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When a lot of water vapour fills the air, it begins to change and condense into droplets of water. These droplets fall back onto Earth as precipitation, which can take many forms - rain, hail, snow, sleet, fog, dew. So rain is just one form of precipitation.

The difference between Rainfall and Precipitation is that the Rainfall is a liquid water in the form of droplets that have condensed from atmospheric water vapor and then precipitated and Precipitation is a product of the condensation of atmospheric water vapour that falls under gravity.

Rainfall

Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then becomes heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.

The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes.

The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth it is much higher at 990 mm (39 in). Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar.

Rain is also known or suspected on other planets, where it may be composed of methane, neon, sulfuric acid, or even iron rather than water.

Precipitation

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and “precipitates”. Thus, fog and mist are not precipitation but suspensions, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called “showers.”

Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is typically active when freezing rain occurs. A stationary front is often present near the area of freezing rain and serves as the foci for forcing and rising air. Provided necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely stratus and cumulonimbus. Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extra tropical cyclones. Lake-effect snowfall can be locally heavy. Thunder snow is possible within a cyclone’s comma head and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation occurs within the tropics and is caused by convection. The movement of the monsoon trough, or inter-tropical convergence zone, brings rainy seasons to savannah climes.

Precipitation is a major component of the water cycle, and is responsible for depositing the fresh water on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of waterfalls as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of it over the oceans and 107,000 cubic kilometres (26,000 cu mi) over land. Given the Earth’s surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in), but over land it is only 715 millimetres (28.1 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes.

Precipitation may occur on other celestial bodies, e.g. when it gets cold, Mars has precipitation which most likely takes the form of frost, rather than rain or snow.

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When water evaporates it forms the gaseous water vapour. The amount of water vapour in the air at any one time is known as its humidity. AS more and more water vapour saturates the air, humidity increases, eventually resulting in rain, fog or mist, depending on the heat and temperature of the place.

Humidity is the amount of moisture or water vapour or water molecules present in the atmospheric gas. The more water in the vapour, the higher the humidity. Humidity arises from water evaporating from places like lakes and oceans. Warm water evaporates quickly. That’s why; you may find the most humid regions near to warm water bodies in places like the Red Sea, the Persian Gulf, and Miami.

Types of Humidity:

1. Relative humidity: A meteorologist uses the term ‘relative humidity’. The relative humidity is a comparison of the amount of moisture present in the air to the amount of moisture air can hold. The amount of moisture the atmosphere can hold totally depends on the temperature.

1. Specific humidity: We define specific humidity as the mass of water vapour present in a given unit mass of moist air.

Specific humidity is equal to the ratio of water vapour mass and the air parcel’s total (including dry) air mass.

Specific humidity is also known as the humidity ratio. It does not change with the expansion or compression of an air parcel.

We usually express specific heat as grams of vapour per kg of air, or in air conditioning as grains per pound.

The specific humidity has great usage in meteorology.

3. Absolute humidity: We define the absolute humidity in the two following sentences:

Absolute humidity is equal to the mass of water vapour per unit of volume of air, i.e., grams of water/cm3 of air. The formula for the absolute humidity is given by:

             Absolute humidity = Mass of water/volume in cm3

Absolute humidity does not take temperature into consideration.

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The stratosphere has a layer of ozone gas, which acts like a thick umbrella covering the layers beneath. By absorbing most of the harmful UV radiation from the Sun, the ozone layer prevents it from reaching the surface of the Earth, thus enabling the survival of life on the planet.

Stratosphere could be aptly called the ‘protection blanket’ of Earth. It extends up to 600 kms from the surface of the earth and it is the second layer of the Earth’s atmosphere, right above troposphere.

Stratosphere houses in it the most important layer called Ozone (O3), which acts as an absorber of the harmful UV radiations of the Sun (of about 90%) and thereby protecting us from diseases like Cancer, skin burn etc.

Its non-turbulance and stable, non-convection character makes it possible for the jets to cruise easily, hence they are flown here.

