Molecules of two elements, nitrogen and oxygen, comprise about 99 percent of the air. The remaining hoity toity 1% includes small amounts celestial seasoning luxurious riches as argon and carbon dioxide. (Other gases such as neon, helium, and methane are present in trace amounts.) Oxygen is the life-giving element in the air.

Earth's atmosphere is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.03% carbon dioxide with very small percentages of other elements. Our atmosphere also contains water vapor. In addition, Earth's atmosphere contains traces of dust particles, pollen, plant grains and other solid particles.

Even when the air seems to be completely clear, it is full of atmospheric particles - invisible solid and semisolid bits of matter, including dust, smoke, pollen, spores, bacteria and viruses. Some atmospheric particles are so large that you will feel them if they strike you. However, particles this large rarely travel far before they fall to the ground. Finer particles may be carried many miles before settling during a lull in the wind, while still tinier specks may remain suspended in the air indefinitely. The finest particles are jostled this way and that by moving air molecules and drift with the slightest currents. Only rain and snow can wash them out of the atmosphere. These tiny particles are so small that scientists measure their dimensions in microns - a micron is about one 25-thousandth of an inch. They include pollen grains, whose diameters are sometimes less than 25 microns; bacteria, which range from about 2 to 30 microns across; individual virus particles, measuring a very small fraction of a micron; and carbon smoke particles, which may be as tiny as two hundredths of a micron.

Particles are frequently found in concentrations of more than a million per cubic inch of air. A human being's daily intake of air is about 450,000 cubic inches. This means that we inhale an astronomical numbers of foreign bodies. Particles larger than about 5 microns are generally filtered from the air in the nasal passages. Other large particles are caught by hairlike protuberances in the air passages leading to the lungs and are swept back toward the mouth. Most of the extremely fine particles that do reach the lungs are exhaled again - although some of this matter is deposited in the minute air sacs within the lungs. From these air sacs, particles may go into solution and pass through the lung walls into the bloodstream. If the material is toxic, harmful reactions may occur when it enters the blood. Fine particles retained in the lungs can cause permanent tissue damage, as with Coal workers' pneumoconiosis (black lung disease), caused by buildup of coal dust in the lungs, and with silicosis, which is caused by the buildup of silicon dust.

If the air is still, given sufficient time, all but the smallest airborne particles will settle to the ground under their own weight. Their rate of fall is closely proportional to particle size and density.
For example, vast amounts of fine volcanic ash were thrown into the air by the eruption of the Indonesian volcano Krakatoa, in 1883, and again by the Alaskan volcano Katmai, in 1912. In both instances, the finer dust reached the stratosphere and spread around the world high above the rains and storms that tend to cleanse the lower atmosphere. In fact, many years elapsed before these volcanic dusts entirely disappeared from the atmosphere. Since a two-micron dust particle may require about four years to fall 17 miles in the atmosphere, the lingering effect is not in the least surprising.
Dust storms are also prolific producers of airborne debris. Europe is sometimes showered with dust originating in the Sahara. In March 1901, for instance, an estimated total of two million tons of Sahara dust fell on North Africa and the Europe. Two years later, in February 1903, Britain received a deposit estimated at ten million tons. On many occasions, Sahara dust has fallen in muddy rain and reddish snow over much of southwestern Europe. During North America's droughts of the 1930s, dust storms blew ten million tons of dust at a time aloft in the heart of the continent. Occasionally, high winds swept the dust eastward 1800 miles to darken skies along the continent's Atlantic coast.

When the wind strikes the crest of an ocean wave, or a calm sea is agitated by rain or by air bubbles bursting at the surface, the finer droplets that enter the air quickly evaporate, leaving tiny salt crystals suspended in the air. Winds carry these salt crystals over all the Earth. Normally, airborne salt particles from the sea are less than a micron in diameter. It would take a million of them to weigh a pound.
Salt particles play an important part in weather processes because they are hygroscopic - they absorb water. Raindrops usually form around tiny particles that act as nuclei for condensation. Generally, each fog and cloud droplet also collects around a particle of some type at its center. Tiny crystals of sea salt make better condensation nuclei than other natural particles found in the air. Thus, salt particles in the air help make rain.

