雅思阅读 176: The Day Three Ancestors Walked（三位远祖走过的那一天）

改编自 Smithsonian Human Origins / Nature (McNutt et al.)（2021-2025年）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://humanorigins.si.edu/evidence/behavior/footprints/laetoli-footprint-trails

Reading Passage

A. Most fossils are bones. They tell us about an animal at the moment of death — its teeth, its limb structure, the shape of its skull. What they rarely tell us is what the animal did, moment by moment, while it was alive. That gap is filled, occasionally and miraculously, by trace fossils: footprints, tail drags, bite marks, the impressions left in soft sediment that later hardened into rock. Among these, footprints hold a special place. A skeleton is a snapshot of anatomy; a footprint is a snapshot of behaviour. It shows not how an animal was built but how it moved, how fast it walked, whether it paused, whether two individuals walked side by side. At Laetoli, in northern Tanzania, a fall of wet volcanic ash from the nearby Sadiman volcano preserved exactly such a snapshot 3.66 million years ago, and it has been arguing with anthropologists ever since. The tracks are, in a literal sense, an instant — a few seconds of walking, paused in mud — yet they have outlasted the mountains that once surrounded the walkers.

B. The Laetoli trackway was discovered in 1976, with the principal excavation following in 1978, by a team led by Mary Leakey. Across a surface now just under 27 metres long, the prints of at least three individuals were preserved, walking in the same direction across the fresh ash. A subsequent ash fall covered them, protected them from weather, and potassium-argon dating later fixed their age at 3.66 million years. The prints themselves were striking: a non-divergent big toe, a defined longitudinal arch, and a clear heel-to-toe sequence of weight transfer. In every measurable respect, the gait they recorded was that of a biped walking at a leisurely pace — so much so that some observers noted that, undated, the prints would be indistinguishable from those of a modern human walking barefoot. The implication was electrifying: obligate bipedalism, the habit of walking on two feet as the ordinary mode of travel, was established far earlier than the fossil bones had previously suggested, and at a time when the brains of the walkers were still the size of a chimpanzee's.

C. The reason the trackway preserved so much detail lies in a chain of unlikely coincidences. The ash was carbonatitic — unusual, fine-grained, and just damp enough to take an impression without collapsing. It set rapidly, like cement, as the sun dried it and subsequent rain flushed through. Before it could be eroded by animals or water, a second eruption buried it. In a sense, the walkers had to pick exactly the right moment: not so soon after the eruption that the ash was still hot, not so late that it had hardened. Palaeontologists call the conditions required for such preservation "exceptional", and they are rare precisely because they require catastrophe to arrive and then to cover its own tracks. Without the volcano, the three walkers would have left impressions that rain, wind and other animals would have erased within hours. Without the rapid cementation, the prints would have slumped. The trackway is thus not only a record of behaviour; it is also a record of a geological moment. Without that conjunction — a fresh wet ash fall, a walk across it, a quick drying, another fall — none of it would have survived. The palaeontologist's favourite phrase, "fossilisation bias", is not a technical abstraction here; it is the reason three ancient afternoons still speak to us across three million years.

D. For decades, the interpretation of Laetoli settled into a comfortable shape: three members of Australopithecus afarensis — the species to which the famous Lucy skeleton belonged — walked upright across the ash, proving that bipedalism preceded brain enlargement in human evolution. A more controversial trackway, discovered even earlier at a nearby locality called site A, had been partly excavated in the 1970s but set aside because its prints bore a strange resemblance to those of a bear walking on its hind legs. In 2021, a team led by researchers at Dartmouth College returned, reopened the excavation, and showed that the site-A prints were in fact made by a hominin — but one whose gait was subtly different from the famous G-trackway. The stride was shorter, the footfall less mechanical, suggesting that early hominin walking was not yet the uniform, fully modern pattern that the neat Laetoli trail had implied. There may have been locomotor diversity 3.66 million years ago, just as there was anatomical diversity, and the single famous trackway had disguised it.

E. The discovery raises a larger point about how we read the fossil record. Bones, when they are found, are often isolated and broken; assigning them to a species and a locomotor habit involves inference that can be contested for decades. Footprints, by contrast, are unambiguous about what they record: some creature, at some moment, placed its foot there. They cannot be misidentified as belonging to another species in the same way a jaw fragment can. Yet they remain hard to date precisely and even harder to connect to a particular skeleton. The walkers at Laetoli cannot be matched, bone for bone, to Lucy or to any other specimen, because their bones are not there. What the new site-A work demonstrates is that the most famous trace fossil in human evolution is also, inevitably, an incomplete sample: a few metres of ash, a few individuals, a few seconds of a long history. Each time the trenches are reopened, the picture grows richer — and the confident old generalisations about when and how our ancestors stood upright grow, appropriately, more cautious. The three walkers themselves left no bones, no tools, no names; they left only that brief trail across the ash, which is perhaps the most intimate record a species can hope to inherit.

Questions 1-4

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

List of Headings i. Why footprints matter more than bones ii. The discovery at Laetoli and what the prints show iii. The improbable geological chain that preserved the tracks iv. A long-dismissed trackway revived — and what it changes v. The bear that walked on two legs in modern zoos vi. Why the fossil record of footprints is incomplete vii. The dating of Sadiman volcano

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. Why are footprints considered more informative than bones about an animal's life? A. Bones are always broken, whereas footprints are always complete. B. Footprints record behaviour and movement, while bones mainly record anatomy. C. Footprints are easier to date than bones. D. Bones never reveal the age of the animal.

6. What is the approximate age of the Laetoli trackway? A. About 3.66 million years. B. About 100,000 years. C. About 36 million years. D. About 3,600 years.

7. Why did the site-A trackway stay neglected for decades? A. It was thought to belong to a walking bear rather than a hominin. B. It was destroyed by rain in the 1970s. C. It was never excavated at all. D. It was too young to be interesting.

8. What did the 2021 reopened excavation suggest? A. All hominins walked identically at that time. B. There may have been locomotor diversity among early hominins. C. The original Laetoli prints were faked. D. Lucy was not a hominin.

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 Laetoli trackway preserves the prints of at least three individuals walking in the same direction.
10. The brain size of the Laetoli walkers was substantially larger than that of a chimpanzee.
11. The ash that preserved the prints was unusually coarse and sandy.
12. The 2021 study concluded that the site-A prints were made by a hominin with a subtly different gait.
13. The Laetoli walkers have been matched bone-for-bone to the Lucy skeleton.

Questions 14-15

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

The Laetoli prints show a non-divergent big toe and a clear heel-to-toe sequence of weight transfer, indicating that (14) __________ walking was established long before the brain enlarged. Preservation depended on a chain of coincidences: fine carbonatite ash that set rapidly like (15) __________ and a second eruption that buried the prints before they could erode.

答案与解析

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