When Volcanic eruptions occur, the ejected material reaches as high as Stratosphere and it stays there for long period, as it doesn’t allow the circulation, there by leading to stratifying the volcanic particles and cooling down of the Earth surface.

However, such an important layer is being perforated by us through the extensive use of the chloro-fluro-carbons which happen to destroy the ozone molecules.

There is also an idea which the scientists are considering that could result in the slowing down of the Earth’s heating, i.e., by adding the man-made materials to stratosphere. Though the feasibility of this idea is yet to be verified. Thus, is the importance of the Stratosphere layer.

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Yes, the atmosphere has five layers. The lowest layer, closest to the surface of the Earth, is the troposphere. This is where weather is made, and most of the atmosphere’s gases are concentrated in it. Above it is the Stratosphere. No winds blow in this layer, nor are there any clouds. Beyond it lies the cold mesosphere, with very few gases. It is followed by the thermosphere, the thickest and hottest layer of the atmosphere, and lastly, the exosphere, on the edge of outer space.

Earth’s atmosphere is all around us. Most people take it for granted. Among other things, it shields us from radiation and prevents our precious water from evaporating into space. It keeps the planet warm and provides us with oxygen to breathe. In fact, the atmosphere makes Earth the livable, lovable home sweet home that it is.

The atmosphere extends from Earth’s surface to more than 10,000 kilometers (6,200 miles) above the planet. Those 10,000 kilometers are divided into five distinct layers. From the bottom layer to the top, the air in each has the same composition. But the higher up you go, the further apart those air molecules are.

Troposphere: Earth’s surface to between 8 and 14 kilometers (5 and 9 miles)

This lowest layer of the atmosphere starts at the ground and extends 14 kilometers (9 miles) up at the equator. That’s where it’s thickest. It’s thinnest above the poles, just 8 kilometers (5 miles) or so. The troposphere holds nearly all of Earth’s water vapor. It’s where most clouds ride the winds and where weather occurs. Water vapor and air constantly circulate in turbulent convection currents. Not surprisingly, the troposphere also is by far the densest layer. It contains as much as 80 percent of the mass of the whole atmosphere. The further up you go in this layer, the colder it gets.

Stratosphere: 14 to 64 km (9 to about 31 miles)

Unlike the troposphere, temperatures in this layer increase with elevation. The stratosphere is very dry, so clouds rarely form here. It also contains most of the atmosphere’s ozone, triplet molecules made from three oxygen atoms. At this elevation, ozone protects life on Earth from the sun’s harmful ultraviolet radiation. It’s a very stable layer, with little circulation. For that reason, commercial airlines tend to fly in the lower stratosphere to keep flights smooth. This lack of vertical movement also explains why stuff that gets into in the stratosphere tends to stay there for a long time. That “stuff” might include aerosol particles shot skyward by volcanic eruptions, and even smoke from wildfires. This layer also has accumulated pollutants, such as chlorofluorocarbons. Better known as CFCs, these chemicals can destroy the protective ozone layer, thinning it greatly. By the top of the stratosphere, called the stratopause, air is only a thousandth as dense as at Earth’s surface.

Mesosphere: 64 to 85 km (31 to 53 miles)

Scientists don’t know quite as much about this layer. It’s just harder to study. Airplanes and research balloons don’t operate this high and satellites orbit higher up. We do know that the mesosphere is where most meteors harmlessly burn up as they hurtle towards Earth.

The mesopause is also known as the Karman line. It’s named for the Hungarian-born physicist Theodore von Kármán. He was looking to determine the lower edge of what might constitute outer space. He set it at about 80 kilometers (50 miles) up.

The ionosphere is a zone of charged particles that extends from the upper stratosphere or lower mesosphere all the way to the exosphere. The ionosphere is able to reflect radio waves; this allows radio communications.

