雅思阅读 6: A Bushy Tree, Not a Ladder（进化不是梯子，而是灌木）

改编自 Arizona State University / Nature（2025-2026）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://sciencedaily.com/releases/2026/05/260515234644.htm

Reading Passage

A. For more than a century, popular imagination has pictured human evolution as a steady march — an ape-like creature gradually straightening its posture, enlarging its brain and transforming into Homo sapiens in a single, uninterrupted line. Textbooks reinforced this image with diagrams that ran from left to right, from knuckle-walking primates to upright modern humans. Fossils emerging from Ethiopia's Ledi-Geraru research site over the past decade have dismantled that picture piece by piece, and a 2025 study in Nature has delivered one of the most decisive blows yet: evidence that two distinct hominin genera — early Homo and an as-yet-unnamed species of Australopithecus — lived side by side in the same region between 2.6 and 2.8 million years ago. Human evolution, the authors conclude, was not a ladder but a dense, branching bush, populated by overlapping lineages, some of which led nowhere. That visual tradition is so deeply embedded that it survives even in museums and documentaries that know better, because a single straight line is easier to draw, and to remember, than a branching diagram whose branches mostly end in extinction. The Ledi-Geraru findings do not merely add a twig to that familiar image; they suggest that the image itself was long built from a handful of specimens chosen to tell a tidy story.

B. The key evidence came from thirteen fossil teeth recovered from ancient sediments at Ledi-Geraru, a rugged badland in Ethiopia's Afar region already famous as the site of the earliest known Homo jaw, discovered in 2013. The new study confirmed that teeth from both Homo and a previously unknown Australopithecus species were embedded in sediments dating to the same 2.6-to-2.8-million-year window. Crucially, the team determined that the Australopithecus teeth did not belong to Australopithecus afarensis — the species immortalised by the fossil "Lucy" — supporting the growing consensus that Lucy's lineage had disappeared by around 2.95 million years ago. The mysterious Australopithecus species remains unnamed; researchers need more cranial and skeletal material before they can formally describe it and place it on the family tree. Teeth are, in practice, the palaeoanthropologist's best friend: they fossilise far more readily than fragile limb bones, and their shape and surface texture carry species-specific information that trained researchers can read almost at a glance. The thirteen teeth were assigned to genera through comparison with reference collections from Ethiopia and Kenya, a process that relies as much on years of expert judgment as on automated measurement.

C. Dating such fossils relies not on the bones themselves but on the volcanic ash in which they are embedded. Ethiopia's Afar region is an active rift zone, where tectonic forces have produced repeated eruptions over millions of years. Ash from those eruptions contains feldspar crystals that can be radiometrically dated with precision. Because the fossils lie between layers of volcanic ash, researchers can bracket their age by dating the deposits above and below. This technique does more than establish a timeline; it allows geologists to reconstruct the world those ancient hominins inhabited. Two to three million years ago, Ledi-Geraru was not the arid badland seen today. Ancient rivers traversed a greener landscape, feeding shallow lakes that expanded and contracted with the climate. The environmental diversity may itself help explain how multiple hominin lineages managed to coexist without immediately outcompeting one another. The reconstruction also undermines a subsidiary assumption: that early Homo evolved in open, drying savannahs to which its body was gradually adapted. A landscape of riverside forest and lakeside woodland suggests instead that the first members of our genus emerged in a patchwork of habitats, and that the supposed march onto the open plains was one episode among many rather than the driving narrative of human origins.

D. The Ledi-Geraru finds fit into a rapidly broadening picture. A 2025 study reported Homo fossils at 2.78 and 2.59 million years ago alongside Australopithecus at 2.63 million years ago, and noted that as many as four hominin lineages — early Homo, Paranthropus, Australopithecus garhi, and the new Ledi-Geraru species — may have lived in eastern Africa between 3.0 and 2.5 million years ago. In 2026, a University of Chicago team added a 2.6-million-year-old Paranthropus jaw from the Afar region, pushing that lineage's known range hundreds of kilometres north. Together, these discoveries suggest that early human relatives were far more geographically widespread and ecologically flexible than the old ladder model allowed. The question of how these species shared resources — whether they ate different foods, occupied different niches, or simply ignored one another — remains open. The research team is now analysing tooth enamel chemistry to reconstruct ancient diets. They are also examining the microscopic scratches on tooth surfaces, which record whether a given individual ate mainly soft fruits, tough leaves or grit-covered underground tubers. If two species living in the same landscape turn out to have eaten different foods, that would offer a partial answer to the coexistence puzzle; if their diets overlapped heavily, the question becomes sharper still.

E. The significance of these findings extends beyond palaeoanthropology. The old linear model, in which one species steadily replaced another, implied a single, deterministic path leading inevitably to Homo sapiens. The bushy tree that has now replaced it emphasises contingency and divergence: multiple experiments in hominin form, most of which ended in extinction, with one surviving lineage ultimately giving rise to us. That perspective carries a humbling implication. "Because we're the survivors," notes ASU palaeoecologist Kaye Reed, "we know that it happened to us" — but there is no guarantee that it had to. More fossils will be needed to trace the precise shape of that ancient bush, and many sites across eastern Africa remain unexplored. But the era in which a single, simple narrative of human origins sufficed is, by any measure, over. That shift carries practical consequences too. Museums, textbooks and popular accounts are now being rewritten to replace the familiar parade of upright figures with messier diagrams in which several species overlap and most die out. It is a harder story to tell, because it refuses to flatter the reader with a sense of inevitability; but it is, on the evidence the rocks now preserve, the truer one.

Questions 1-4

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

List of Headings i. How volcanic ash pins down the age of fossils ii. Why the linear model was originally proposed iii. Thirteen teeth reveal a crowded landscape iv. What this means for how we understand our own origins v. New fossils expand the number of coexisting lineages vi. The economic uses of ancient volcanic deposits vii. How Lucy's species migrated out of Africa

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. What did the Ledi-Geraru study demonstrate? A. Lucy's species survived until 2.6 million years ago. B. Homo and a new Australopithecus species lived contemporaneously. C. Human evolution proceeded in a single straight line. D. Paranthropus originated in southern Africa.

6. How do researchers date the Ledi-Geraru fossils? A. By analysing DNA extracted from the teeth. B. By comparing the shape of tooth enamel with known samples. C. By dating volcanic ash layers above and below the fossils. D. By measuring the chemical composition of surrounding soil.

7. What did the 2026 Paranthropus jaw discovery contribute? A. It confirmed that Paranthropus was identical to early Homo. B. It extended the known geographic range of that lineage. C. It proved that four species coexisted at a single site. D. It showed that hominins left Africa earlier than thought.

8. The writer suggests that the bushy-tree view of human evolution A. confirms that modern humans were inevitable. B. implies that survival was a matter of chance as well as adaptation. C. resolves all outstanding questions about hominin diet. D. shows that extinct species were ecologically identical.

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. The Ledi-Geraru Australopithecus has been formally named as a new species.
10. Volcanic ash at Ledi-Geraru can be dated using radiometric methods.
11. The environment at Ledi-Geraru 2.6 million years ago was similar to today's.
12. Researchers have already determined what the coexisting species ate.
13. Many archaeological sites in eastern Africa remain unexcavated.

Questions 14-15

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

The old model of human evolution resembled a (14) __________ leading inevitably to modern humans. Current evidence instead suggests a (15) __________ tree in which multiple lineages coexisted and most became extinct.

答案与解析

← 上一篇 | 返回雅思焦点 | 下一篇 →