what is life
WHAT IS LIFE?
Life resists a simple, one-sentence definition. We can recognize life without defining it, and we recognize life by what living things do. According to Margulis and Sagan (1995: 14):
The question “What is life?” is thus a linguistic trap. To answer according to the rules of grammar, we must supply a noun, a thing. But life on Earth is more like a verb. It repairs, maintains, recreates itself...”
PROPERTIES OF LIFE
A high degree of Order and Complexity. Organisms are, as the name suggests: “organized.”
Diversity and Uniformity.
Reproduction: Life comes only from life—cells from cells—an axiom known as biogenesis.
Growth and development:
Energy utilization: organisms take in energy and transform it to do many kinds of work.
Metabolism: featuring a uniform set of carbon-based macromolecules including: Enzymes—RNA/DNA—Phospholipids in membranes—Sugars—ATP
Responsiveness to the environment:
Homeostasis: regulatory mechanisms maintain an organism’s internal environment within tolerable limits, even though the external environment may fluctuate.
Evolutionary adaptation: Life evolves as a result of the interaction between organisms and their environments.
THE HIERARCHY OF STRUCTURAL LEVELS
Biological organization is based on a hierarchy of structural levels, each level building on the levels below it. Atoms, the chemical building blocks of all matter, are ordered into complex biological molecules such as proteins. The molecules of life are arranged into minute structures called organelles, which are in turn the components of cells. Some organisms consist of single cells, but others, including plants and animals, are aggregates of many specialized types of cells. In such multicellular organisms, similar cells are grouped into tissues, and specific arrangements of different tissues form organs. In the hierarchy of biological organization, there are tiers beyond the individual organism.
A population is a localized group of organisms belonging to the same species; populations of species living in the same area make up a biological community; and community interactions that include nonliving features of the environment, such as soil and water, form an ecosystem. Biomes are whole geographical regions with similar climates, like arctic tundra and tropical rain forests. The biosphere (“Gaia”) is the sum of all Earth’s biomes. A biosphere differs from an organism in that it is essentially closed to influx and egress of materials.
Biological processes transcend this hierarchy, with causes and effects at several organizational levels. A narrow focus on a single level of biological organization depreciates the fun and the power of biology. Each level of biological organization has emergent properties With each step upward in the hierarchy of biological order, novel properties emerge that were not present at the simpler levels of organization. These emergent properties result form interactions between components. A molecule such as a protein has attributes not exhibited by any of its component atoms, and a cell is certainly much more than a bag of molecules. An organism is a living, synergistic whole: greater than the sum of its parts.
HOLISM VS. REDUCTIONISM
A trained biologist must appreciate both aspects simultaneously. Reductionism is reducing complex systems to simpler components that are more manageable to study—a powerful strategy in biology and science in general.
Life resists a simple, one-sentence definition. We can recognize life without defining it, and we recognize life by what living things do. According to Margulis and Sagan (1995: 14):
The question “What is life?” is thus a linguistic trap. To answer according to the rules of grammar, we must supply a noun, a thing. But life on Earth is more like a verb. It repairs, maintains, recreates itself...”
ENTROPY
Life conforms to all the chemical and physical properties of matter, but with a unique twist. The generally entropic universe—which has tended towards increased disorder and simplicity, ever since the big bang—is pocked by rare, local regions of intense ordering. This ordering is life.
AUTOPOESIS
Life changes to stay the same. Autopoesis is an elegant term introduced by Chilean biologists, Matrurana and Varela which refers to life’s continuous production of itself. Here are soms observations from Margulis and Sagan (1995: 17-18):
Every five days you get a new stomach lining. You get a new liver every two months. Your skin replaces itself every six weeks. Every year, ninety-eight percent of the atoms of your body are replaced...
An autopoetic entity metabolizes continuously... Life entails energy expenditure and the making messes..
Replication is not nearly as fundamental a characteristic of life as is autopoesis. Consider: the mule, offspring of a donkey and a horse, cannot “replicate.” It is sterile, but it metabolizes with as much vigor as either of its parents...
DEATH
Bacteria grow and divide and they can be killed; but they do not age or naturally die. Aging and dying is a programmed internal process. For Margulis and Sagan (1995: 137) It:
arose in our microbial ancestors at some time during the evolution of sexual individuals. Strange to say, death itself, evolved. Indeed, it was the first—and is still—the most serious sexually transmitted “disease.”
CELLS VS ORGANISMS
While cells are the units of organisms, it is organisms that are the units of life. It’s an important distinction. A single-celled organism such as an Amoeba is analogous not to one of your cells, but to your whole body. Unlike the Amoeba, none of your cells could live for long on its own. The organism we recognize as an animal or plant is not a collection of unicells, but a multicellular cooperative with the emergent properties of “whole organism.”
