The First Eyes

Putting a date on the first appearance of eyes depends on what one means by eye. If the term refers to a multicellular organ, even if it has just a few cells, then by definition, eyes could not form before there were multicellular animals. But many protists (animal-like, plantlike, or fungus-like unicellular organisms that require a water-based environment) can detect light by using aggregations of pigment molecules, and they use this information to modify their metabolic activity or motility  (the ability to move spontaneously and independently). One of the familiar living examples, probably known to anyone who has taken a biology class, is the aquatic protozoan Euglena, which has an eyespot near its motile flagellum (hair-like structure). Some living protists are very like their ancestral forms embedded in ancient sedimentary rocks, and this similarity 

suggests that the ability to detect light and modify behavior in response to light has been around for a very long time. Animals arose from one of such unicellular creatures, perhaps from one already specialized for a primitive kind of vision.

 

 

An eye is a collection of cells that are specialized for light detection through the presence of photosensitive pigment as well as a means of restricting the direction of incoming light that will strike the photosensitive cells. This definition says nothing about image formation, lenses, eye movements, or any of the other features we associate with our own eyes, but it does recognize the simplest form of functional and anatomical specialization, namely, detection of light. Everything else can be built up from this simple beginning, and some animals appear to have had eyes almost from the beginning of the animal kingdom.

 

 

Animals were scarce 600 million years ago in the geological era called the Precambrian. There are very few fossil remains from that time (though more keep turning up), and most evidence of the presence of animals is indirect, such as small tunnels in rock that could be ancient worm burrowings. But just 50 million years or so later, fossilized bits and pieces of animals abound, suggesting that a great burst of evolutionary creativity occurred in the 50-million-year interval. This surge of new life, marked by an abundance of animals, is called the Cambrian explosion.

 

 

The first direct evidence for the early origin of eyes comes from fossils that are about 530 million years old, a time shortly after the Cambrian explosion; they were found on a mountainside in British Columbia in a deposit known as the Burgess Shale. The Burgess Shale fossils are extraordinarily important because among them are remains of soft-bodied creatures, many of them lacking shells and other hard parts that fossilize easily. Consequently, their preservation is little short of miraculous (as are the delicate methods used to reconstruct three-dimensional structure from these flattened fossils), and 

they are one of the few known repositories of early soft-bodied animals.

 

 

Not all of the Burgess animals had eyes. However, some did. (Gross features location, size, and hemispheric shape are responsible for the designation of some structures as eyes). The reconstructed eyes of these Burgess animals look superficially like eyes of some living crustaceans, particularly those of shrimp and crabs whose eyes are mounted on stalks that improve the range of vision by raising the eyes above the surface of the head. The eyes of some Burgess organisms sat on stalks; those of others were on or a part of the body surface. One animal, Opabinia, had five eyes: two lateral pairs and 

a single medial eye; at least one of the lateral pairs had stalks that could have been movable. And some trilobite-like animals in the Burgess Shale had faceted eyes much like those of later fossil trilobites.

 

 

Although the presence of eyes on some of the Burgess animals indicates that eyes have been around for a very long time, it is unlikely that these were the first eyes; they seem much too large and (potentially) well developed to be brand new inventions. The best we can do is put the origin of eyes somewhere between the beginning of the Cambrian explosion, about 600 million years ago, and the death of the Burgess animals, some 530 million years ago.

 

【Paragraph 1】

Putting a date on the first appearance of eyes depends on what one means by eye. If the term refers to a multicellular organ, even if it has just a few cells, then by definition, eyes could not form before there were multicellular animals. But many protists (animal-like, plantlike, or fungus-like unicellular organisms that require a water-based environment) can detect light by using aggregations of pigment molecules, and they use this information to modify their metabolic activity or motility (the ability to move spontaneously and independently). One of the familiar living examples, probably known to anyone who has taken a biology class, is the aquatic protozoan Euglena, which has an eyespot near its motile flagellum (hair-like structure). Some living protists are very like their ancestral forms embedded in ancient sedimentary rocks, and this similarity suggests that the ability to detect light and modify behavior in response to light has

been around for a very long time. Animals arose from one of such unicellular creatures, perhaps from one already specialized for a primitive kind of vision.

 

1. Paragraph 1 supports all of the following statements about protists EXCEPT:

A. Some are multicellular.

B. Some are able to move.

C. Some have pigment molecules.

D. They live in environments that contain moisture.

 

2. According to paragraph 1, what have scientists concluded from the fact that some living protists are very like their ancestral forms？

A. The eye did not evolve until multicellular organisms arose.

B. The ability to detect light and change behavior in response to light has existed for a long time.

C. The ancestral forms of these living protists likely had an eyespot near the motile flflagellum.

D. The ancestral forms of these living protists depended primarily on light as the mechanism for modifying their metabolic activity or motility.

 

【Paragraph 2】

An eye is a collection of cells that are specialized for light detection through the presence of photosensitive pigment as well as a means of restricting the direction of incoming light that will strike the photosensitive cells. This definition says nothing about image formation, lenses, eye movements, or any of the other features we associate with our

own eyes, but it does recognize the simplest form of functional and anatomical specialization, namely, detection of light. Everything else can be built up from this simple beginning, and some animals appear to have had eyes almost from the beginning of the animal kingdom.