Thermosphere: 85 to 600 km (53 to 372 miles)

The next layer up is the thermosphere. It soaks up x-rays and ultraviolet energy from the sun, protecting those of us on the ground from these harmful rays. The ups and downs of that solar energy also make the thermosphere vary wildly in temperature. It can go from really cold to as hot as about 1,980 ºC (3,600 ºF) near the top. The sun’s varying energy output also causes the thickness of this layer to expand as it heats and to contract as it cools. With all the charged particles, the thermosphere is also home to those beautiful celestial light shows known as auroras. This layer’s top boundary is called the thermopause.

Exosphere: 600 to 10,000 km (372 to 6,200 miles)

The uppermost layer of Earth’s atmosphere is called the exosphere. Its lower boundary is known as the exobase. The exosphere has no firmly defined top. Instead, it just fades further out into space. Air molecules in this part of our atmosphere are so far apart that they rarely even collide with each other. Earth’s gravity still has a little pull here, but just enough to keep most of the sparse air molecules from drifting away. Still, some of those air molecules — tiny bits of our atmosphere — do float away, lost to Earth forever.

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The atmosphere, with all its layers, extends up to 10,000 km from Earth’s surface. Temperatures vary in the different layers: the mesosphere can go down to -90 °C, the exosphere much, much lower, whereas the thermosphere can be as hot as 2000 °C!

Temperature varies greatly at different heights relative to Earth's surface and this variation in temperature characterizes the four layers that exist in the atmosphere. These layers include the troposphere, stratosphere, mesosphere, and thermosphere.

The troposphere is the lowest of the four layers, extending from the surface of the Earth to about 11 km (6.8 mi) into the atmosphere where the tropopause (the boundary between the troposphere stratosphere) is located. The width of the troposphere can vary depending on latitude, for example, the troposphere is thicker in the tropics (about 16 km (9.9 mi)) because the tropics are generally warmer, and thinner at the poles (about 8 km (5.0 mi)) because the poles are colder. Temperatures in the atmosphere decrease with height at an average rate of 6,5°C (11,7 °F) per kilometer. Because the troposphere experiences its warmest temperatures closer to Earth's surface, there is great vertical movement of heat and water vapour, causing turbulence. This turbulence, in conjunction with the presence of water vapour, is the reason that weather occurs within the troposphere.

Following the tropopause is the stratosphere. This layer extends from the tropopause to the stratopause which is located at an altitude of about 50 km (31 mi). Temperatures remain constant with height from the tropopause to an altitude of 20 km (12 mi), after which they start to increase with height. This happening is referred to as an inversion and it is because of this inversion that the stratosphere is not characterized as turbulent. The stratosphere receives its warmth from the sun and the ozone layer which absorbs ultraviolet radiation.

The next layer is called the mesosphere which extends from the stratopause to the mesopause, located at an altitude of 85 km (53 mi). Temperatures in the mesospere decrease with altitude and are in fact the coldest in the Earth's atmosphere.This decrease in temperature can be attributed to the diminishing radiation received from the Sun, after most of it has already been absorbed by the thermosphere.

The fourth layer of the atmosphere is known as the thermosphere which extends from the mesopause to the 'top' of the collisional atmosphere. Some of the warmest temperatures can be found in the thermosphere, due to its reception of strong ionizing radiation at the level of the Van Allen radiation belt.

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The troposphere can be found between the ground and an altitude of 7 to 20 kilometers (4 to 12 miles). The lesser thickness is found at the Polar Regions, since colder temperatures lead to a decrease in gas volume. The vast majority of the world's weather is formed in the troposphere and this layer also contains 80 percent of the atmosphere's mass. The temperature within the troposphere drops with altitude, since it is essentially being warmed by the ground. The pressure also drops within the troposphere as altitude increases, and this explains why mountaineers require oxygen masks.

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It is another layer, overlapping the mesosphere, thermosphere and exosphere, where radio waves are reflected.

