雅思阅读 119: The Ghosts in Our Genes（我们基因中的幽灵）

改编自 University of California, Berkeley / Science（2024年12月）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://news.berkeley.edu/2024/12/12/a-new-timeline-for-neanderthal-interbreeding-with-modern-humans/

Reading Passage

A. Open any biology textbook written before 2010 and you will find a simple story. Modern humans arose in Africa, spread out into the world, and replaced the archaic peoples they met — Neanderthals in Europe, Denisovans in Asia — without, it was assumed, interbreeding. That story collapsed almost overnight when ancient DNA began to be recovered from fossil bones. Comparing the genome of a Neanderthal with those of living people, geneticists found that the two had not met as strangers at all. Almost every human alive today whose ancestors came from outside Africa carries about two per cent of Neanderthal DNA. The extinct relatives did not vanish without issue; they live on, a little, in the rest of us. Later work added a second ghost. People from Melanesia and Australian Aboriginal backgrounds carry an extra two to five per cent of DNA from Denisovans, a mysterious Asian cousin known mainly from a few bones and teeth. We are, it turns out, a mixed species, and the mixing happened deep in prehistory. The textbooks of the 1990s are not just wrong in detail; they are wrong in spirit, because they assumed the story of our species was one of triumph over replacement rather than of quiet, repeated union.

B. For years, one large question dominated the field: when, exactly, did this mixing happen? The trouble is that a fragment of Neanderthal DNA carried in a living person is the survivor of an event thousands of generations old, and dating it requires mathematical models rather than a calendar. In December 2024, a team at the University of Berkeley sharpened the estimate using DNA from ancient modern humans unearthed across Europe and Asia. They concluded that the main episode of interbreeding began about fifty thousand five hundred years ago and lasted roughly seven thousand years, only ending as Neanderthal populations began to disappear. That window brackets the moment when the ancestors of all non-African people alive today were still a single, closely related group. Earlier genomes from Africa and the Near East, the researchers note, show no such Neanderthal segments, which means the mixing took place after our own kind had left the continent but before it spread into the far corners of Eurasia.

C. The picture is more tangled than a single romantic encounter. A separate analysis, published the same year, argued that gene flow between Neanderthals and modern humans had recurred, on and off, across more than two hundred thousand years — and that it ran in both directions. Not only did Neanderthal DNA enter our ancestors, but traces of modern human DNA also turned up in Neanderthal remains, as though the two lineages exchanged genes whenever their ranges overlapped. Cave deposits make the same point in stone. In one Siberian cave, the remains of an ancient Denisovan girl were found to be more closely related to the Denisovan DNA carried by living people than to the earlier Denisovan bones also recovered there, suggesting that one population replaced another over time. In the same cave, Denisovans were succeeded, for a while, by Neanderthals, and then by modern humans. The picture is not two clean species meeting once; it is a slow, repeated, messy overlap of peoples who traded genes as readily as they traded territory. Bones alone can never show us the meetings themselves — a shared campfire, a joint hunt, the children of such encounters — but the genome records that they happened, over and over, across hundreds of thousands of years.

D. Why does this two-per-cent matter? Because natural selection has been working on it ever since. Most of the Neanderthal DNA that entered our ancestors was slowly removed by evolution, which tells us that large chunks of it were harmful in a modern-human body. But a few segments were kept, and they tend to cluster around genes affecting the immune system, the skin and hair, and the way the body responds to extreme cold. The Neanderthal variants that survive today are, in other words, not random souvenirs; they are the pieces that helped a tropical species survive when it moved into an ice-age world. Studies of modern Indian genomes, for example, recovered a surprisingly large fraction of the Neanderthal genome from living people alone — about half of it — even though no Neanderthal bone has ever been dug up on the subcontinent. The archaic DNA, thinned by fifty thousand years of recombination, still carries readable signatures of the encounters that put it there. In that sense, a living human genome is a palimpsest: most of it records the recent African story, but the under-drawing is older, and stranger.

E. The deepest lesson is a humbling one for the idea of species. Biologists have long defined a species as a group that cannot produce fertile young with another, but the genome keeps complicating the rule. If Neanderthals and Denisovans were separate species, they were species that successfully interbred whenever they met — and their offspring became us. The people we used to picture as hulking, dim cousins turned out to be close enough to our ancestors that their genes flow in our veins, shaping how our immune systems fight infection and how our skin adapts to cold. As more ancient genomes are recovered, the family tree of humanity stops looking like a set of clean branches and starts looking like a web, with threads crossing and recrossing for hundreds of thousands of years. We did not simply replace the peoples we met. We absorbed some of them — and a small, surviving trace of those encounters is still, today, part of what it means to be human outside of Africa. The Neanderthal who left a gene in your immune system is not a stranger in your family album; is, in a literal sense, an ancestor.

Questions 1-4

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

List of Headings i. Dating the mixing — a sharper timeline ii. A messier overlap than a single meeting iii. Why the surviving two per cent was kept iv. A web rather than a tree v. How to extract DNA from fossils vi. The extinction of the dinosaurs vii. Why Neanderthals were stronger than humans

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 ancient DNA reveal about Neanderthals? A. They were completely unrelated to modern humans. B. Non-African people today carry about two per cent Neanderthal DNA. C. They were a subspecies of chimpanzee. D. They never met modern humans.

6. According to the 2024 Berkeley study, when did the main interbreeding episode begin? A. About 50,500 years ago. B. About 10,000 years ago. C. About 2 million years ago. D. In the last century.

7. What does the two-way gene flow suggest? A. Mixing happened only once, in one direction. B. Neanderthals also carried traces of modern human DNA. C. The two species never overlapped in time. D. Modern humans left no genetic mark on Neanderthals.

8. Why did natural selection keep some Neanderthal segments? A. They helped the body respond to cold and infection. B. They made people taller. C. They had no effect at all. D. They caused diseases.

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. People of Melanesian and Australian Aboriginal ancestry carry additional Denisovan DNA.
10. The main interbreeding episode lasted about seven thousand years.
11. Most of the Neanderthal DNA that entered modern humans was preserved unchanged.
12. A Neanderthal fossil has been found in India.
13. Neanderthals were less intelligent than modern humans.

Questions 14-15

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

Most Neanderthal DNA was removed by natural selection, but the surviving segments cluster around genes affecting the (14) __________ system, the skin and hair, and the body's response to extreme (15) __________.

答案与解析

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