Margulis and Sagan (1995: 18) remind us that “The tiniest bacteria (about one ten-millionth of a meter in diameter) are the minimal autopoetic units known today.” Furthermore (1995: 137),
Minimal life on earth today is a system, a minute membrane-bound sphere, a bacterial cell, requiring many interacting molecules. Some 500 to 5,000 genes make a similar number of proteins.
VIRUSES
Viruses are not alive. They must utilize the cytoplasmic architecture and metabolic pathways of a host cell.Again, here is what Margulis and Sagan (1995: 18) have to say about them:
[V]iruses... are too small and to simple self-maintain, they do not metabolize. Viruses do nothing until they enter a autopoietic entity, a bacterial cell, the cell of an animal, or of another live organism. Viruses must utilize the cytoplasmic architecture and metabolic pathways of a host cell.
LIFE EVOLVES…
Just as an individual has a family history, each species is one tip on a branching tree of life extending back in time through ancestral species more and more remote. Species that are very similar, such as the horse and zebra, share a common ancestor that represents a relatively recent branch point on the tree of life. Trace evolution back far enough, and there are only the primeval prokaryotes that inhabited Earth more than three billion years ago. All of life is connected.
Margulis, Lynn and Sagan, Dorion (1995) What is Life? Simon and Schuster, New York.
THE ESSENTIAL QUESTIONS OF BIOLOGY:
How is it built?
How does it work?
How did it evolve?
SLIPPERY QUESTIONS ABOUT THE PROBLEM OF DEFINING LIFE
1. Why does life resist a simple, one-sentence definition?
2. DNA replicates and so do viruses. Why are they not alive?
3. Flames, whirlpools and tornadoes are dissipative structures. They are recognizable, dynamic organized units of matter, formed by energy flow. What are the similarities and differences between these dissipative structures and living entities?
4. Copper sulphate crystals can be grown in from seed crystals in a beaker. They reproduce but they are classified as inorganic, non-living matter. Why?
5. Trains move, use oxygen and fuel, and produce wastes and heat. Which of the characteristics of life are satisfied by trains and other fuel burning machines? Which characteristics categorically do not apply?
Life resists a simple, one-sentence definition. We can recognize life without defining it, and we recognize life by what living things do. According to Margulis and Sagan (1995: 14):
The question “What is life?” is thus a linguistic trap. To answer according to the rules of grammar, we must supply a noun, a thing. But life on Earth is more like a verb. It repairs, maintains, recreates itself...”
PROPERTIES OF LIFE
A high degree of Order and Complexity. Organisms are, as the name suggests: “organized.”
Diversity and Uniformity.
Reproduction: Life comes only from life—cells from cells—an axiom known as biogenesis.
Growth and development:
Energy utilization: organisms take in energy and transform it to do many kinds of work.
Metabolism: featuring a uniform set of carbon-based macromolecules including: Enzymes—RNA/DNA—Phospholipids in membranes—Sugars—ATP
Responsiveness to the environment:
Homeostasis: regulatory mechanisms maintain an organism’s internal environment within tolerable limits, even though the external environment may fluctuate.
Evolutionary adaptation: Life evolves as a result of the interaction between organisms and their environments.
THE HIERARCHY OF STRUCTURAL LEVELS
Biological organization is based on a hierarchy of structural levels, each level building on the levels below it. Atoms, the chemical building blocks of all matter, are ordered into complex biological molecules such as proteins. The molecules of life are arranged into minute structures called organelles, which are in turn the components of cells. Some organisms consist of single cells, but others, including plants and animals, are aggregates of many specialized types of cells. In such multicellular organisms, similar cells are grouped into tissues, and specific arrangements of different tissues form organs. In the hierarchy of biological organization, there are tiers beyond the individual organism.
A population is a localized group of organisms belonging to the same species; populations of species living in the same area make up a biological community; and community interactions that include nonliving features of the environment, such as soil and water, form an ecosystem. Biomes are whole geographical regions with similar climates, like arctic tundra and tropical rain forests. The biosphere (“Gaia”) is the sum of all Earth’s biomes. A biosphere differs from an organism in that it is essentially closed to influx and egress of materials.
Biological processes transcend this hierarchy, with causes and effects at several organizational levels. A narrow focus on a single level of biological organization depreciates the fun and the power of biology. Each level of biological organization has emergent properties With each step upward in the hierarchy of biological order, novel properties emerge that were not present at the simpler levels of organization. These emergent properties result form interactions between components. A molecule such as a protein has attributes not exhibited by any of its component atoms, and a cell is certainly much more than a bag of molecules. An organism is a living, synergistic whole: greater than the sum of its parts.