Dust from meteor showers may occasionally affect world rainfall. When the Earth encounters a swarm of meteors, those meteors that get to the upper reaches of the Earth's atmosphere are vaporized by heat from friction. The resulting debris is a fine smoke or powder. This fine dust then floats down into the cloud system of the lower atmosphere, where it can readily serve as nuclei around which ice crystals or raindrops can form. Increases in world rainfall come about a month after the Earth encounters meteor systems in space. The delay of a month allows sufficient time for the meteoric dust to fall through the upper atmosphere. Occasionally, large meteors leave visible trains of dust. Most often their trails disappear rapidly, but in a few witnessed cases a wake of dust has remained visible for an hour or so.
In one extreme instance-a great meteor that broke up in the sky over Siberia in 1908-the dust cloud traveled all the way around the world before it dissipated.

Large forest fires are among the more spectacular producers of foreign particles in the atmosphere.
Because these fires create violent updrafts, smoke particles are carried to great heights, and, being small, are spread over vast distances by high altitude winds. In the autumn of 1950, forest fires in Alberta, Canada produced smoke that drifted east over North America on the prevailing wind and crossed the North Atlantic, reaching Britain and continental Europe. The light-scattering properties of this dense smoke made the Sun look indigo and the Moon blue to observers in Scotland and other northern lands.

Wind-pollinated plants are the most prolific sources of foreign particles in the air. This is a problem for people with allergies.

Spores are closely related to pollens. Spores are the reproductive bodies of fungi, which include molds, yeasts, rusts, mildews, puffballs and mushrooms. Tiny spores are adrift everywhere in the air, even over the oceans. Although they resemble pollens in general appearance, spores are not fertilizing agents. Instead, they are like seeds, and give rise to new organisms wherever they take hold. Spores have been found as high as 14 miles in the air over the entire globe. Most fungi depend on the wind for spore dissemination. Once airborne, spores are carried easily by the slightest air currents.

Once, physicians were taught that infectious microorganisms quickly settle out of the air and die. Today, the droplets ejected, say, by a sneeze, are known to evaporate almost immediately, leaving whatever microorganisms they contain to drift through the air. Only a relatively small fraction of microorganism’s human beings breathe cause disease. In fact, most bacteria are actually helpful. Some, for example, convert atmospheric nitrogen into usable plant food. Pathogenic, or disease-producing, microorganisms, however, can be very dangerous. Most propagate by subdivision-each living cell splits into two cells. Each of the new cells then grows and divides again into two more cells. Provided with ideal conditions, populations multiply quickly. Fortunately microorganisms do not thrive very well in the air. Unless there is enough humidity in the air, many desiccate and die. Short exposure to the ultraviolet radiation of the Sun also kills most microorganisms. Low temperatures greatly decrease their activity, and elevated temperatures destroy them rapidly. Still, many microorganisms survive in the air, despite these hazards. Among the tiniest of airborne particles are viruses, which are on the borderline between living matter and lifeless chemical substances.

Earth is the only planet we know of that can support life. This is an amazing fact, considering that it is made out of the same matter as other planets in our solar system, was formed at the same time and through the same processes as every other planet, and gets its energy from the sun. To a universal traveler, Earth may seem to be a harmless little planet in the far reaches of one of billions of spiral galaxies in the universe. It has an average size star of average brightness and is joined by seven other planets — which support no known life forms — in its solar system. While this may be fitting for a passage from The Hitchhikers Guide to the Galaxy, by Douglas Adams, in the grand scheme of the universe, it would be a fairly accurate description. However, Earth is a planet teeming with vitality and is home to billions of plants and animals that share a common evolutionary track. How and why did we get here? What processes had to take place for this to happen? And where do we go from here? The fact is, no one has been able to come close to knowing exactly what led to the origins of life, and we may never know. After 5 billion years of Earth’s formation and evolution, the evidence may have been lost. But scientists have made significant progress in understanding what chemical processes that may have led to the origins of life. There are many theories, but most have the same general perspective of how things came to be the way they are. Following is an account of life’s beginnings based on some of the leading research and theories related to the subject, and of course, fossil records dating back as far as 3.5 billion years ago.

The solar system was created from gas clouds and dust that remained from the Sun's formation some 6-7 billion years ago. This material contained only about .2% of the solar system's mass with the Sun holding the rest. Earth began to form over 4.6 billion years ago from the same cloud of gas (mostly hydrogen and helium) and interstellar dust that formed our sun, the rest of the solar system and even our galaxy. In fact, Earth is still forming and cooling from the galactic implosion that created the other stars and planetary systems in our galaxy. This process began about 13.6 billion years ago when the Milky Way Galaxy began to form. As our solar system began to come together, the sun formed within a cloud of dust and gas that continued to shrink in upon itself by its own gravitational forces. This caused it to undergo the fusion process and give off light, heat and other radiation. During this process, the remaining clouds of gas and dust that surrounded the sun began to form into smaller lumps called planetesimals, which eventually formed into the planets we know today.