 

3. Paragraph 2 implies which of the following about the early eyes

A. They were able to detect simple movements almost from the beginning of their evolution.

B. They were not as sensitive to light as once thought.

C. They could not form images.

D. Their cells had more photosensitive pigment than do human eyes.

 

【Paragraph 3】

Animals were scarce 600 million years ago in the geological era called the Precambrian. There are very few fossil remains from that time (though more keep turning up), and most evidence of the presence of animals is indirect, such as small tunnels in rock that could be ancient worm burrowings. But just 50 million years or so later, fossilized bits and pieces of animals abound, suggesting that a great burst of evolutionary creativity occurred in the 50-million-year interval. This surge of new life, marked by an abundance of animals, is called the Cambrian explosion.

 

 

4. 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.

A. There are few fossils from the Precambrian, though more keep turning up.

B. Most evidence of animals in the fossil record is indirect and little of it is from the Precambrian.

C. Tunnels in Precambrian rocks that may have been made by worms provide indirect evidence of these animals existing

at that time.

D. There are very few fossils of animals from the Precambrian and most evidence of animal life from that period is indirect.

 

【Paragraph 4】

The first direct evidence for the early origin of eyes comes from fossils that are about 530 million years old, a time shortly after the Cambrian explosion; they were found on a mountainside in British Columbia in a deposit known as the Burgess Shale. The Burgess Shale fossils are extraordinarily important because among them are remains of soft

bodied creatures, many of them lacking shells and other hard parts that fossilize easily. Consequently, their preservation is little short of miraculous (as are the delicate methods used to reconstruct three-dimensional structure from these flattened fossils), and they are one of the few known repositories of early soft-bodied animals.

 

5. The phrase “little short of miraculous” is closest in meaning to

A. very highly valued

B. amazing because almost impossible

C. causing controversy

D. almost but not quite complete

 

6. According to paragraph 4, all of the following are true of the Burgess Shale EXCEPT

A. Its fossils were in a flattened condition when discovered.

B. Its fossils provide direct evidence about the origin of eyes.

C. It contains fossils of both Precambrian and Cambrian animals.

D. It contains fossilized remains of soft-bodied organisms.

 

【Paragraph 5】

Not all of the Burgess animals had eyes. However, some did. (Gross features location, size, and hemispheric shape are responsible for the designation of some structures as eyes). The reconstructed eyes of these Burgess animals look superficially like eyes of some living crustaceans, particularly those of shrimp and crabs whose eyes are mounted on stalks that improve the range of vision by raising the eyes above the surface of the head. The eyes of some Burgess organisms sat on stalks; those of others were on or a part of the body surface. One animal, Opabinia, had five eyes: two lateral pairs and

a single medial eye; at least one of the lateral pairs had stalks that could have been movable. And some trilobite-like animals in the Burgess Shale had faceted eyes much like those of later fossil trilobites.

 

7. Why does the author point out that the eyes of some Burgess organisms sat on stalks

A. To suggest that some Burgess organisms had a greater range of vision than do living shrimp and crabs

B. To explain why it is thought that one of the lateral pairs of eyes in Opabinia may have been movable

C. To explain why the eyes of some Burgess animals were not recognizable as such before they were reconstructed

D. To support the statement that the reconstructed eyes of Burgess animals look superfificially like the eyes of some living crustaceans

 

 

 

【Paragraph 6】

Although the presence of eyes on some of the Burgess animals indicates that eyes have been around for a very long time, it is unlikely that these were the first eyes; they seem much too large and (potentially) well developed to be brand new inventions. The best we can do is put the origin of eyes somewhere between the beginning of the Cambrian

explosion, about 600 million years ago, and the death of the Burgess animals, some 530 million years ago.

8. Paragraph 6 suggests that the first eyes probably

A. came into existence long before 600 million years ago

B. came into existence at a late point in the Cambrian period

C. existed before the animals of the Burgess Shale existed

D. were larger than those of animals found in the Burgess Shale

 

【Paragraph 5】

Not all of the Burgess animals had eyes. However, some did. (Gross features location, size, and hemispheric shape are responsible for the designation of some structures as eyes). ■The reconstructed eyes of these Burgess animals look superficially like eyes of some living crustaceans, particularly those of shrimp and crabs whose eyes are mounted on stalks that improve the range of vision by raising the eyes above the surface of the head. ■The eyes of some Burgess organisms sat on stalks; those of others were on or a part of the body surface. ■One animal, Opabinia, had five eyes: two lateral pairs and a

single medial eye; at least one of the lateral pairs had stalks that could have been movable. ■And some trilobite-like animals in the Burgess Shale had faceted eyes much like those of later fossil trilobites.

 

9. Look at the four squares【■】that indicate where the following sentence could be added to the passage.

Molaria spinifera and H. optata, both of which lived in water levels beyond the reach of light, fit into this category.

Where would the sentence best fifit？

 

 

10. 【Directions】 An introductory sentence for a brief summary of the passage is provided below. Complete the summary

by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong

in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

The first eyes originated between the beginning of the Cambrian explosion and the death of the Burgess animals.

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Answer Choices

A. The ability of some unicellular organisms to detect light and change their behavior accordingly suggests that eyes did not originate with multicellular animals.

B. The earliest eyes apparently contained molecules that were capable of forming and focusing images.

C. Too few fossils from the Precambrian have been found to determine which if any Precambrian organisms had eyes.

D. Evidence from the Burgess Shale suggests that eyes of some early animals were similar to the eyes of living crustaceans.

E. Fossil evidence suggests that organisms in the Burgess Shale with faceted eyes developed later than organisms in the Burgess Shale with nonfaceted eyes.

F. The large size and possible complexity of the eyes of some organisms in the Burgess Shale suggest that their eyes were not the fifirst eyes.

 

 

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