Monday, June 1, 2009
Sunday, March 22, 2009
Life
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization, metabolism, growth, adaptation, response to stimuli and reproduction.[34] Life may also be said to be simply the characteristic state of organisms.
Properties common to terrestrial organisms (plants, animals, fungi, protists, archaea and bacteria) are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, and reproduce. An entity with these properties is generally considered life. However, not every definition of life considers all of these properties to be essential. Human-made analogs of life may also be considered to be life.
The biosphere is the part of Earth's outer shell – including air, land, surface rocks and water – within which life occurs, and which biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). Currently the entire Earth contains over 75 billion tons (150 trillion pounds or about 6.8 x 1013 kilograms) of biomass (life), which lives within various environments within the biosphere.[35]
Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.[36] More than 2 million species of plant and animal life have been identified to date,[37] and estimates of the actual number of existing species range from several million to well over 50 million.[38][39][40] The number of individual species of life is constantly in some degree of flux, with new species appearing and others ceasing to exist on a continual basis.[41][42] The total number of species is presently in rapid decline.
Properties common to terrestrial organisms (plants, animals, fungi, protists, archaea and bacteria) are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, and reproduce. An entity with these properties is generally considered life. However, not every definition of life considers all of these properties to be essential. Human-made analogs of life may also be considered to be life.
The biosphere is the part of Earth's outer shell – including air, land, surface rocks and water – within which life occurs, and which biotic processes in turn alter or transform. From the broadest geophysiological point of view, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere (rocks), hydrosphere (water), and atmosphere (air). Currently the entire Earth contains over 75 billion tons (150 trillion pounds or about 6.8 x 1013 kilograms) of biomass (life), which lives within various environments within the biosphere.[35]
Over nine-tenths of the total biomass on Earth is plant life, on which animal life depends very heavily for its existence.[36] More than 2 million species of plant and animal life have been identified to date,[37] and estimates of the actual number of existing species range from several million to well over 50 million.[38][39][40] The number of individual species of life is constantly in some degree of flux, with new species appearing and others ceasing to exist on a continual basis.[41][42] The total number of species is presently in rapid decline.
Microbes
The first form of life to develop on the Earth were microbes, and they remained the only form of life on the planet until about a billion years ago when multi-cellular organisms began to appear.[48] Microorganisms are single-celled organisms that are generally smaller than the human eye can see. They include Bacteria, Fungi, Archaea and Protista.
These life forms are found in almost every location on the Earth where there is liquid water, including the interior of rocks within the planet.[49] Their reproduction is both rapid and profuse. The combination of a high mutation rate and a horizontal gene transfer[50] ability makes them highly adaptable, and able to survive in new environments, including outer space.[51] They form an essential part of the planetary ecosystem. However some microorganisms are pathogenic and can post health risk to other organisms.
These life forms are found in almost every location on the Earth where there is liquid water, including the interior of rocks within the planet.[49] Their reproduction is both rapid and profuse. The combination of a high mutation rate and a horizontal gene transfer[50] ability makes them highly adaptable, and able to survive in new environments, including outer space.[51] They form an essential part of the planetary ecosystem. However some microorganisms are pathogenic and can post health risk to other organisms.
Evolution
Life, as it is currently understood, is only known to exist on the planet Earth. The origin of life is still a poorly understood process, but it is thought to have occurred about 3.9 to 3.5 billion years ago during the hadean or archean eons on a primordial earth that had a substantially different environment than is found at present.[46] These life forms possessed the basic traits of self-replication and inheritable traits. Once life had appeared, the process of evolution by natural selection resulted in the formation of ever-more diverse life forms.
Species that were unable to adapt to the changing environment and competition from other life forms became extinct. However, the fossil record retains evidence of many of these older species. Current fossil and DNA evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms.[46]
The advent of photosynthesis in very basic forms of plant life worldwide allowed the sun's energy to be harvested to create conditions allowing for more complex life. The resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of yet more complex cells called eukaryotes.[47] Cells within colonies became increasingly specialized, resulting in true multicellular organisms. With the ozone layer absorbing harmful ultraviolet radiation, life colonized the surface of Earth.
Species that were unable to adapt to the changing environment and competition from other life forms became extinct. However, the fossil record retains evidence of many of these older species. Current fossil and DNA evidence shows that all existing species can trace a continual ancestry back to the first primitive life forms.[46]
The advent of photosynthesis in very basic forms of plant life worldwide allowed the sun's energy to be harvested to create conditions allowing for more complex life. The resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of yet more complex cells called eukaryotes.[47] Cells within colonies became increasingly specialized, resulting in true multicellular organisms. With the ozone layer absorbing harmful ultraviolet radiation, life colonized the surface of Earth.
Ecosystems
All forms of life interact with the environment in which they exist, and also with other life forms. In the 20th century this premise gave rise to the concept of ecosystems, which can be defined as any situation where there is interaction between organisms and their environment.
Ecosystems are composed of a variety of abiotic and biotic components that function in an interrelated way.[55] The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: soil, atmosphere, radiation from the sun, water, and living organisms.
Each living organism has a continual relationship with every other element that makes up its environment. Within the ecosystem, species are connected and dependent upon one another in the food chain, and exchange energy and matter between themselves as well as with their environment.[56]
An aerial view of a human ecosystem. Pictured is the city of Chicago
Every species has limits of tolerance to factors that affect its survival, reproductive success and ability to continue to thrive and interact sustainably with the rest of its environment, which in turn may have effects on these factors for many other species or even on the whole of life.[57] The concept of an ecosystem is thus an important subject of study, as such study provides information needed to make decisions about how human life may interact in a way that allows the various ecosystems to be sustained for future use rather than used up or otherwise rendered ineffective. For the purpose of such study, a unit of smaller size is called a microecosystem. For example, an ecosystem can be a stone and all the life under it. A macroecosystem might involve a whole ecoregion, with its drainage basin.