HOLISM VS. REDUCTIONISM
A trained biologist must appreciate both aspects simultaneously. Reductionism is reducing complex systems to simpler components that are more manageable to study—a powerful strategy in biology and science in general.
Life resists a simple, one-sentence definition. We can recognize life without defining it, and we recognize life by what living things do. According to Margulis and Sagan (1995: 14):
The question “What is life?” is thus a linguistic trap. To answer according to the rules of grammar, we must supply a noun, a thing. But life on Earth is more like a verb. It repairs, maintains, recreates itself...”
ENTROPY
Life conforms to all the chemical and physical properties of matter, but with a unique twist. The generally entropic universe—which has tended towards increased disorder and simplicity, ever since the big bang—is pocked by rare, local regions of intense ordering. This ordering is life.
AUTOPOESIS
Life changes to stay the same. Autopoesis is an elegant term introduced by Chilean biologists, Matrurana and Varela which refers to life’s continuous production of itself. Here are soms observations from Margulis and Sagan (1995: 17-18):
Every five days you get a new stomach lining. You get a new liver every two months. Your skin replaces itself every six weeks. Every year, ninety-eight percent of the atoms of your body are replaced...
An autopoetic entity metabolizes continuously... Life entails energy expenditure and the making messes..
Replication is not nearly as fundamental a characteristic of life as is autopoesis. Consider: the mule, offspring of a donkey and a horse, cannot “replicate.” It is sterile, but it metabolizes with as much vigor as either of its parents...
DEATH
Bacteria grow and divide and they can be killed; but they do not age or naturally die. Aging and dying is a programmed internal process. For Margulis and Sagan (1995: 137) It:
arose in our microbial ancestors at some time during the evolution of sexual individuals. Strange to say, death itself, evolved. Indeed, it was the first—and is still—the most serious sexually transmitted “disease.”
CELLS VS ORGANISMS
While cells are the units of organisms, it is organisms that are the units of life. It’s an important distinction. A single-celled organism such as an Amoeba is analogous not to one of your cells, but to your whole body. Unlike the Amoeba, none of your cells could live for long on its own. The organism we recognize as an animal or plant is not a collection of unicells, but a multicellular cooperative with the emergent properties of “whole organism.”
Margulis and Sagan (1995: 18) remind us that “The tiniest bacteria (about one ten-millionth of a meter in diameter) are the minimal autopoetic units known today.” Furthermore (1995: 137),
Minimal life on earth today is a system, a minute membrane-bound sphere, a bacterial cell, requiring many interacting molecules. Some 500 to 5,000 genes make a similar number of proteins.
VIRUSES
Viruses are not alive. They must utilize the cytoplasmic architecture and metabolic pathways of a host cell.Again, here is what Margulis and Sagan (1995: 18) have to say about them:
[V]iruses... are too small and to simple self-maintain, they do not metabolize. Viruses do nothing until they enter a autopoietic entity, a bacterial cell, the cell of an animal, or of another live organism. Viruses must utilize the cytoplasmic architecture and metabolic pathways of a host cell.
LIFE EVOLVES…
Just as an individual has a family history, each species is one tip on a branching tree of life extending back in time through ancestral species more and more remote. Species that are very similar, such as the horse and zebra, share a common ancestor that represents a relatively recent branch point on the tree of life. Trace evolution back far enough, and there are only the primeval prokaryotes that inhabited Earth more than three billion years ago. All of life is connected.
Margulis, Lynn and Sagan, Dorion (1995) What is Life? Simon and Schuster, New York.
THE ESSENTIAL QUESTIONS OF BIOLOGY:
How is it built?
How does it work?
How did it evolve?
SLIPPERY QUESTIONS ABOUT THE PROBLEM OF DEFINING LIFE
1. Why does life resist a simple, one-sentence definition?
2. DNA replicates and so do viruses. Why are they not alive?
3. Flames, whirlpools and tornadoes are dissipative structures. They are recognizable, dynamic organized units of matter, formed by energy flow. What are the similarities and differences between these dissipative structures and living entities?
4. Copper sulphate crystals can be grown in from seed crystals in a beaker. They reproduce but they are classified as inorganic, non-living matter. Why?
5. Trains move, use oxygen and fuel, and produce wastes and heat. Which of the characteristics of life are satisfied by trains and other fuel burning machines? Which characteristics categorically do not apply?
