that increase global warming, most experts point to carbon dioxide as the worst pollutant in this case. This gas is released into the atmosphere mainly through burning of fossil fuels, such as coal, gasoline, and diesel. The gas also forms from the destruction of natural vegetation, such as the burning of forests to turn into grazing meadows for livestock. In this case, the carbon dioxide releases in two ways. First, the destruction of plant life through human actions causes less carbon dioxide to be absorbed out of the atmosphere; and secondly, rotting vegetation in clear-cut forests releases carbon dioxide.
What is ozone and how did it benefit the early Earth?
Ozone (O3)—compared to the oxygen (O2) we breathe—usually refers to a blanket of gas found between 9 and 25 miles (15 and 40 kilometers) in the layer of Earth’s atmosphere called the stratosphere. The so-called “ozone layer” is produced by the interaction of the Sun’s radiation with certain air molecules. The blue-tinged ozone gas is also found in the lower atmosphere. While beneficial in the stratosphere, ozone forms photochemical smog at ground level. This smog is a secondary pollutant produced by the photochemical reactions of certain air pollutants, usually from industrial activities and cars.
The stratosphere’s ozone layer is important to all life on the planet because it protects organisms from the Sun’s damaging ultraviolet radiation. Scientists believe that about two billion years ago, oxygen was being produced by shallow water marine plants. This sudden—geologically speaking—outpouring of oxygen helped to build up the ozone layer. As the oxygen levels increased, ocean animals began to evolve. Once the protective ozone layer was in place in the atmosphere, it allowed the marine plants and animals to spread onto land, safe from the Sun’s radiation.
The formation of the ozone layer in the upper atmosphere early in Earth’s history created a radiation boundary that protects life on the planet. Today, scientists are concerned about the hole in the ozone that has appeared over the South Pole, as seen in this 1987 satellite image (National Oceanographic and Atmospheric Administration).
BEGINNINGS OF LIFE
When did life first begin on Earth?
No one knows the precise time that life began on Earth. One reason is that early life consisted of single-celled organisms. Because the soft parts of an organism are the first to decay and disappear after death, it is almost impossible to find the remains. In addition, because the organisms were so small, they are now difficult to detect in ancient rocks. Some modern viruses are only about 18 nanometers (18 billionths of a meter) across and modern bacteria typically measure 1,000 nanometers across, which is much larger than the early organisms.
In addition, because scientists have found so little fossil evidence, it is difficult to know all the true shapes of the earliest life. Scientists believe that early life was composed of primitive single-cells and started in the oceans. The reason is simple: life needed a filter to protect it from the incoming ultraviolet energy from the Sun—and the ocean waters gave life that protection.
Could life have arrived from outer space?
There is another theory of how the precursors of life were brought to Earth—known as panspermia. Scientists theorize that comets and asteroids bombarded the early Earth, bringing complex organic materials, many of which survived the fall to our planet.
Scientists know there are such organic materials in space. In the late 1960s, radio astronomers discovered organic molecules in dark nebulae. Since that time, other sources have been found, including organic molecules existing in space bodies such as asteroids, comets, and meteorites. In 1969, analysis of a meteorite showed at least 74 amino acids within the chunk of rock. Scientists began to speculate that the organic molecules could have traveled to Earth via meteorites, cometary dust, or, during the early years of Earth, by way of comets and asteroids.
Although many scientists argue that the heat from the impact of a giant asteroid or comet would destroy any organic passengers, many other scientists disagree. They propose that only the outer layers of a large body would be affected, or that the fine, unheated dust of comets could have brought the necessary amino acids to Earth. If this theory is true, we are apparently all—from dinosaurs to humans—made of “star stuff.”
Despite such gaps in knowledge, scientists estimate that the first life began about four billion years ago. These organisms did not survive on oxygen, but carbon dioxide.
What were the conditions on the early Earth that scientists believe may have led to life?
Two major theories explain how life could have grown on early Earth. The first theory states that life grew from a primordial “soup,” a thick stew of biomolecules and water. Chemical reactions were then triggered by the Sun’s ultraviolet rays, lightning, or perhaps even the shockwaves from violent meteor strikes that were more common at the time. These reactions produced various carbon compounds, including amino acids, which make up the proteins found in all living organisms. This theory was postulated after a famous experiment performed at the University of Chicago in 1954 by then-graduate student Stanley Miller (1930–2007), and his advisor, chemist Harold Urey (1893–1981). They showed that the amino acids could be formed from chemicals thought to exist in the early Earth atmosphere when they were combined with water and zapped by lightning.
One of the earliest forms of life to appear on Earth was cyanobacteria (inset), which have left behind unusual fossil rocks called stromatolites. (iStock).
The second theory of life centers around a discovery made within the last half century: hydrothermal vents, which are cracks caused by volcanic magma seeping through the deep ocean floor. There were probably many more hydrothermal vents during the early history of Earth, as the crust was newer, and thus thinner, than today’s cooled, thicker crust. The organisms around these vents did not need to rely on photosynthesis for energy. Scientists know that today’s volcanic vent organisms live off the bacteria around the vents, which in turn extract energy from the hot, hydrogen sulfide-rich water found around the sunless cracks in the ocean floor. Early organisms could have survived in much the same way.
In actuality, the conditions described by both theories could have existed simultaneously to produce the planet’s early life.
What are the oldest-known fossils found in rock on Earth?
The oldest-known fossils in rock have been found in Australia. One set of fossils found in Western Australia is dated between 3.45 and 3.55 billion years old. They show evidence of layered mounds of limestone sediment called stromatolites, which were formed by primitive microorganisms similar to blue-green algae called cyanobacteria. Scientists know that stromatolites exist today. The fossils look amazingly like the stromatolites from the shallow waters off the coast of modern Australia.
There are other contenders for the oldest-known fossils. Tiny, simple cells have also been found in ancient cherts (crystalline-rich sedimentary rocks) from Australia, and there are similar ones in Africa. These cells are preserved by the silica from the chert, and appear to show a cell wall of some kind.
When did the basic form of life develop on Earth?
It is thought that as far back as about 3.8 billion years ago, a basic form of life was present on Earth. This life took the form of tiny cells, which were surrounded by membranes to isolate and protect their interiors from the surrounding environment. The cells had a basic genetic system similar to those in modern cells, and this allowed the cells to self-replicate. We classify these earliest life-forms as prokaryotes, which includes such organisms as bacteria and cyanobacteria.
When did larger cells develop?
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