Index
A SHORT HISTORY OF THE EARTH ... 1
FOSSILS, ROCKS AND TIME ... 4
The geologic time scale ... 4
Relative dating ... 5
Absolute dating ... 6
Index Fossils ... 6
Using Absolute and Relative Age ... 6
A STORY OF HISTORY OF THE EARTH IN GEOLOGIC TIME DIVISIONS ... 7
Precambrian Time: The Dawn of Life ... 7
Paleozoic Era: The Origin of Complex Life-Forms ... 7
Mesozoic Era: The Age of Dinosaurs... 7
Cenozoic Era: The Age of Mammals ... 8
LIFE ON EARTH ... 9
How did life begin on Earth? ... 9
Organic evolution ... 9
Past extinctions ... 9
A SHORT HISTORY OF THE EARTH
Our planet has existed for 4.6 billion years. From leaps forward in evolution to devastating asteroid impacts, these were the turning points that shaped our world.
4.6b years ago EARTH IS BORN
Earth grew from a cloud of dust and rocks surrounding the young Sun. Earth formed when some of these rocks collided. Eventually they were massive enough to attract other rocks with the force of gravity, and vacuumed up all the nearby junk, becoming the Earth. The Moon probably formed soon after, when a planet-sized chunk of rock smashed into the Earth and threw up a huge cloud of debris. This condensed into the Moon.
4-3.5b years ago THE ORIGIN OF LIFE
Nobody knows exactly when life began. The oldest confirmed fossils, of single-celled microorganisms, are 3.5 billion years old. Life may have begun a bit earlier than that, but probably not while huge rocks were still raining down on Earth.
3.4b years ago LIFE HARNESSES THE POWER OF SUNLIGHT (Photosynthesis)
All life needs energy to survive, and the biggest source of energy for life on Earth is the Sun. Some of the early microorganisms evolved a way to use the energy from sunlight to make sugars out of simpler molecules. This process is called photosynthesis. The first photosynthetic bacteria absorbed near-infrared rather than visible light and produced sulfur or sulfate compounds rather than oxygen. Their pigments were predecessors to chlorophyll.
3b years ago? THE BEGINNING OF PLATE TECTONICS
Today, Earth's surface is divided into a few dozen plates of rock, one of which sometimes ploughs under another to be destroyed in the planet's molten heart. This process, called plate tectonics, is thought to have begun around 3 billion years ago.
2.4b years ago THE GREAT OXIDATION EVENT (Breathable air)
For the first half of Earth's history, there was hardly any oxygen in the air. But then some bacteria began harnessing sunlight to make sugar from carbon dioxide and water, just like green plants today. These microbes pumped out oxygen as a waste product, creating the oxygen-rich atmosphere we have today. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally, the free oxygen reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the longest snowball Earth episode in the Earth's history.
2-1b years ago ENDOSYMBIOSIS (Complex cells)
The first organisms were simple cells like modern bacteria, but some of them became much more internally complex. These 'eukaryotes' developed lots of specialised equipment within their cells. They also had a new source of energy: sausage-shaped objects called mitochondria that were once free-living bacteria, but which were absorbed in a process called endosymbiosis. Every animal and plant you've ever seen is a eukaryote.
1.2b years ago? THE FIRST SEX (Origin of mating)
stopped simply dividing in two and started sex. But it was definitely going on 1.2 billion years ago: there are fossils of red algae from that time that were clearly forming specialised sex cells such as spores.
1b years ago? MULTICELLULAR LIFE (Big organisms)
For the first time, life was not just made up of single cells. Now cells were teaming up to form larger organisms with things like mouths, limbs and sense organs. It's hard to say when this happened: there are fossils of large organisms dating back 2.1 billion years, but these may simply have been colonies of bacteria. Different groups of organisms probably evolved multicellularity independently.
850-635m years ago SNOWBALL EARTH (A frozen world)
Earth froze over again, twice, in the space of 200 million years. The ice may well have stretched all the way from the poles to the equator. This second Snowball period may have triggered the evolution of the first complex animals. The first complex organisms, weird tube- and frond-shaped things called the Ediacarans, appeared soon after.
535m years ago THE CAMBRIAN EXPLOSION
Soon after animals evolved, evolution went through a major growth spurt. In the Cambrian Explosion, it seems almost every group of modern animals appeared within tens of millions of years. This apparent 'explosion' may be partly down to better fossilisation, as many animals now had hard shells.
465m years ago PLANTS COLONISE THE LAND
Some animals ventured onto land as far back as 500 million years ago, but they only visited briefly – perhaps to lay eggs in a place without predators. Plants were the first to take up permanent residence on land. The first land plants were relatives of green algae, but they rapidly diversified.
460-430m years ago THE FIRST MASS EXTINCTION
The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The deep freeze led to the first-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common.
375m years ago FISH THAT WALK ON LAND
With plants well-established on land, the next step was for animals to move out of the water. Insects were among the first, around 400 million years ago. But they were followed soon after by big, backboned animals such fish that looked a bit like a salamander. Fish like that would eventually evolve four limbs, and give rise to amphibians, reptiles and mammals. Soon afterwards the Late Devonian Extinction wiped out many marine animals, including some terrifying-looking armoured fish.
320m years ago DAWN OF THE REPTILES
When the first reptiles appeared, Earth was in the middle of a long cold snap called the Late Paleozoic Ice Age. Reptiles evolved from newt-like amphibians. Unlike their ancestors they had tough, scaly skin and laid eggs with hard shells that did not have to be left in water. Thanks to these advantages, they quickly became the dominant land animals.
300m years ago PANGAEA
For the last time, all Earth's continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents.
252m years ago THE GREAT DYING (Permian extinction)
96% of marine species and similar numbers of land animals. We don't know for sure what caused it. In the aftermath, the first dinosaurs evolved.
220m years ago THE FIRST MAMMALS
At the same time that the dinosaurs were spreading and diversifying, the first mammals evolved. Early mammals were small and probably only active at night. This may have spurred them to evolve warm-bloodedness: the ability to keep their body temperature constant. 201m years ago THE TRIASSIC EXTINCTION
The dinosaurs were flourishing on land, and in the sea giant reptiles called ichthyosaurs had become the top predators. Then another disaster struck. We’re not sure what caused the Triassic extinction, but it killed off around 80% of species. In the aftermath, the dinosaurs became the dominant land animals and eventually reached titanic sizes.
160m years ago THE FIRST BIRDS
Birds evolved from feathered dinosaurs – modern birds are essentially Velociraptors with beaks instead of snouts and wings instead of arms. The most famous early bird, Archaeopteryx, lived 150 million years ago.
130m years ago FLOWER POWER
This may sound strange, but flowers are a recent invention. There have been land plants for 465 million years, yet there were no flowers for over two-thirds of that time. Flowering plants only appeared in the middle of the dinosaur era. The equally-familiar grasses appeared even more recently. The oldest fossil grasses are just 70 million years old, although grass may have evolved a bit earlier than that.
65m years ago DEATH OF THE DINOSAURS
65 million years ago, a huge chunk of rock from outer space smashed into what is now Mexico. The explosion was devastating, but the longer-term effects were worse. Dust was thrown into the upper atmosphere and blocked out sunlight, and in the ensuing cold and darkness Earth suffered its fifth and last mass extinction. The dinosaurs were the most famous casualties.
60-55m years ago THE FIRST PRIMATES EVOLVE
Almost immediately after the dinosaurs were wiped out, mammals evolved the ability to nourish their young inside their wombs using a placenta, just like modern humans. Soon, some of these early placental mammals evolved into the first primates. They would ultimately give rise to monkeys, apes and humans. But the first ones were small creatures. They lived in the hot and humid rainforests of Asia.
32-25m years ago SUPERCHARGED PLANTS (C4 photosynthesis)
Plants have been busily harnessing sunlight to make sugar for hundreds of millions of years – a process called photosynthesis. But fairly recently, some plants have found a better way to do it. C4 photosynthesis is far more efficient than normal photosynthesis, allowing C4 plants to cope with harsh conditions.
13-7m years ago THE FIRST HOMININS
The first apes appeared in Africa around 25 million years ago. Then at some point, the group split into the ancestors of modern humans and the ancestors of modern apes. It's hard to say exactly when, but thanks to modern genetics and a host of fossil discoveries, we have a rough idea.
200,000 years ago THE HUMAN RACE
FOSSILS, ROCKS AND TIME
We study our Earth for many reasons: to find water to drink or oil to run our cars or coal to heat our homes, to know where to expect earthquakes or landslides or floods, and to try to understand our natural surroundings. Earth is constantly changing nothing on its surface is truly permanent. Rocks that are now on top of a mountain may once have been at the bottom of the sea. Thus, to understand the world we live on, we must add the dimension of time. We must study Earth's history.
When we talk about recorded history, time is measured in years, centuries, and tens of centuries. When we talk about Earth history, time is measured in millions and billions of years. We keep track of time with the calendar, which is based on the movements of Earth in space. People who study Earth's history also use a type of calendar, called the geologic time scale. It looks very different from the familiar calendar. In some ways, it is more like a book, and the rocks are its pages. Some of the pages are torn or missing, and the pages are not numbered, but geology gives us the tools to help us read this book.
The
geologic
time
scale
Fossils are the recognizable remains of past life on Earth. Long before geologists had the means to recognize and express time in numbers of years before the present, they developed the geologic time scale. This time scale was developed gradually, mostly in Europe, over the eighteenth and nineteenth centuries. Earth's history is subdivided into eons, which are subdivided into eras, which are subdivided into periods, which are subdivided into epochs. The names of these subdivisions, like Paleozoic or Cenozoic, may look daunting, but to the geologist there are clues in some of the words. For example, zoic refers to animal life, and pa/eo means ancient, meso means middle, and ceno means recent. So the relative order of the three youngest eras, first Paleozoic, then Mesozoic, then Cenozoic, is straightforward.
shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth. Fossils are fundamental to the geologic time scale. The names of most of the eons and eras end in zoic, because these time intervals are often recognized on the basis of animal life1.
Relative dating
We study Earth's history by studying the record of past events that is preserved in the rocks. The layers of the rocks are the pages in our history book. Most of the rocks exposed at the surface of Earth are sedimentary formed from particles of older rocks that have been broken apart by water or wind. The gravel, sand, and mud settle to the bottom in rivers, lakes, and oceans. These sedimentary particles may bury living and dead animals and plants on the lake or sea bottom. With the passage of time and the accumulation of more particles, and often with chemical changes, the sediments at the bottom of the pile become rock and the animal skeletons and plant pieces can become fossils. To tell the age of most layered rocks, scientists study the fossils these rocks contain. Fossils provide important evidence to help determine what happened in Earth history and when it happened.
Relative Dating of rocks is determining the age of materials by putting them in a sequence in order of which event took place from most recent to oldest (comparative not exact).
Law of Superposition: the lower layers in any particular cross section of rock are older than the upper layers in that cross section.
Principle of original horizontality: sedimentary layers are horizontal, or nearly so, when originally deposited. Cross sections that are not horizontal have been deformed by movements of the Earth’s crust (folds, faults, erosion).
Principle of crosscutting relations: geologic features, such as faults, and igneous intrusions are younger than the rocks they cut.
Principle of faunal succession: groups of fossil plants and animals occur in the geologic record in a definite and determinable order. If there are fossils embedded in the rock, these fossils (inclusions) are younger than the rock they are embedded in but older than the rock above them. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called index fossils.
Principle of inclusion: a rock body that contains inclusions of preexisting rocks is younger that the rocks from which the inclusions came from.
Example: pPut in order from oldest to youngest. Remember to include any erosion, faulting, tilting or folding! D – oldest – at the bottom
A – next to bottom Tilt – plate tectonics E – intrusion
Erosion – not horizontal/straight line B
C – intrusion – Law of cross-cutting Erosion – Horizontality
DABEC (old to young)
E and C are igneous (magma rising) A, B, D are sedimentary (layers)
1 Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms, such as bacteria, algae, and wormlike
Absolute dating
Thus far we have been discussing the relative time scale. How can we add numbers to our time scale?
Nineteenth-century geologists and paleontologists believed that Earth was quite old, but they had only crude ways of estimating just how old. The assignment of ages of rocks in thousands, millions, and billions of years was made possible by the discovery of radioactivity. Now we can use minerals that contain naturally occurring radioactive elements to calculate the numeric age of a rock in years.
The basic unit of each chemical element is the atom. An atom consists of a central nucleus, which contains protons and neutrons, surrounded by a cloud of electrons. Isotopes of an element are atoms that differ from one another only in the number of neutrons in the nucleus. For example, radioactive atoms of the element potassium have 19 protons and 21 neutrons in the nucleus (potassium 40); other atoms of potassium have 19 protons and 20 or 22 neutrons (potassium 39 and potassium 41). A radioactive isotope (the parent) of one chemical element naturally converts to a stable isotope (the daughter) of another chemical element by undergoing changes in the nucleus. The change from parent to daughter happens at a constant rate, called the half-life. The half-life of a radioactive isotope is the length of time required for exactly one-half of the parent atoms to decay to daughter atoms. Each radioactive isotope has its own unique half-life. Precise laboratory measurements of the number of remaining atoms of the parent and the number of atoms of the new daughter produced are used to compute the age of the rock. For dating geologic materials, four parent/daughter decay series are especially useful: carbon to nitrogen, potassium to argon, rubidium to strontium, and uranium to lead.
Radioactive dating works best with igneous rocks. Sedimentary rocks are formed from material that came from other rocks. For this reason, any measurements would show when the original rocks were formed, not when the sedimentary rock itself formed. Just as uranium 235 can be used to date igneous rocks, carbon 14 can be used to find the ages of the remains of some things that were once alive.
Index Fossils
Fossils contained within sedimentary rock can offer clues about the age of the rock. An organism that was fossilized in rock must have lived during the same time span in which the rock formed. Using information from rocks and other natural evidence, scientists have determined when specific fossilized organisms existed. If people know how long ago a fossilized organism lived, then they can figure out the age of the rock in which the fossil was found. Fossils of organisms that were common, that lived in many areas, and that existed only during specific spans of time are called index fossils. These characteristics of index fossils make them especially useful for figuring out when rock layers formed.
Using Absolute and Relative Age
Scientists must piece together information from all methods of determining age to figure out the story of Earth’s past. • Radioactive dating of igneous rocks reveals their absolute age.
• Interpreting layers of sedimentary rock shows the relative order of events. • Fossils help to sort out the sedimentary record.
A STORY OF HISTORY OF THE EARTH IN GEOLOGIC TIME DIVISIONS
Precambrian Time: The Dawn of Life
Geologic history begins with Earth’s formation 4.6 billion years ago, which is the start of Precambrian time. Earth’s oldest rocks are just under 4 billion years old. However, some mineral inclusions have been dated at 4.4 billion years. Scientific evidence indicates that Earth and the solar system formed at the same time. However, our planet is too active to preserve its oldest surface materials. The processes of plate tectonics along with metamorphism, weathering, and erosion have destroyed Earth’s original crust.
At one time, fossil collectors could not recognize fossils in rocks older than those from the Cambrian Period. Some concluded that older rocks (Precambrian) did not contain fossils. However, scientists now know that Precambrian rocks do contain fossils. Organisms alive during the Precambrian time did not have hard parts, like shells and skeletons. Therefore, they did not form easily identifiable fossils. Precambrian time makes up about 88 percent of Earth’s history. Living things evolved in the oceans. The ocean water provided them with food and protected them from harmful solar radiation. Earth’s early atmosphere probably was mostly carbon dioxide and nitrogen. It was similar in composition to the present atmosphere of Venus and Mars. Single-celled Precambrian organisms developed photosynthesis. They used carbon dioxide to store energy, releasing oxygen as a waste product. The addition of oxygen to the atmosphere had two very important results. The ozone layer formed. Oxygen in the form of ozone absorbs harmful radiation, such as ultraviolet rays, from outside Earth. Oxygen also is necessary for air-breathing animals. These developments, roughly 2 billion years ago, allowed living things to move out of the oceans.
Events of the Precambrian time are not as well known as more recent events. This is because most of the Precambrian rocks have been covered by later rocks, or the rocks were recycled by weathering and erosion, or changed by metamorphism.
Paleozoic Era: The Origin of Complex Life-Forms
Paleozoic means “the time of early life.” This era began a little more than half a billion years ago. The presence of many fossils marks the beginning of the Paleozoic Era. This abundance of life was a result of rapid evolution. Skeletons and shells allowed some organisms to move rapidly in search of food. These parts allowed other organisms to protect themselves from becoming food.
In the oceans, trilobites and the first fish appeared early in the Paleozoic Era. Trilobites evolved many variations in shape before they became extinct at the end of the Paleozoic Era. Plants and amphibians that inhabited the land also appeared in the Paleozoic Era. Amphibians lay their eggs in water; but as adults, many move onto the land. Reptiles, which can lay eggs on land, followed the amphibians.
At the end of the Paleozoic Era, 95 percent of living species went extinct. Scientists do not know the cause of this extinction. They suspect some dramatic change in the world’s climates. The change might have been related to the formation of the supercontinent Pangaea. It may have been the impact of a large meteorite. There may have been great volcanic eruptions. Whatever the cause, this catastrophic event led to the appearance of new lifeforms in the next era.
Mesozoic Era: The Age of Dinosaurs
Mesozoic means “middle life.” This era began about 251 million years ago, following the end of the Paleozoic Era. Some forms of fish, insects, and reptiles had survived the Paleozoic Era extinction. Mammals appeared in the Mesozoic Era. However, they remained small creatures. The first birds may have evolved from flying dinosaurs, such as Archaeopteryx, in the Mesozoic Era.
years. Some may have been remarkably intelligent. Dinosaurs became extinct at the end of the Mesozoic Era. Recent evidence links that extinction to the impact of an asteroid and climatic change.
Cenozoic Era: The Age of Mammals
Cenozoic means “recent life.” The Cenozoic Era began 65.5 million years ago and continues to the present. With the extinction of dinosaurs, mammals evolved as the most successful group of vertebrate animals. Mammals inhabit nearly every terrestrial environment. Whales, dolphins, and seals live in the seas, and bats fly through the air. The first humans evolved in the late Cenozoic Era. The oldest human fossils were found in Africa and are about 2 to 4 million years old.
LIFE ON EARTH
How did life begin on Earth?
Scientists are studying how life began on Earth. They know that chemical reactions in Earth’s early environment could have made amino acids. Living organisms are made of amino acids, which are the building blocks of proteins. However, scientists do not understand how amino acids become organized to make even the simplest life-forms. Experiments to produce living material from nonliving processes have not been conclusive. Millions of years of chemical changes on the primitive Earth might have produced results that cannot be reproduced in short-term laboratory experiments.
Life could have started in places not usually considered good for living organisms. Recently, scientists discovered primitive forms of life in nearly solid rock many kilometers below Earth’s surface. They have found living organisms near hot water vents in the deep ocean bottom beyond the reach of sunlight. Perhaps conditions in one of these places gave rise to the first life-forms.
It appears that the impact of objects from space, such as meteorites, blasted rock fragments from the surface of Mars. Some of these rocks fell to Earth. Scientists have found pieces of rock that came from Mars in Antarctica. These rocks provide another clue to the ability of life to exist in extreme environments. Microscopic examination of these rocks found tiny objects that some scientists think are evidence of life. Could very simple forms of life travel from planet to planet as spores in rock fragments like these? While this is an interesting possibility, scientists do not know for sure. They need more investigations to help decide how life began on Earth.
Scientists look at ancient rocks to find evidence of early life. However, there are very few rocks left from Earth’s earliest history. Erosion and metamorphism have changed most of the oldest rocks that might have contained fossils. However, some scientists think that they have found evidence of carbon compounds made by organisms living nearly 4 billion years ago. Stronger evidence comes from patterns of life-forms. The oldest fossils are clumps of primitive bacteria called stromatolites. These fossils are about 3.5 billion years old.
Organic evolution
Fossils older than 2 billion years are remains of tiny single-celled organisms. These life-forms did not contain a nucleus. They reproduced by splitting into two or more new cells. The first important advance in the development of more complex life-forms is found in rocks about 1.4 billion years old. Cells with a nucleus appeared. These cells could reproduce sexually and inherit characteristics from two parent cells.
Less than a billion years ago, colonies of single-celled organisms developed into simple multicellular organisms, such as jellyfish and worms. The first organisms with shells and skeletons appeared about 545 million years ago. These organisms eventually gave rise to the complex life-forms that exist today. The gradual change in living organisms from generation to generation is biological, or organic, evolution.
Past extinctions
Most of the life-forms found as fossils have become extinct. The major global biotic turnovers were all caused by physical events that lay outside the normal climatic and other physical disturbances which species, and entire ecosystems, experience and survive. What caused them?
First major extinction (c. 440 mya): Climate change (relatively severe and sudden global cooling) seems to have been at work at the first of these-the end-Ordovician mass extinction that caused such pronounced change in marine life (little or no life existed on land at that time). 25% of families lost (a family may consist of a few to thousands of species).
Second major extinction (c. 370 mya): The next such event, near the end of the Devonian Period, may or may not have been the result of global climate change. 19% of families lost.
Third major Extinction (c. 245 mya): Scenarios explaining what happened at the greatest mass extinction event of them all (so far, at least!) at the end of the Permian Period have been complex amalgams of climate change perhaps rooted in plate tectonics movements. Very recently, however, evidence suggests that a bolide impact similar to the end-Cretaceous event may have been the cause. 54% of families lost.
Fifth major extinction (c. 65 mya): Most famous, perhaps, was the most recent of these events at the end-Cretaceous. It wiped out the remaining terrestrial dinosaurs and marine ammonites, as well as many other species across the phylogenetic spectrum, in all habitats sampled from the fossil record. Consensus has emerged in the past decade that this event was caused by one (possibly multiple) collisions between Earth and an extraterrestrial bolide (probably cometary). Some geologists, however, point to the great volcanic event that produced the Deccan traps of India as part of the chain of physical events that disrupted ecosystems so severely that many species on land and sea rapidly succumbed to extinction. 17% of families lost.
The Sixth Extinction
The current mass extinction is caused by humans: about 30,000 species go extinct annually. At first glance, the physically caused extinction events of the past might seem to have little or nothing to tell us about the current Sixth Extinction, which is a patently human-caused event. For there is little doubt that humans are the direct cause of ecosystem stress and species destruction in the modern world through such activities as: transformation of the landscape; overexploitation of species, pollution or the introduction of alien species. And because Homo sapiens is clearly a species of animal (however behaviorally and ecologically peculiar an animal), the Sixth Extinction would seem to be the first recorded global extinction event that has a biotic, rather than a physical, cause.
Human impact on the planet is a direct analogue of the Cretaceous cometary collision. Sixty-five million years ago that extraterrestrial impact — through its sheer explosive power, followed immediately by its injections of so much debris into the upper reaches of the atmosphere that global temperatures plummeted and, most critically, photosynthesis was severely inhibited — wreaked havoc on the living systems of Earth. That is precisely what human beings are doing to the planet right now: humans are causing vast physical changes on the planet.
We can divide the Sixth Extinction into two discrete phases:
phase one began when the first modern humans began to disperse to different parts of the world about 100,000 years ago
phase two began about 10,000 years ago when humans turned to agriculture. The fossil record attests to human destruction of ecosystems:
Wherever early humans migrated, other species became extinct. Humans arrived in large numbers in North America roughly 12,500 years ago-and sites revealing the butchering of mammoths, mastodons and extinct buffalo are well documented throughout the continent. The Caribbean lost several of its larger species when humans arrived some 8000 years ago. Extinction struck elements of the Australian megafauna much earlier-when humans arrived some 40,000 years ago. Madagascar-something of an anomaly, as humans only arrived there two thousand years ago-also fits the pattern well: the larger species (elephant birds, a species of hippo, plus larger lemurs) rapidly disappeared soon after humans arrived.
Indeed only in places where earlier hominid species had lived (Africa, of course, but also most of Europe and Asia) did the fauna, already adapted to hominid presence, survive the first wave of the Sixth Extinction pretty much intact. The rest of the world’s species, which had never before encountered hominids in their local ecosystems, were naively unwary.
The invention of agriculture accelerated the pace of the Sixth Extinction. Agriculture appears to have been invented several different times in various different places, and has, in the intervening years, spread around the entire globe. Agriculture represents the single most profound ecological change in the entire 3.5 billion-year history of life. With its invention: humans did not have to interact with other species for survival, and so could manipulate other species for their own use. All other species, including our ancestral hominid ancestors, all pre-agricultural humans, and remnant hunter-gatherer societies still extant exist as semi-isolated populations playing specific roles (i.e., have “niches”) in local ecosystems. This is not so with post-agricultural revolution humans
Indeed, to develop agriculture is essentially to declare war on ecosystems - converting land to produce one or two food crops, with all other native plant species all now classified as unwanted “weeds” — and all but a few domesticated species of animals now considered as pests.