A dense layer of molecules and electrically charged particles, called the ionosphere, hangs in the Earth’s upper atmosphere starting at about 35 miles (60 kilometers) above the planet's surface and stretching out beyond 620 miles (1,000 km). Solar radiation coming from above buffets particles suspended in the atmospheric layer. Radio signals from below bounce off the ionosphere back to instruments on the ground. Where the ionosphere overlaps with magnetic fields, the sky erupts in brilliant light displays that are incredible to behold.

Several distinct layers make up Earth's atmosphere, including the mesosphere, which starts 31 miles (50 km) up, and the thermosphere, which starts at 53 miles (85 km) up. The ionosphere consists of three sections within the mesosphere and thermosphere, labeled the D, E and F layers, according to the UCAR Center for Science Education.

Extreme ultraviolet radiation and X-rays from the sun bombard these upper regions of the atmosphere, striking the atoms and molecules held within those layers. The powerful radiation dislodges negatively charged electrons from the particles, altering those particles' electrical charge. The resulting cloud of free electrons and charged particles, called ions, led to the name "ionosphere." The ionized gas, or plasma, mixes with the denser, neutral atmosphere.

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You can consider most satellites to be in space, but in terms of the Earth's atmosphere, they occupy regions called the thermosphere and the exosphere. The layer through which a satellite orbits depends on the satellite's function and the kind of orbit it has. Since the launch of Sputnik in the 1950s, spacefaring countries have put thousands of satellites into orbit around the Earth and even other planets. They serve many different purposes, from complex space stations like the International Space Station to the Global Positioning System that helps you find your way home.

Thermosphere: High Temperatures

The thermosphere is a region of very high temperature that extends from the top of the mesosphere at around 85 kilometers (53 miles) up to 640 kilometers (400 miles) above the Earth's surface. It is called the thermosphere because temperatures can reach up to 1,500 degrees Celsius (2,732 degrees Fahrenheit). However, despite the high temperatures, the pressure is very low, so satellites don't suffer heat damage.

Exosphere: Farthest Reaches

Above the thermosphere sits a final layer called the exosphere, which extends up to 10,000 kilometers (6,200 miles) above the Earth, depending on how it is defined. Some definitions of the exosphere include all space up until the point where atoms get knocked away by solar wind. No distinct upper boundary exists since the exosphere has no pressure and molecules float freely here. Eventually, the exosphere gives way to space outside of the Earth's influence.

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The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.

Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.

An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.

Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Honolulu is a city at sea level. Denali, also known as Mount McKinley, is the highest peak in North America.

As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.

Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly.

Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.

Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.

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Covering the surface of Earth like a thin blanket is a layer of gases that forms the atmosphere. It is made up of 78 per cent nitrogen, 21 per cent oxygen and 0.04 per cent carbon dioxide. The minute, remaining percentage is made of some other gases, water vapour and dust. We barely notice this all-enveloping atmosphere, but without it the Earth would be lifeless as the Moon.

Earth’s atmosphere is composed of about 78% nitrogen, 21% oxygen, 0.9% argon, and 0.1% other gases. Trace amounts of carbon dioxide, methane, water vapor, and neon are some of the other gases that make up that remaining 0.1%. While the earth’s atmosphere is mainly gases, it also contains tiny particles such as dust and pollen. Some unnatural particles also collect in the atmosphere and cause air pollution. These include anything from aerosols to carbon emissions from vehicles and power plants.

As humans, we rely on the atmosphere around us for life. We breathe it, we live in it—without it, we wouldn’t survive. Not only does the atmosphere around us need to be made of a certain composition for us to thrive, but it also needs to be one in which plants and food can grow, and one that protects us from the elements. Having oxygen we can breathe is just as important as being protected from the harsh sun rays, or the open expanses of space, and Earth has just the right location and atmospheric chemical composition to sustain life for humans and all other life forms that call Earth home.

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Earth’s surface was probably molten for many millions of years after its formation. Life did not exist for the first 400-800 million years, and first began in water after the forming of the oceans.

Life seems to have started on Earth almost as soon as the surface cooled off enough to make it possible. However, complex animal life—everything from insects to fish to humans - took a lot longer to show up. Given that modern animals are a phenomenally diverse group that evolved relatively quickly, why were they so slow to get going?

