Origins of Life on Earth

The first experimental demonstration that complex molecules could have evolved naturally from simpler ingredients found on the primitive Earth came in 1953.Scientists Harold Urey and Stanley Miller took a mixture of the materials thought to be present on Earth long ago-water,methane,carbon dioxide,and ammonia -and energized it by passing an electrical discharge through it, simulating the effects of lightning.After a few days,they found their mixture contained many of the same amino acids found today in all living things on Earth.About a decade later,scientists succeeded in constructing nucleotide bases,carbon-based molecules that are the building blocks of life as we know it,in similar fashion.

None of these experiments has ever produced a living organism, and the molecules created are far less complex than a strand of DNA,which contains the nucleic acids that carry genetic information in cells and are associated with the formation of proteins.Nevertheless,they demonstrate conclusively that “biological'”molecules can be synthesized by strictly nonbiological means,using raw materials available on the early Earth.More advanced experiments have fashioned protein-like blobs (tiny drops)that behave to some extent like true biological cells.They resist dissolution in water and tend to cluster into small droplets called microspheres — a little like oil globules floating on the surface of water.The walls of these laboratory-made droplets permit the inward passage of small molecules,which then combine within the droplet to produce more complex molecules too large to pass back out through the walls.As the droplets“grow,”they tend to “reproduce,”forming smaller droplets.

Can we consider these protein-like microspheres to be alive? Almost certainly not.The microspheres contain many of the basic ingredients needed to form life,but are not life itself.They lack the hereditary molecule DNA.However,they do have similarities to ancient cells found in the fossil record.Thus,while no actual living cells have yet been created from scratch,many biochemists feel that many key steps in chemical evolution-the chain of events leading from simple nonbiological molecules to the building blocks of life itself-have now been demonstrated.

Recently,a dissenting view has emerged.Some scientists argue that there may not have been sufficient energy available to power the chemical reactions in Earth’s primitive atmosphere,and there may not have been enough raw material for the reactions to occur at a significant rate in any case.Instead,they suggest,most of the organic material that combined to form the first living cells was produced in interstellar space and subsequently arrived on Earth in the form of interplanetary dust and meteors that did not burn up during their descent through the atmosphere.

To test this hypothesis,NASA researchers carried out their own version of the Urey-Miller experiment,exposing an icy mixture of water, methanol, ammonia, and carbon monoxide- representative of many interstellar grains-to ultraviolet radiation to simulate the energy from a nearby newborn star.When they later placed the irradiated ice in water and examined the results,they found that it had formed droplets,surrounded by membranes, containing complex organic molecules.No amino acids,proteins,or DNA were found in the mix,but the results,repeated numerous times,clearly show that even the harsh,cold vacuum of interstellar space can be a suitable medium for the formation of complex molecules and primitive cellular structures.

Interstellar molecular clouds are known to contain many complex molecules,and large amounts of organic material were detected on comets Halley and Hale-Bopp when they last visited the inner solar system.Many scientists think cometary impacts early in our planet’s history were responsible for most of Earth’s water.Perhaps those comets carried complex molecules too.In addition,a small fraction of meteorites that survive the plunge to Earth’s surface have been shown to contain organic campounds. Such discoveries strongly suggest that complex molecules can form in an interplanetary or interstellar environment and could have reached Earth’s surface unharmed after their fiery descent.The moderately large molecules found in meteorites and in interstellar clouds are our only evidence that chemical evolution has occurred elsewhere in the universe.However,most researchers regard this organic matter as prebiotic-matter that could eventually lead to life,but that has not yet done so.Nevertheless,the hypothesis that organic matter is constantly raining down on Earth from space in the form of interplanetary debris is quite plausible.

1

The first experimental demonstration that complex molecules could have evolved naturally from simpler ingredients found on the primitive Earth came in 1953.Scientists Harold Urey and Stanley Miller took a mixture of the materials thought to be present on Earth long ago-water,methane,carbon dioxide,and ammonia -and energized it by passing an electrical discharge through it, simulating the effects of lightning.After a few days,they found their mixture contained many of the same amino acids found today in all living things on Earth.About a decade later,scientists succeeded in constructing nucleotide bases,carbon-based molecules that are the building blocks of life as we know it,in similar fashion.

