雅思阅读 118: Fossils Reset the Clock on Complex Animal Evolution

改编自 Scientific American 2026 年真实文章，去广告后按雅思阅读题型出题。

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Reading Passage

Around 540 million years ago, the ocean erupted with complex life. Creatures rapidly transformed from simple, soft-bodied, seafloor-dwelling organisms into animals with shells, cartilage, mouths, and the ability to swim, burrow or hunt. Scientists call this dramatic burst the Cambrian explosion. But the discovery of a trove of bizarre fossils in southern China is challenging that consensus.

Hundreds of fossils uncovered in Yunnan province reveal that at least some life-forms previously thought to have arisen during the Cambrian period were alive millions of years earlier, in an era known as the Ediacaran period. Many of the fossils look alien: wormlike creatures tethered to the ground, a sausage-shaped animal, and fingerlike organisms with tentacles. The findings were published in the journal Science.

The discovery was partly accidental. Researchers at Yunnan University were looking for algal fossils in the region's cliff faces when they stumbled upon the trove. More than 700 specimens from the Ediacaran period were recovered. Some were mere algae, but hundreds were animals in a variety of different forms.

The most common animal found was about the size of a human index finger. It had a wormlike body and a disk that kept it rooted to the seafloor. More than 100 of these unnamed creatures were found, suggesting they once densely populated the ocean floor. "It was unlike anything I've ever seen," says Frances Dunn, a senior researcher at the Oxford University Museum of Natural History and a co-author.

What was most surprising, Dunn says, was that so many of the fossils looked like they belonged to the Cambrian period rather than the Ediacaran. Some, including the abundant worm, were bilaterians. This term refers to animals with bilateral symmetry, meaning one side of the body mirrors the other. This critical evolutionary adaptation helped early life move through sediment or water, develop a nervous system, and eventually dominate the animal kingdom. Most animals today are bilaterians, including humans.

Before this discovery, scientists believed bilaterians primarily arose during the Cambrian and were rare in the Ediacaran. The new fossils offer a glimpse into a transitional world where simple, soft-bodied life was already experimenting with body plans that would later define the Cambrian explosion. The Cambrian explosion, it seems, may have been less explosive than previously thought. The stage for complex life may have been set much earlier than anyone realized.

The findings also raise questions about what environmental changes triggered the eventual burst of diversification. If these animals already existed 550 million years ago, what held them back for millions of years, and what finally allowed them to flourish?

Questions 1-5

Choose the correct letter, A, B, C or D.

1. What is the Cambrian explosion?

   • A. A mass extinction of sea creatures.
   • B. A rapid burst of complex animal evolution.
   • C. The arrival of humans on land.
   • D. The formation of Earth oceans.

2. Where were the new fossils found?

   • A. In northern China.
   • B. In Yunnan province, southern China.
   • C. In Oxford, England.
   • D. In the deep sea near Australia.

3. How many specimens were recovered?

   • A. About 100.
   • B. About 700.
   • C. About 50.
   • D. About 5,000.

4. What is a bilaterian?

   • A. An animal with no nervous system.
   • B. An animal with bilateral body symmetry.
   • C. A type of algae.
   • D. A soft-bodied seafloor organism.

5. What does the discovery suggest?

   • A. The Cambrian explosion was more explosive than thought.
   • B. Bilaterians appeared much earlier than previously believed.
   • C. Complex life began only 540 million years ago.
   • D. The Ediacaran period had no animal life.

Questions 6-8

Do the following statements agree with the information given in the passage? Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

6. The researchers were specifically searching for animal fossils when they found the trove.
7. The most common animal found was about the size of a human index finger.
8. Most animals alive today, including humans, are bilaterians.

Questions 9-13

Complete the summary below. Choose ONE WORD ONLY from the passage for each answer.

The Yunnan fossils suggest that bilaterian animals existed millions of years before the Cambrian. The most common creature was a wormlike organism rooted to the seafloor. The findings imply the Cambrian explosion was less explosive than scientists once believed.

Answers

1. B
2. B
3. B
4. B
5. B
6. FALSE
7. TRUE
8. TRUE
9. bilaterian
10. wormlike
11. explosive

Key Vocabulary

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