Fossil formations like the Burgess Shale show that evolutioncannot always be thought of as a slow progression. The Cambrian explosioninvolved rapid evolutionary diversification, followed by the extinction of manyunique animals. Why was this evolution so rapid? No one really knows. Manyzoologists believe that it was because so many ecological niches were availablewith virtually no competition from existing species. Will zoologists ever knowthe evolutionary sequences in the Cambrian explosion? Perhaps another ancientfossil bed of soft-bodied animals from 600-million-year-old seas is awaitingdiscovery.

　　Paragraph1: The geologic timescale is marked bysignificant geologic and biological events, including the origin of Earth about4.6 billion years ago, the origin of life about 3.5 billion years ago, theorigin of eukaryotic life-forms (living things that have cells with true nuclei)about 1.5 billion years ago, and the origin of animals about 0.6 billion yearsago. The last event marks the beginning of the Cambrian period. Animalsoriginated relatively late in the history of Earth – in only the last 10 percentof Earth’s history. During a geologically brief 100-million-year period, allmodern animal groups (along with other animals that are now extinct) evolved.This rapid origin and diversification of animals is often referred to as “theCambrian explosion。

1. The word significant in the passage is closest inmeaning to

　　numerous

　　important

　　unexplained

　　sudden

　　2. Theword relatively in the passage is closest in meaningto

　　surprisingly

　　collectively

　　comparatively

　　characteristically

　　3.The word diversification in the passage is closest in meaning to

　　emergenceof many varieties

　　steady decline in number

　　gradual increase in bodysize

　　sudden disappearance

　　Paragraph2: Scientists have asked importantquestions about this explosion for more than a century. Why did it occur so latein the history of Earth? The origin of multicellular forms of life seems arelatively simple step compared to the origin of life itself. Why does thefossil record not document the series of evolutionary changes during theevolution of animals? Why did animal life evolve so quickly? Paleontologistscontinue to search the fossil record for answers to these questions.

　　4.The period discussed in the passage is referred to as an explosion becauseit

　　occurred 0.6 billion years ago, late in Earth’s history

　　wascharacterized by the unusually fast evolution of many new life-forms.

　　Wascharacterized by widespread animal extinction

　　Was characterized by violentvolcanic eruptions

　　5. According to Paragraph2, which of the following isNOT a question that paleontologists asked about the Cambrian explosion?

　　Whywas the origin of life a simple step in Earth’s history?

　　Why did it take solong for multicellular organisms to develop?

　　Why did animal life evolve sorapidly?

　　Why does the fossil record lack evidence of animal evolution duringthat time?