Paragraph 1 indicates that in their experiments in 1953,Urey and Miller were able to

• Acreate nucleotide bases,the building blocks of life
• Bcreate many of the amino acids present in living organisms today
• Cuse a laboratory method developed by earlier scientists to produce carbon-based molecules
• Ddiscover that lightning on primitive Earth provided as much natural energy as it does today

2

None of these experiments has ever produced a living organism, and the molecules created are far less complex than a strand of DNA,which contains the nucleic acids that carry genetic information in cells and are associated with the formation of proteins.Nevertheless,they demonstrate conclusively that “biological'”molecules can be synthesized by strictly nonbiological means,using raw materials available on the early Earth.More advanced experiments have fashioned protein-like blobs (tiny drops)that behave to some extent like true biological cells.They resist dissolution in water and tend to cluster into small droplets called microspheres — a little like oil globules floating on the surface of water.The walls of these laboratory-made droplets permit the inward passage of small molecules,which then combine within the droplet to produce more complex molecules too large to pass back out through the walls.As the droplets“grow,”they tend to “reproduce,”forming smaller droplets.

Paragraph 2 indicates that the protein-like blobs scientists created behave like biological cells in all of the following ways EXCEPT:

• AThey do not dissolve in water.
• BThey can absorb small oil globules floating on the surface of the water.
• CThey allow small molecules to pass inside and combine.
• DAs they get larger,they produce smaller versions of themselves.

3

Can we consider these protein-like microspheres to be alive? Almost certainly not.The microspheres contain many of the basic ingredients needed to form life,but are not life itself.They lack the hereditary molecule DNA.However,they do have similarities to ancient cells found in the fossil record.Thus,while no actual living cells have yet been created from scratch,many biochemists feel that many key steps in chemical evolution-the chain of events leading from simple nonbiological molecules to the building blocks of life itself-have now been demonstrated.

Why does the author mention that the microspheres“lack the hereditary molecule DNA“?

• ATo provide an example of how the microspheres are similar to ancient cells found in the fossil record
• BTo argue against the idea that the building blocks of life could be generated from nonbiological molecules
• CTo support the claim that the microspheres are not alive
• DTo emphasize the idea that more research is needed to fully understand chemical evolution

4

Recently,a dissenting view has emerged.Some scientists argue that there may not have been sufficient energy available to power the chemical reactions in Earth’s primitive atmosphere,and there may not have been enough raw material for the reactions to occur at a significant rate in any case.Instead,they suggest,most of the organic material that combined to form the first living cells was produced in interstellar space and subsequently arrived on Earth in the form of interplanetary dust and meteors that did not burn up during their descent through the atmosphere.

The word“dissenting“in the passage is closest in meaning to

• Amore detailed
• Bnew
• Cintriguing
• DOpposing

5

Recently,a dissenting view has emerged.Some scientists argue that there may not have been sufficient energy available to power the chemical reactions in Earth’s primitive atmosphere,and there may not have been enough raw material for the reactions to occur at a significant rate in any case.Instead,they suggest,most of the organic material that combined to form the first living cells was produced in interstellar space and subsequently arrived on Earth in the form of interplanetary dust and meteors that did not burn up during their descent through the atmosphere.

According to paragraph 4,some scientists believe which of the following about the organic molecules that formed the first living cells?

• AThey were mostly the result of chemical reactions present in Earth’s primitive atmosphere.
• BThey were mostly produced using a combination of organic molecules present on Earth and inorganic material available in interplanetary space.
• CThey did not require as much energy as was once thought to generate the chemical reactions needed to form living cells.
• DThey were for the most part brought to Earth by interplanetary dust and meteors.

6

To test this hypothesis,NASA researchers carried out their own version of the Urey-Miller experiment,exposing an icy mixture of water,methanol,ammonia,and carbon monoxide- representative of many interstellar grains-to ultraviolet radiation to simulate the energy from a nearby newborn star.When they later placed the irradiated ice in water and examined the results,they found that it had formed droplets,surrounded by membranes, containing complex organic molecules.No amino acids,proteins,or DNA were found in the mix,but the results,repeated numerous times,clearly show that even the harsh,cold vacuum of interstellar space can be a suitable medium for the formation of complex molecules and primitive cellular structures.

