasingly humid conditions that would hamper their ability to throw off heat.  These same conditions would benefit cold-blooded species by insulating the environment and keeping temperatures consistently hot.

In the Jurassic (200 to 145 million years ago), therapsids were forced out of most ecological niches by an explosion of diversity in dinosaurs, and were only able to compete under limited conditions at much smaller sizes.  This shrinking may have been a physiological response to increasingly humid conditions - an environment far different from the one they evolved in - but it was likely also just a default result of being unable to compete with large dinosaurs.  As a consequence, most therapsids went extinct, leaving behind only a few mammal-like lines - one of which would give rise to the first mammals during this period, although they were small and probably nocturnal.  An example of Jurassic mammalian life, the ironically-named Megazostrodon:

The heirs of therapsids had fallen quite far from their ancestors' dominant position in the Permian.  Jurassic dinosaurs came to fill most niches on land above the size of insects, took over the seas, and penetrated the skies with the rise of pterosaurs.  The period gave birth to many iconic herbivores, like the universally-known stegosaurus and the giant, long-necked sauropods.  Apex predators of the time also continued to grow in size with the allosaurus.   Archaeopteryx appears late in the period, hailing the evolution of birds.

During the Cretaceous (145 to 65 million years ago), the ancestors of birds begin to impinge on the domain of pterosaurs until the latter are reduced to a few niches, and two of the Mesozoic Era's most recognizable lifeforms appear: Tyrannosaurus rex and Triceratops.  Flowering plants arrive, and with them the emergence of the plant-insect symbiosis that has served both well ever since.  Mammals of the period are marsupials, with placental mammals (Eutheria) appearing only at the very end.  A Cretaceous mammal, Eomaia:

5.  K-T Boundary Event, Cenozoic Era, and Evolutionary Acceleration 2

The Age of the Dinosaurs ended abruptly when a carbonaceous chondrite asteroid 10 km in diameter impacted the Earth in what is now the Yucatan peninsula, creating the Chicxulub crater and triggering a mass-extinction called the Cretaceous-Tertiary (KT) extinction event.  The energy of the impact was 2 million times more powerful than the highest-yield thermonuclear bomb ever detonated by humankind, sent tsunamis high enough to overwhelm small mountains, set an entire region of the Earth's surface on fire, and sent shockwaves through the crust that triggered earthquakes and volcanoes around the world.

The superheated ash cloud literally baked much of the biosphere to death, but soon afterward the rising of the dust into the upper atmosphere blotted out the Sun and inflicted a worldwide nuclear winter.  Layers upon layers of ash fell on to the surface, choking animals to death, preventing photosynthesis, and poisoning waters.  With the death of photosynthesizing plants, herbivorous dinosaurs died; with their deaths, the large carnivores died.  Land species that survived were largely carrion-eaters, insectivores (like our mammalian ancestors, and birds), and other "bottom-feeders" - i.e., precisely the type to profit from a sudden glut of very large corpses.  However, the extinctions seem to have largely been a land phenomenon, with relatively modest impacts on the marine biosphere.  A good popular-science program about the catastrophe:

With the near-total collapse of the land biosphere and the death of the dinosaurs, the Mesozoic Era ends and the Cenozoic (65 million years ago to today) begins - we are now in the present, on the hundred-million-year Era timescale.  Mammals emerge from the ashes of the K-T boundary and spread throughout the world, diversifying to fill the niches vacated by extinct reptiles.  Large herbivores and highly-evolved predators appear, as do numerous smaller species that continue the lifestyles of their skittish tree-dwelling ancestors.  Meanwhile, birds fill the skies, the trees, and the high rocks, keeping the dinosaur legacy alive in smaller forms.  

Although their physiologies are considerably different, the outward anatomy of the dominant mammals begins to resemble some of the therapsids that had not been seen since the Triassic or even Permian - a genetic line that had been shoved aside 150 million years ago, now restored and flourishing.  Some now-extinct mammals from earlier times in the Cenozoic:

Even with the coming of humanity's direct primate ancestors in the most recent few million years of the Cenozoic, it may not have been a fundamentally different situation from what existed in the age of therapsids.  But an absolute change occurred when the ability to use simple tools extended, with the hominids, to the creation and use of fire.  At that point, a fundamentally new form of life was born - not only oxygen-breathing as a matter of metabolism, but as a matter of combustion.  The individual human body breathes air, but humans collectively breathe fire: At first as camp fires, then hearths, and eventually in machines.  Today, in the thinnest sliver of the geological clock at the head of an unimaginable ocean of time, a new cycle begins as humanity's fire-breathing destabilizes the climate and its growth changes all around it.  Earth has entered the Anthropecene epoch.

An animation of continental motions over the past several hundred million years: