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4.5 billion years ago, the earth was formed by gases (hydrogen sulfide, methane, and carbon dioxide), dust, etc. Later, it turned into a rock, which then started pulling in more rocks dust, and gasses and formed a ball, which then became a planet

How do scientists know about it?:

Scientists used modern rocks, moon samples, and meteorites to figure out when and how the Earth and moon formed and what they might once have looked like.

Why is this event important?:

Because without the earth, nothing would’ve existed, such as humans, animals, etc

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A planet named Theia, which is the size of Mars, crashed right into the early Earth.  The collision threw huge chunks of matter around the earth's orbit, and gravity-bound them together.

How do scientists know about this?:

The massive impact hypothesis, commonly accepted by scientists, claims that an object colliding with early Earth created the moon. Earth evolved from the leftover cloud of gas and dust around the newborn sun, like the other planets. Many planets that formed but never reached full planetary status did not survive the harsh conditions of the early solar system. One could have crashed into Earth not long after the young planet was created.

Why is this event important?:

Because the moon makes the earth a more livable planet, it moderates our planet's wobble axis, which makes for a stable climate. Another reason is that the moon was the only source of light during the night

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3.8 billion years ago, most of Earth's water came from meteoroids (asteroids become known as meteors when they collide with a planet) that struck the Earth

How do scientists know about this?:

Scientists know how water came to earth because of the interactions between the magma ocean and a molecular hydrogen proto-atmosphere

Why is this event important?:

H20 is essential for a living organism, so without water, we would not have any living organisms.

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It appears likely that a massive stream of falling comets and asteroids may have prevented life from forming on Earth's surface in the first place, however, a considerably less intense comet rain may have deposited the very elements necessary for life to form some 3.5–3.8 billion years ago.  These lives were single cells that started floating in a vast ocean.

How do scientists know about this?:

In an enclosed glass container, two scientists named Miller and Urey introduced water vapor, ammonia, and methane to imitate the conditions that were thought to have been in Earth's early atmosphere at the time. Then, in order to imitate lightning, they shot electrical sparks through the container. Protein-building ingredients, known as amino acids, quickly emerged. Miller and Urey recognized that the chemicals required to create life might have been made possible by this mechanism.

Why is this event important?:

Prokaryotes play an important part in the subsystem of soil decomposition that breaks down plant and animal waste into organic matter and releases nutrients into food webs

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by which green plants and certain other organisms transform light energy into chemical energy. The air was mostly carbon dioxide, but then little green cells started making oxygen.

How do scientists know about this?:

A scientist named Ingenhousz placed one plant in the sunlight and one in the shade, but he noticed that the plants that were in the sunlight produced small bubbles.  Ingenhous later concluded that plants use light to produce oxygen.

Why is this events important?:

Photosynthesis is very important because it produces all the oxygen we breathe. It is also important because it produces glucose, which is the main energy source for plants

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Multicellular organisms consist of several cells, with cell categories getting to perform specific tasks. In simple terms, a multicellular organism is an organism that compose many cells

How do scientists know about this?:

Scientists identified mutations that can turn single-celled organisms into multicellular organisms

Why is this event important?:

The multi cellular cell is important because they is extremely complex and must have specialized cells that function for a particular area of the body

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The ozone layer is a thin part of Earth's atmosphere that absorbs almost all of the sun's harmful ultraviolet light

How do scientists know about this?:

In conclusion, the spectrum of the missing radiation was identified as ozone, the only known molecule. It was discovered by Schönbein in 1867

Why is this event important?:

A portion of the sun's radiation is absorbed by the stratosphere's ozone layer, keeping it from reaching the planet's surface.

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The Cambrian explosion was an explosion of new life. This happened during the Cambrian period when a bunch of different types of animals came to life.

How do scientists know about this?:

the strongest evidence to support the Cambrian evolutionary explosion of animal forms is the first clear appearance, in the Early Cambrian, of skeletal fossils representing members of many marine bilaterian animals

Why is this event important?:

It is important because it is the time when most of the major groups of animals first appear in the fossil record

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Scientists believe that the earliest dinosaurs appeared during the Triassic period, which was 250–200 mya. The first dinosaur that existed was the Nyasasaurus parringtoni. Fun fact: the T-rex is closer to us than the Stegosaurus.

How do scientists know about this?:

Paleontologists study fossil evidence found in ancient rock to determine the length of time and behavior of extinct species.

Why is this event important?:

Researching dinosaurs was important to the development of evolutionary theory as well as other scientific ideas like plate tectonics and biogeography.

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In 66 MYA, an asteroid came, killing 75% of the species. This is how dinosaurs went extinct. But in simple terms, extinction means that at least half of all species die in a really short amount of time

How do scientists know about this?:

Mass extinctions were first identified by the obvious traces they left in the fossil record

Why is this event important?:

Because if dinosaurs were still here to this day, no humans would be alive as dinosaurs are very huge and can easily kill anyone

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In the late Triassic period, a lot of mammals came onto Earth, such as armadillos, giant whales, wolves, hippos, mammoths, lions, and many more

How do scientists know about this?:

The discovery of extensive plant and animal fossils allows us to paint a more detailed picture of how mammals arose after the demise of the dinosaurs 66 million years ago.

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The first human ancestors appeared between five million and seven million years ago, probably when some apelike creatures in Africa began to walk habitually on two legs.

They were flaking crude stone tools 2.5 million years ago. Then some of them spread from Africa into Asia and Europe two million years ago

How do scientists know about this?:

Archaeological evidence and early human fossils provide the most significant hints about this ancient past. Bones, tools, and any other evidence (such as footprints) are all included in these leftovers.

Why is this event important?:

Because humans coming on earth is the start of new life, and without humans, we wouldnt have alot of the things we have right now