A large number of small objects, called planetesimals, began to form around the Sun early in the formation of the solar system. These objects were the building blocks for the planets that exist today. The Earth went through a period of catastrophic and intense formation during its earliest beginnings 4.6-4.4 billion years ago. By 3.8 to 4.1 billion years ago, Earth had become a planet with an atmosphere (not like our atmosphere today) and an ocean. This period of Earth’s formation is referred to as the Precambrian Period. The Precambrian is divided into three parts: the Hadean, Archean and Proterozoic Periods.

The Earth formed under so much heat and pressure that it formed as a molten planet. For nearly the first billion years of formation (4.5 to 3.8 billion years ago) — called the Hadean Period (or hellish period) — Earth was bombarded continuously by the remnants of the dust and debris — like asteroids, meteors and comets — until it formed into a solid sphere, pulled into orbit around the sun and began to cool down. Earth's early atmosphere most likely resembled that of Jupiter's atmosphere, which contains hydrogen, helium, methane and ammonia, and is poisonous to humans. (Photo: NASA, from Voyager 1). As Earth began to take solid form, it had no free oxygen in its atmosphere. It was so hot that the water droplets in its atmosphere could not settle to form surface water or ice. Its first atmosphere was also so poisonous, comprised of helium and hydrogen, that nothing would have been able to survive.
Earth’s second atmosphere was formed mostly from the outgassing of such volatile compounds as water vapor, carbon monoxide, methane, ammonia, nitrogen, carbon dioxide, nitrogen, hydrochloric acid and sulfur produced by the constant volcanic eruptions that besieged the Earth. It had no free oxygen. About 4.1 billion years ago, the Earth’s surface — or crust — began to cool and stabilize, creating the solid surface with its rocky terrain. Clouds formed as the Earth began to cool, producing enormous volumes of rainwater that formed the oceans. For the next 1.3 billion years (3.8 to 2.5 billion years ago), the Archean Period, first life began to appear and the world’s land masses began to form. Earth’s initial life forms were bacteria, which could survive in the highly toxic atmosphere that existed during this time. Toward the end of the Archean Period and at the beginning of the Proterozoic Period, about 2.5 billion years ago, oxygen-forming photosynthesis began to occur. The first fossils were a type of blue-green algae that could photosynthesize.

Earth's atmosphere was first supplied by the gasses expelled from the massive volcanic eruptions of the Hadean Era. These gases were so poisonous, and the world was so hot, that nothing could survive. As the planet began to cool, its surface solidified as a rocky terrain, much like Mars' surface (center photo) and the oceans began to form as the water vapor condensed into rain. First life came from the oceans. Some of the most exciting events in Earth’s history and life occurred during this time, which spanned about two billion years until about 550 million years ago. The continents began to form and stabilize, creating the supercontinent Rodinia about 1.2 billion years ago. Although Rodinia is composed of some of the same land fragments as the more popular supercontinent, Pangea, they are two different supercontinents. Pangea formed some 225 million years ago and would evolve into the seven continents we know today. Free oxygen began to build up around the middle of the Proterozoic Period — around 1.8 billion years ago — and made way for the emergence of life as we know it today. This increased oxygen created conditions that would not allow most of the existing life to survive and thus made way for the more oxygen-dependent life forms. By the end of the Proterozoic Period, Earth was well along in its evolutionary processes leading to our current period, the Holocene Period,  or Anthropocene Period, also known as the Age of Man. Thus, about 525 million years ago, the Cambrian Period began. During this period, life “exploded,” developing almost all of the major groups of plants and animals in a relatively short time. It ended with the massive extinction of most of the existing species about 500 million years ago, making room for the future appearance and evolution of new plant and animal species. About 498 million years later — 2.2 million years ago — the first modern human species emerged.

Did You Know? The first modern human being was called **** habilis, the first of the **** genus. This species developed stone tools for use in daily life. **** habilis means “Handy Man.” He existed from about 2.2 to 1.5 million years ago. There are earlier species related to modern man, called hominids. The images show the skull shape and probable appearance of **** habilis.

The PreCambrian Period — accounts for about 90 percent of Earth’s history. It lasted for about four billion years until about 550 million years ago. About 70 percent of the world’s land masses were created in the Archean Era, between 3.8 and 2.5 million years ago. Rodinia, widely recognized as the first supercontinent, formed during the Proterozoic Era, about 2.5 billion years ago. It is believed that the oldest human family member was discovered in Ethiopia and lived 4.4 million years ago. It was named “Ardi,” short for Ardipithecus ramidus.