The answer may be that animals are greedy: they need a lot of oxygen to grow big and complicated. Early Earth didn’t have much oxygen, but microbes changed the chemical content of the atmosphere over time from something alien and poisonous to us into the breathable air we have today. A new paper showed that the oxygen level as recently as 800 million years ago was only a tiny fraction of today’s - far too low to support oxygen-breathers like our ancestors and their relatives.

Life on Earth has always belonged mainly to microorganisms. Clouds are full of microbes; they have been found in deep mines and on the ocean floor. They outnumber and may even outweigh all other forms of life. If all animals vanished, most bacteria would still live on, but if all bacteria disappeared, we would die quickly.

The history of life on Earth reflects this as well. The first single-celled organisms appeared about 3.8 billion years ago, while the first known multi-cellular organisms evolved 2.1 billion years ago. However, these were “primitive” in our human-centric eyes: they didn’t have specialized organs for breathing or eating, much less brains for the wasteful activity we call “thinking”.

Then in the Cambrian era, around 570 million years ago, recognizably complex animal life evolved, including vertebrate ancestors. This change was relatively rapid in evolutionary terms, and a lot of diverse critters came out of it. Thus, something significant must have changed between 2.1 billion years and 570 million years to let animals diversify and complexity.

To explain this great change, scientists consider several possible explanations. One environmental (as opposed to genetic) idea: animals breathe in oxygen, so there needs to be enough oxygen in the air and water. Corals, sponges, and the like require less oxygen than crabs or fish, so oxygen levels limit what sorts of animals can evolve in a particular environment.

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Woolly mammoths were closely related to today's Asian elephants. They looked a lot like their modern cousins, except for one major difference. They were covered in a thick coat of brown hair to keep them warm in their home on the frigid Arctic plains. They even had fur-lined ears. 

Their large, curved tusks may have been used for fighting. They also may have been used as a digging tool for foraging meals of shrubs, grasses, roots and other small plants from under the snow.

Though woolly mammoths went extinct around 10,000 years ago, humans know quite a bit about them because of where they lived. The permafrost of the Arctic preserved many woolly mammoth bodies almost intact. When the ground around riverbanks and streams erodes, it often reveals the corpse of a long-dead mammoth that looks much like it did when it died.

Woolly mammoths were around 13 feet (4 meters) tall and weighed around 6 tons (5.44 metric tons), according to the International Union for Conservation of Nature (IUCN). Some of the hairs on woolly mammoths could reach up to 3 feet (1 m) long, according to National Geographic.

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Primitive life forms may have first appeared on Earth about 3800 million years ago. These bacteria lived in the oceans and built up solid mats of calcium carbonate, also known as lime. The deposits from the bacteria are known as stromatolites.

Stromatolites are living fossils and the oldest living lifeforms on our planet. The name derives from the Greek, stroma, meaning “mattress”, and lithos, meaning “rock”. Stromatolite literally means “layered rock”. The existence of these ancient rocks extends three-quarters of the way back to the origins of the Solar System.

With a citizen scientist’s understanding, stromatolites are stony structures built by colonies of microscopic photosynthesising organisms called cyanobacteria. As sediment layered in shallow water, bacteria grew over it, binding the sedimentary particles and building layer upon millimetre layer until the layers became mounds. Their empire-building brought with it their most important role in Earth’s history. They breathed. Using the sun to harness energy, they produced and built up the oxygen content of the Earth’s atmosphere to about 20%, giving the kiss of life to all that was to evolve.

Living stromatolites are found in only a few salty lagoons or bays on Earth. Western Australia is internationally significant for its variety of stromatolite sites, both living and fossilised. Fossils of the earliest known stromatolites, about 3.5 billion years old, are found about 1,000km north, near Marble Bar in the Pilbara region. With Earth an estimated 4.5 billion years old, it’s staggering to realise we can witness how the world looked at the dawn of time when the continents were forming. Before plants. Before dinosaurs. Before humans.