Ecosystems are composed of a variety of abiotic and biotic components that function in an interrelated way.[55] The structure and composition is determined by various environmental factors that are interrelated. Variations of these factors will initiate dynamic modifications to the ecosystem. Some of the more important components are: soil, atmosphere, radiation from the sun, water, and living organisms.
Each living organism has a continual relationship with every other element that makes up its environment. Within the ecosystem, species are connected and dependent upon one another in the food chain, and exchange energy and matter between themselves as well as with their environment.[56]
An aerial view of a human ecosystem. Pictured is the city of Chicago
Every species has limits of tolerance to factors that affect its survival, reproductive success and ability to continue to thrive and interact sustainably with the rest of its environment, which in turn may have effects on these factors for many other species or even on the whole of life.[57] The concept of an ecosystem is thus an important subject of study, as such study provides information needed to make decisions about how human life may interact in a way that allows the various ecosystems to be sustained for future use rather than used up or otherwise rendered ineffective. For the purpose of such study, a unit of smaller size is called a microecosystem. For example, an ecosystem can be a stone and all the life under it. A macroecosystem might involve a whole ecoregion, with its drainage basin.
Plants and animals
The distinction between plant and animal life is not sharply drawn, with some categories of life that stand between or across the two. Originally Aristotle divided all living things between plants, which generally do not move, and animals. In Linnaeus' system, these became the kingdoms Vegetabilia (later Plantae) and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts. Bacterial life is sometimes included in flora,[52][53] and some classifications use the term bacterial flora separately from plant flora.
Among the many ways of classifying plants are by regional floras, which, depending on the purpose of study, can also include fossil flora, remnants of plant life from a previous era. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and terrain.
Regional floras commonly are divided into categories such as native flora and agricultural and garden flora, the latter of which are intentionally grown and cultivated. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced. This is an example of how human interaction with nature can blur the boundary of what is considered nature.
Another category of plant has historically been carved out for weeds. Though the term has fallen into disfavor among botanists as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature. Similarly, animals are often categorized in ways such as domestic, farm animals, wild animals, pests, etc. according to their relationship to human life.
Among the many ways of classifying plants are by regional floras, which, depending on the purpose of study, can also include fossil flora, remnants of plant life from a previous era. People in many regions and countries take great pride in their individual arrays of characteristic flora, which can vary widely across the globe due to differences in climate and terrain.
Regional floras commonly are divided into categories such as native flora and agricultural and garden flora, the latter of which are intentionally grown and cultivated. Some types of "native flora" actually have been introduced centuries ago by people migrating from one region or continent to another, and become an integral part of the native, or natural flora of the place to which they were introduced. This is an example of how human interaction with nature can blur the boundary of what is considered nature.
Another category of plant has historically been carved out for weeds. Though the term has fallen into disfavor among botanists as a formal way to categorize "useless" plants, the informal use of the word "weeds" to describe those plants that are deemed worthy of elimination is illustrative of the general tendency of people and societies to seek to alter or shape the course of nature. Similarly, animals are often categorized in ways such as domestic, farm animals, wild animals, pests, etc. according to their relationship to human life.
Human interrelationship
Although humans currently comprise only about one-half of one percent of the total living biomass on Earth,[61] the human effect on nature is disproportionately large. Because of the extent of human influence, the boundaries between what we regard as nature and "made environments" is not clear cut except at the extremes. Even at the extremes, the amount of natural environment that is free of discernible human influence is presently diminishing at an increasingly rapid pace, or, according to some, has already disappeared.
The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human civilization remains closely linked to changes in the environment. There exists a highly complex feedback-loop between the use of advanced technology and changes to the environment that are only slowly becoming understood.[62] Manmade threats to the Earth's natural environment include pollution, deforestation, and disasters such as oil spills. Humans have contributed to the extinction of many plants and animals.
Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains the primary component of the world's economic system. Some activities, such as hunting and fishing, are used for both sustenance and leisure, often by different people. Agriculture was first adopted around the 9th millennium BCE. Ranging from food production to energy, nature influences economic wealth.
Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing,[63] most modern human use of plants is through agriculture. The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion.
The development of technology by the human race has allowed the greater exploitation of natural resources and has helped to alleviate some of the risk from natural hazards. In spite of this progress, however, the fate of human civilization remains closely linked to changes in the environment. There exists a highly complex feedback-loop between the use of advanced technology and changes to the environment that are only slowly becoming understood.[62] Manmade threats to the Earth's natural environment include pollution, deforestation, and disasters such as oil spills. Humans have contributed to the extinction of many plants and animals.
Humans employ nature for both leisure and economic activities. The acquisition of natural resources for industrial use remains the primary component of the world's economic system. Some activities, such as hunting and fishing, are used for both sustenance and leisure, often by different people. Agriculture was first adopted around the 9th millennium BCE. Ranging from food production to energy, nature influences economic wealth.
Although early humans gathered uncultivated plant materials for food and employed the medicinal properties of vegetation for healing,[63] most modern human use of plants is through agriculture. The clearance of large tracts of land for crop growth has led to a significant reduction in the amount available of forestation and wetlands, resulting in the loss of habitat for many plant and animal species as well as increased erosion.
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