雅思阅读 56: The Cambrian Explosion: When Life Got Mobile（寒武纪生命大爆发：当生命学会运动）

改编自 Science Advances / University of Cambridge（2025-2026）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://www.cam.ac.uk/research/news/a-star-is-born-half-billion-year-old-fossil-rewrites-the-origin-story-of-starfish

Reading Passage

A. For most of Earth's history, animals as we know them simply did not exist. For nearly three billion years the planet was populated by single cells, and even after large, soft-bodied creatures finally appeared in the Ediacaran period, the ocean floor was, by all accounts, a strangely silent place: organisms lay where they had grown, grazing mats of algae in slow, immobile assemblages. Then, roughly 539 million years ago, something changed. In a geological eyeblink — perhaps as little as 20 million years — almost all of the major animal body plans that still populate the Earth first appear in the rocks: creatures with legs, eyes, shells, guts, hearts and brains. Palaeontologists call this event the Cambrian explosion, and it remains one of the most conspicuous unsolved puzzles in evolutionary biology. Darwin himself noted the absence of obvious ancestors in older rocks and fretted that it might undermine his theory. A century and a half later, new fossil discoveries and genomic comparisons are finally beginning to explain how a world of garden-like stillness became one of hunters, burrowers and escape artists.

B. The turning point is now being traced in unprecedented detail. In 2025, a team at the Nanjing Institute of Geology and Palaeochemistry reported what they call the oldest complex three-dimensional burrow system yet found, in the Shibantan biota near Yichang in Hubei, China. The traces date to the Ediacaran-Cambrian transition, about 539 million years ago, and record something the older rocks do not: animals actively moving through, rather than simply resting on, the sediment. In the Ediacaran seafloor, the researchers note, the bottom was covered by what they describe as an almost motionless "silent world"; the burrows at Shibantan are the fossilised footprints of creatures that had begun to mine the sediment from within, to chase prey, and to burrow away from predators. Behaviour, in other words, appears in the rock record before most of the familiar Cambrian skeletons do. The find, published in Science Advances, offers rare behavioural evidence for the moment the ecosystem was rewired.

C. Why did so much change so fast? A recent hypothesis argues that the trigger was not external — a meteorite, a sudden spike in oxygen — but biological, and internal. Single-celled ancestors, the argument runs, began to chase and eat one another, and the stress of failed predation — a prey escaping, a predator being damaged — exposed cells to oxidative and enzymatic stress. That stress triggered error-prone DNA repair and the activation of mobile genetic elements, which in turn produced chromosomal rearrangements, gene duplications and rewiring of the regulatory networks that govern development. Chief among those rewired networks are the Hox gene clusters, the genetic architects of segmentation and body-axis layout. Once those networks were flexible, evolution could experiment with new numbers of limbs, new arrangements of eyes, new shapes of mouth. The hypothesis elegantly links the rise of motility to the rise of morphological variety: an animal that can move to chase or escape lives in a world that rewards faster reactions, sharper senses and more specialised bodies.

D. The exceptional fossil sites where these early experiments are preserved give texture to the argument. The Burgess Shale in Canada continues to yield creatures whose body plans defy neat classification; in 2025, researchers described a new radiodont, Mosura fentoni, with up to 26 trunk segments — the most yet recorded for that group — despite being among the smallest known. Half a world away, in the Bright Angel Formation of the Grand Canyon, carbonaceous fossils dating to about 507 to 502 million years preserve suspension-feeding crustaceans, substrate-scraping molluscs and exotic worms alongside algae and cyanobacteria, evidence that the Cambrian food web was already functionally sophisticated. Not every experiment survived. The so-called Sinsk Event, an early Cambrian extinction, pruned some of these early lineages, and the recovery that followed reorganised which traits were successful. The picture that emerges is not a single sudden leap but a radiation interrupted, reshaped and then refined by ecological competition.

E. Why does any of this matter now? Because the Cambrian explosion is not a story about extinct oddities. The same fossil beds record the first appearance of the phylum that includes human beings, and the 2026 announcement of a half-billion-year-old starfish-like animal from the Cambrian, reported by Cambridge researchers in Current Biology, helps explain why two closely related lineages — echinoderms like starfish and chordates like us — ended up with such wildly different body plans. A growing body of work also argues that animal brains grew more complex in parallel with the explosion, as nervous systems became regionalised to handle a more complicated sensory world. Read together, these findings suggest that the Cambrian was not merely the origin of shells and skeletons but the origin of the dynamic, predatory, information-rich ecology in which every later animal — including us — would evolve. The silent world ended half a billion years ago, but its consequences, wired into our own genes and nerves, have never gone away. Debates about the explosion are not merely antiquarian. They determine how we think about the rate at which evolution can move, and whether the apparent suddenness of the fossil record is an illusion of imperfect preservation or a real signal that ecological crises, once crossed, can open doors that never close again. They also, not least, remind us that the body plan of a human being — a chordate, with a dorsal nerve cord and a segmented backbone — was worked out in principle by animals swimming in Cambrian seas, in a burst of experimentation whose basic options have not been substantially improved upon since. Every fish, every bird, every reader of these lines is still, in that literal sense, a Cambrian experiment that worked.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The oldest fossil burrows and the move into sediment ii. A biological explanation for the sudden genetic change iii. Exceptional fossil faunas and an early extinction iv. Why the Cambrian still matters for us v. A history of Victorian fossil collecting vi. How shale is mined commercially today vii. The chemistry of modern seawater

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

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

5. Roughly how long did the main Cambrian radiation last? A. About 20 million years. B. About 2,000 years. C. About 2 billion years. D. About 20 years.

6. What did the Nanjing team report finding at the Shibantan site? A. The oldest complex three-dimensional burrow systems. B. The first dinosaur bones in China. C. A 539-million-year-old human footprint. D. No trace fossils at all.

7. According to the cellular-predation hypothesis, what drove the genomic innovation? A. The stress of failed predation, leading to DNA repair and chromosomal rearrangement. B. A single large meteorite impact. C. The sudden arrival of flowering plants. D. A global drop in oxygen levels.

8. What was the Sinsk Event? A. An early extinction that interrupted the Cambrian radiation. B. A volcanic eruption in modern Russia. C. The naming of a new dinosaur. D. The discovery of the Burgess Shale.

Questions 9-13

Do the following statements agree with the claims of the writer?

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

9. Almost all major animal body plans first appear in the rocks during the Cambrian radiation.
10. The Ediacaran seafloor was dominated by actively swimming predators chasing one another.
11. The Shibantan discovery was published in the journal Science Advances.
12. Hox gene clusters have no connection to animal segmentation and body architecture.
13. Scientists have agreed unanimously that the explosion had a single cause.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

The move from a motionless Ediacaran seafloor is recorded in the oldest complex (14) __________ systems, while researchers argue that predation stress rewired developmental gene networks known as (15) __________ clusters.

答案与解析

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