雅思阅读 086: Two Origins of Life — The Half-Alive Ancestor

Adapted from "Life's last universal common ancestor may predate life itself" by Philip Ball, Scientific American, 20 August 2026.

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Reading Passage

How did life begin? This question has tantalised scientists since Charles Darwin mused on it in 1871. A new study now suggests that the answer may require us to rethink what "life" actually means.

In a paper published in Science Advances, evolutionary biologist Bill Martin of Heinrich Heine University Düsseldorf and his colleagues argue that life may have begun not once, but twice. They propose that the two earliest branches of the tree of life — bacteria and archaea — arose independently from a single source that was itself not truly alive.

That source is known as LUCA: the Last Universal Common Ancestor. Traditionally, LUCA is pictured as a primitive, bacteria-like cell from which all living things descend. But in the new view, LUCA was not a complete organism. It was a chemical system formed in the mineral-rich waters of deep-sea hydrothermal vents — part organic, part rock.

The researchers reached this conclusion by comparing the metabolic networks of bacteria and archaea. They found that LUCA possessed genes for only about half of modern metabolism. Some essential reactions were catalysed not by protein enzymes, as in every modern organism, but by simple metallic elements found in vent minerals — nickel, iron, cobalt and palladium.

These metals are commonly used as industrial catalysts today. On the early Earth, which had almost no oxygen, they did not rust and would have been abundant in vent rocks.

The researchers also suggest that LUCA used a phosphorus compound called phosphite, rather than the phosphate used by all modern cells, to store energy. Chemical experiments showed that palladium can catalyse phosphite reactions in ways that resemble modern metabolism.

Crucially, LUCA already possessed a genetic encoding system resembling DNA or RNA. The researchers argue that genes and metabolism did not arise one before the other; they co-evolved. LUCA eventually produced the first truly free-living cells — the last bacterial common ancestor and the last archaeal common ancestor — when it evolved a complete set of protein enzymes.

Because current definitions reserve the word "alive" for free-living cells, Martin says: "We are looking at one origin of the genetic code but two origins of life."

Not everyone is convinced. Biochemist Nick Lane of University College London calls the idea insightful but questions whether metals from kilometres beneath the seabed could have reached the surface in sufficient quantities. Other researchers ask whether the missing enzymes might simply be undetected rather than genuinely absent.

Regardless of the details, the study reframes the debate. Instead of asking "when did life begin?", scientists are now asking how and when the various chemical ingredients assembled themselves. The boundary between "living" and "non-living" may be less a line than a gradient.

Questions 1–5

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

1. According to the new study, LUCA was

   • A. the first free-living bacterium.
   • B. a half-alive chemical system at hydrothermal vents.
   • C. a modern archaeal cell.
   • D. a warm pond on land.

2. How much of modern metabolism did LUCA's genes cover?

   • A. Almost all of it
   • About half of it
   • C. Less than ten percent
   • D. None of it

3. What catalysed some metabolic reactions in LUCA?

   • A. Protein enzymes
   • B. DNA
   • C. Metals such as nickel and iron
   • D. RNA

4. What did LUCA likely use to store energy?

   • A. ATP
   • B. Phosphate
   • C. Phosphite
   • D. Glucose

5. The researchers conclude that

   • A. genes came before metabolism.
   • B. metabolism came before genes.
   • C. genes and metabolism co-evolved.
   • D. neither genes nor metabolism existed in LUCA.

Questions 6–10

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

Write:

• YES
• NO
• NOT GIVEN

6. LUCA is traditionally seen as a primitive, bacteria-like cell.
7. LUCA possessed no genetic encoding system.
8. The early Earth had abundant oxygen in its atmosphere.
9. Nick Lane fully agrees with the new theory.
10. The study suggests the boundary between life and non-life may be gradual.

Questions 11–13

Complete the sentences. Choose NO MORE THAN TWO WORDS from the passage.

11. Bacteria and archaea arose from a single source that was not yet truly ________.
12. On the early Earth, metals did not ________ because the atmosphere had little oxygen.
13. The first free-living cells appeared when LUCA evolved a complete set of protein ________.

Answers

1. B — "part organic, part rock" at hydrothermal vents.
2. B — "LUCA only had genes for about half of metabolism."
3. C — "catalysed not by elaborate protein enzymes… but by simple metallic chemical elements."
4. C — "phosphite… could have played the same role."
5. C — "they coevolved."
6. YES — "generally regarded as a kind of ur-organism: a primitive bacteria-like cell."
7. NO — LUCA already had "a primitive form of genetic encoding."
8. NO — The early Earth had "very little oxygen."
9. NO — Lane is supportive but raises questions.
10. YES — "less a line than a gradient."
11. alive
12. rust / oxidise
13. enzymes

Key Vocabulary