PrepTest 105, Section 1, Question 19
Paleontologist: It is widely, but falsely, held that(57 more words not shown)
Each of the following, if(9 more words not shown)
- AAccording to one(19 more words not shown)
- BThe severity of(15 more words not shown)
- CResearch suggests that(19 more words not shown)
- DThe 1.2-billion-year-old rocks(19 more words not shown)
- EUranium testing has(14 more words not shown)
Reading the argument
How does carbon in a rock tell us when life existed on land? The paleontologist gives one route: plants and microbes take carbon 14 from the atmosphere and release it when they die. But is that the only way the carbon could get there?
Perhaps some nonliving process could put carbon into the rock. That is a possibility to investigate, not a new scientific fact we're adding. If another route exists, the carbon wouldn't by itself establish the presence of plants or microbes.
There's a second question: were these rocks always on land? Finding them in North America now doesn't establish that they were never underwater. Even a biological source would need to be connected to life on land, rather than in water.
The claim being challenged has two parts: life began in the ocean, and it didn't exist on land until half a billion years ago. Evidence against either part would help the paleontologist dispute that account. We shouldn't replace that with a stronger claim that all life definitely began on land.
What we’re looking for
This is a strengthen EXCEPT question. Four answers support the paleontologist, and one doesn't. My questions about other sources of carbon and the rocks' history give us useful things to check. Support could also come from other evidence bearing directly on the timing or origin of life.
Working the answers
- A
Fossils of land plants older than half a billion years would directly challenge the claim that life didn't exist on land until then. This provides another line of support; it doesn't have to repair every weakness in the carbon argument. A supports the paleontologist, so it isn't the exception.
- B
Conditions making an ocean origin difficult give some reason to doubt the part of the conventional account saying life began there. Difficult doesn't mean impossible, and B doesn't prove a land origin. But it supplies additional support against the account the paleontologist rejects. That's enough to eliminate it on this EXCEPT question.
- C
Don't stop at the portions that were underwater. The answer also says other portions clearly never were, and the passage says carbon 14 was found throughout the rocks. That includes portions that were never submerged.
If all the relevant carbon could be associated with underwater rock, it would be harder to use it as evidence of land life. C helps with that concern by preserving carbon-bearing portions that were on land. It doesn't resolve our separate question about nonliving sources, but it does support one part of the reasoning.
- DCorrect
This gives the carbon a route into the soil that doesn't require plants or microbes: it entered directly from the atmosphere. That fits the objection we raised. Carbon in these rocks would no longer, by itself, be evidence that living things put it there.
D is more than an unrelated detail about how rocks form. It undercuts the proposed link from carbon to ancient life. We need the answer that doesn't add support, and an alternative nonliving source clearly qualifies.
- E
Confirming the rocks' age helps secure the timeline on which the argument relies. It doesn't prove a biological source or establish that every portion was on land. Still, reliable dating makes the evidence more useful for arguing about life earlier than half a billion years ago. That is support, even though other gaps remain.
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