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?Incorrect choices change the meaning in important ways or leave out essential information.

• ATo test this hypothesis,NASA researchers exposed a combination of materials common in space to ultraviolet radiation to create the effect of the energy of a newborn star.
• BNASA researchers carried out their own version of the Urey-Miller experiment to create an icy mixture of water,methanol, ammonia,and carbon monoxide.
• CTo test this hypothesis about the Urey-Miller experiment,NASA researchers exposed many interstellar grains to ultraviolet radiation from a nearby newborn star.
• DNASA researchers tested Urey and Miller’s hypothesis by exposing a mixture of interstellar grains to ultraviolet radiation.

7

Interstellar molecular clouds are known to contain many complex molecules,and large amounts of organic material were detected on comets Halley and Hale-Bopp when they last visited the inner solar system.Many scientists think cometary impacts early in our planet’s history were responsible for most of Earth’s water.Perhaps those comets carried complex molecules too.In addition,a small fraction of meteorites that survive the plunge to Earth’s surface have been shown to contain organic campounds. Such discoveries strongly suggest that complex molecules can form in an interplanetary or interstellar environment and could have reached Earth’s surface unharmed after their fiery descent.The moderately large molecules found in meteorites and in interstellar clouds are our only evidence that chemical evolution has occurred elsewhere in the universe.However,most researchers regard this organic matter as prebiotic-matter that could eventually lead to life,but that has not yet done so.Nevertheless,the hypothesis that organic matter is constantly raining down on Earth from space in the form of interplanetary debris is quite plausible.

The word“plausible“in the passage is closest in meaning to

• Apopular
• Bimportant
• Cattracive
• Dbelievable

8

Interstellar molecular clouds are known to contain many complex molecules,and large amounts of organic material were detected on comets Halley and Hale-Bopp when they last visited the inner solar system.Many scientists think cometary impacts early in our planet’s history were responsible for most of Earth’s water.Perhaps those comets carried complex molecules too.In addition,a small fraction of meteorites that survive the plunge to Earth’s surface have been shown to contain organic campounds. Such discoveries strongly suggest that complex molecules can form in an interplanetary or interstellar environment and could have reached Earth’s surface unharmed after their fiery descent.The moderately large molecules found in meteorites and in interstellar clouds are our only evidence that chemical evolution has occurred elsewhere in the universe.However,most researchers regard this organic matter as prebiotic-matter that could eventually lead to life,but that has not yet done so.Nevertheless,the hypothesis that organic matter is constantly raining down on Earth from space in the form of interplanetary debris is quite plausible.

Which of the following can be inferred from paragraph 6 about any complex molecules from space that reached early Earth?

• AThese molecules could have been involved in the beginning stages of the eventual development of the first living cells on Earth.
• BThese molecules were structurally very different from those currently thought to be present in interstellar clouds.
• CThese molecules were much larger than those detected on comets such as Halley and Hale-Bopp.
• DThese molecules required a medium very rich in water to be able to survive the descent to Earth.

9

Look at the four squaresthat indicate where the following sentence could be added to the passage

The innovative experimental design had clearly resulted in a groundbreaking moment in research, and other studies followed its lead.

Where would the sentence best fit？Click on a square  sentence to the passage.

10

There are various theories about the origins of life on Earth.

• AUrey and Miller re-created the same carbon-based molecules present on Earth long ago by combining water, methane, carbon dioxide, and ammonia with a mixture of amino acids.
• BSome scientists think much of early Earth’s prebiotic organic material originated in space because Earth did not have enough energy or necessary materials to produce an abundance of organic molecules.
• CScientists think that comets Halley and Hale-Bopp provide evidence suggesting that the inner solar system is more favorable to chemical evolution than other places in the universe.
• DScientists created amino acids,nucleotide bases,and protein-like blobs under conditions similar to those found on primitive Earth, perhaps re-creating some of the key steps in chemical evolution.
• EScientists have created organic molecules in conditions similar to those of interstellar space,and organic molecules have been found in comets,interstellar molecular clouds,and meteorites.
• FSome scientists believe that radiation from the Sun as a relatively young star may have chemically altered particles in Earth’s atmosphere,creating complex molecules that rained down on Earth.

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