Post by Admin on Mar 15, 2024 22:39:01 GMT
How did life on Earth begin? Here are 3 popular theories.
For millions of years, Earth was a hot and rocky planet. Scientists have some hypotheses on how life emerged amid such inhospitable terrain.
www.nationalgeographic.com/science/article/early-life-earth-theories
Earth formed roughly 4.6 billion years ago, and for several hundred million years the planet’s surface was almost certainly too hot and heavily bombarded by comets and asteroids to be hospitable to any kind of lifeform. About a billion years later, life not only existed, but was also leaving evidence of its presence in the form of fossilized microbial mats.
So, what happened in the interim? How did life spring from non-life over the course of a half billion years or so?
1. Sparked by lightning
Atmospheric conditions at the time life appeared were very different from those that exist now, notes Jim Cleaves, chair of the chemistry department at Howard University and co-author of A Brief History of Creation: Science and the Search for the Origin of Life.
In the 1950s, he explains, Nobel Prize-winning chemist Harold Urey noted that most atmospheres in the solar system are dominated by nitrogen and methane; Urey reasoned that early Earth also had this type of atmosphere, and that the presence of life transformed it into one richer in oxygen. Urey also proposed that this earlier atmosphere “could be very efficient at making organic compounds, which could be the precursor to life,” Cleaves explains.
He tasked his research student Stanley Miller with developing an experiment to test this theory. What would become known as the Miller-Urey experiment created a closed system, in which water was heated and combined with molecules of hydrogen, methane, and ammonia. These were then zapped with electricity (to represent lightning) and cooled to allow the mixture to condense and fall back into the water, like rain.
The results were astonishing.
Within a week, the experimental “ocean” had turned reddish brown because of the molecules combining to create amino acids, the building blocks of life.
Subsequent research has shown that the planet’s early atmosphere was somewhat different from the experiment created by Miller, and that the main components were nitrogen and carbon dioxide, with hydrogen and methane present in smaller quantities.
The principles espoused by Miller remain broadly sound, however, with lightning combining with asteroid impacts and ultraviolet radiation from the Sun to create hydrogen cyanide, which then reacted with iron brought up by water from Earth’s crust to form chemicals such as sugars. These chemicals may have combined to create strands of ribonucleic acid, or RNA, a key component of life that stores information; at some point, RNA molecules began replicating themselves, and life was possible.
How did these RNA molecules develop into complex cellular structures surrounded by protective membranes?
The key may be coacervates—droplets that contain proteins and nucleic acids and which are able to bind their components together much as cells do, but without the use of membranes; several researchers have hypothesized that such droplets acted as protocells that concentrated early RNA and other organic compounds.
2. Brought to Earth by outer space
Amino acids, as well as some of the other key building blocks of life such as carbon and water, may have been brought to early Earth from outer space, according to another theory. Comets and meteorites have been found to contain some of the same organic building blocks of life, and their early bombardment of Earth may have increased the availability of amino acids.
According to Nobel Prize-winning chemist Jack Szostak of the University of Chicago, who heads the university’s interdisciplinary Origins of Life initiative, asteroid and comet impacts were almost certainly integral.
He notes that an early atmosphere of nitrogen and carbon dioxide would have been less conducive to some of the proposed chemical reactions that took place in Miller’s concoction of hydrogen, methane and ammonia; but, he explains, a moderate sized impact can create atmospheric hydrogen and methane on a transient basis, allowing for a temporary jolt of compound-creating conditions.
“It’s like having your cake and eating it,” he explains.
Rest in Link.
For millions of years, Earth was a hot and rocky planet. Scientists have some hypotheses on how life emerged amid such inhospitable terrain.
www.nationalgeographic.com/science/article/early-life-earth-theories
Earth formed roughly 4.6 billion years ago, and for several hundred million years the planet’s surface was almost certainly too hot and heavily bombarded by comets and asteroids to be hospitable to any kind of lifeform. About a billion years later, life not only existed, but was also leaving evidence of its presence in the form of fossilized microbial mats.
So, what happened in the interim? How did life spring from non-life over the course of a half billion years or so?
1. Sparked by lightning
Atmospheric conditions at the time life appeared were very different from those that exist now, notes Jim Cleaves, chair of the chemistry department at Howard University and co-author of A Brief History of Creation: Science and the Search for the Origin of Life.
In the 1950s, he explains, Nobel Prize-winning chemist Harold Urey noted that most atmospheres in the solar system are dominated by nitrogen and methane; Urey reasoned that early Earth also had this type of atmosphere, and that the presence of life transformed it into one richer in oxygen. Urey also proposed that this earlier atmosphere “could be very efficient at making organic compounds, which could be the precursor to life,” Cleaves explains.
He tasked his research student Stanley Miller with developing an experiment to test this theory. What would become known as the Miller-Urey experiment created a closed system, in which water was heated and combined with molecules of hydrogen, methane, and ammonia. These were then zapped with electricity (to represent lightning) and cooled to allow the mixture to condense and fall back into the water, like rain.
The results were astonishing.
Within a week, the experimental “ocean” had turned reddish brown because of the molecules combining to create amino acids, the building blocks of life.
Subsequent research has shown that the planet’s early atmosphere was somewhat different from the experiment created by Miller, and that the main components were nitrogen and carbon dioxide, with hydrogen and methane present in smaller quantities.
The principles espoused by Miller remain broadly sound, however, with lightning combining with asteroid impacts and ultraviolet radiation from the Sun to create hydrogen cyanide, which then reacted with iron brought up by water from Earth’s crust to form chemicals such as sugars. These chemicals may have combined to create strands of ribonucleic acid, or RNA, a key component of life that stores information; at some point, RNA molecules began replicating themselves, and life was possible.
How did these RNA molecules develop into complex cellular structures surrounded by protective membranes?
The key may be coacervates—droplets that contain proteins and nucleic acids and which are able to bind their components together much as cells do, but without the use of membranes; several researchers have hypothesized that such droplets acted as protocells that concentrated early RNA and other organic compounds.
2. Brought to Earth by outer space
Amino acids, as well as some of the other key building blocks of life such as carbon and water, may have been brought to early Earth from outer space, according to another theory. Comets and meteorites have been found to contain some of the same organic building blocks of life, and their early bombardment of Earth may have increased the availability of amino acids.
According to Nobel Prize-winning chemist Jack Szostak of the University of Chicago, who heads the university’s interdisciplinary Origins of Life initiative, asteroid and comet impacts were almost certainly integral.
He notes that an early atmosphere of nitrogen and carbon dioxide would have been less conducive to some of the proposed chemical reactions that took place in Miller’s concoction of hydrogen, methane and ammonia; but, he explains, a moderate sized impact can create atmospheric hydrogen and methane on a transient basis, allowing for a temporary jolt of compound-creating conditions.
“It’s like having your cake and eating it,” he explains.
Rest in Link.