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The first land plants appeared during the Silurian period, around 440 million years ago. These simple plants reproduced by releasing spores. Plants produced oxygen and provided food for the first land animals - amphibians. Amphibians first developed in the Devonian period, 420 million years ago, from fish whose fins evolved into limbs.

Botanists now believe that plants evolved from the algae; the development of the plant kingdom may have resulted from evolutionary changes that occurred when photosynthetic multicellular organisms invaded the continents. The earliest fossil evidence for land plants consists of isolated spores, tracheid-like tubes, and sheets of cells found in Ordovician rocks. The abundance and diversity of these fossils increase into the Silurian Period (about 443.8 million to 419.2 million years ago), where the first macroscopic (megafossil) evidence for land plants has been found. These megafossils consist of slender forking axes that are only a few centimetres long. Some of the axes terminate in sporangia that bear trilete spores (i.e., spores that divide meiotically to form a tetrad). Because a trilete mark indicates that the spores are the product of meiosis, the fertile axes may be interpreted as the sporophyte phase of the life cycle.

Fossils of this type could represent either vascular plants or bryophytes. Another possibility is that they are neither but include ancestors of vascular plants, bryophytes, or both. The earliest fossils also include at least one or more additional plant groups that became extinct early in the colonization of the land and therefore have no living descendants. By the early Devonian Period (about 419.2 million to 393.3 million years ago), some of the fossils that consist of forking axes with terminal sporangia also produced a central strand of tracheids, the specialized water-conducting cells of the xylem. Tracheids are a diagnostic feature of vascular plants and are the basis for the division name, Tracheophyta.

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Just as the day is divided into hours, minutes and seconds, geologists divide Earth’s history into time periods. The longest divisions are aeons, which are billions of years long; the shortest are chrons, a few thousand years long. In between come eras, periods, epochs and ages. Scientists divide the last 590 million years into three eras: the Palaeozoic (meaning ‘old life’), Mesozoic (‘middle life’), and Cenozoic (‘new life’).

Humans have only been a species in the most recent chapter of the history of Earth. The Earth was formed 4.6 billion years ago, when the sun in our solar system first formed, creating enough gravitational pull to spin planets into orbit. But how do scientists know how old the Earth is if humans weren't around back when it was formed? When was life first formed on this planet? And what are the paths that life has taken so far over the course of that existence? This lesson will teach about the ages that make up the history of life on Earth, and explore the many changes and forms life has taken in its time on this planet.

Timeline of Geological Eras

The geological timeline of Earth is nearly identical to the history of life on Earth, apart from the Hadean Era. This is because the geological timeline, or the order of geological events, such as oceans forming, volcanoes erupting, how long deserts lasted, and tectonic plate movement, all happened in sequence with the life that has existed on this planet. The history of both life and the geological timeline is arranged within 5 subgroups, arranged from the largest span of time to the smallest: eons, eras, periods, epochs, and ages. To understand the timeline of life that has existed on Earth, it is important to understand how scientists determine the age of both rocks and the remnants of living things, otherwise known as fossils.

Because humans were not around 4.6 billion years ago to record the beginnings of our planet, scientists must rely on evidence from geological and fossil records in order to determine the relative age of both the planet and the life that exists here. Both the geological timeline and the age of life are determined in much the same way. Modern scientists rely on what is called radioactive dating to determine an accurate and precise age of both rocks and fossils. Radioactive dating measures the rate of decay of an element in a rock or in a fossil. Carbon-14 is typically used when dating fossils because all living things are carbon-based, and the flow of carbon can be tracked through the carbon cycle. For geological objects such as rocks and minerals, Rubidium-87 and Potassium-40 are often used. By knowing how long it takes for molecules in an object to decay, scientists can calculate when the object's half-life is. The half-life of an object refers to the amount of time it would take for half of an amount of a substance to radioactively decay, or break down. If the half-life of an object is known, it is possible to calculate when the object was first created, when no decay is evident.

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