The First Eyes

The First EyesPutting a date on the first appearance of eyes depends on what one means byeye. 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 organismsthat require a water-based environment) can detect light byusing aggregations of pigment molecules, and they use this information tomodify their metabolic activity or motility (the ability to move spontaneouslyand independently). One of the familiar living examples, probably known toanyone who has taken a biology class, is theaquatic protozoan Euglena, whichhas an eyespot near its motile flagellum (hairlike structure). Some livingprotists are very like their ancestral forms embedded in ancient sedimentaryrocks, and this similarity suggests that the ability to detect light and modifybehavior in response to light has been around for a very long time. Animalsarose from one of such unicellular creatures, perhaps from one alreadyspecialized for a primitive kind of vision.The word “aggregations” in the passage is closest in meaning toA.PartsB.ReactionsC.GroupsD.TypesParagraph 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.According to paragraph 1, what have scientists concluded from the fact thatsome 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 hasexisted for a long time.C.The ancestral forms of these living protists likely had an eyespot nearthe motile flagellum.D.The ancestral forms of these living protists depended primarily onlight as the mechanism for modifying their metabolic activity or motility.An eye is a collection of cells that are specialized for light detection through thepresence of photosensitive pigment as well as a means of restricting thedirection of incoming light that will strike the photosensitive cells. This definitionsays nothing about image formation, lenses, eye movements, or any of theother features we associate with our own eyes, but it does recognize thesimplest form of functional and anatomical specialisation namely, detection oflight. Everything else can be built up from this simple beginning, and someanimals appear to have had eyes almost from the beginning of the animalkingdom.Paragraph 2 implies which of the following about the early eyesA.They were able to detect simple movements almost from thebeginning 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.Animals were scarce 600 million ye...