This is one of those "Evolutionary Logic" questions.
The biologist is making a big jump from the past to the present.
The Argument:
He says: "Our ancestors lived in dark burrows, so we are biologically built for the dark. Therefore, bright lights today will ruin our sleep."
The Gap:
The gap here is the "So What?" factor.
Just because we are adapted to something, does it mean we require it to function?
Analogy: Humans are adapted to walk barefoot on dirt (that's what our ancestors did). Does that mean wearing Nike sneakers ruins our feet? Not necessarily. We might be perfectly fine adapting to shoes.
For the biologist to claim that bright lights will definitely mess us up, he has to assume that our bodies are strict about these rules—that we can't just shrug off the new environment.
That is exactly what Option (C) says.
(C) Modern mammals’ sleep cycles depend at least in part on environmental conditions to which mammals are naturally adapted.
If you take this away (Negate it), the argument falls apart:
"If our sleep cycles don't depend on what we are adapted to, then who cares if we use bright lights? We’ll be fine."
Since the argument dies without it, this is the assumption.
Here is why the others are traps:
A (The "Other Mammals" Trap): This talks about mammals that live in bright environments (like lions on the savannah). Who cares about them? The argument is about us (the burrow-dwellers). What happens to a lion in the sun doesn't tell us what happens to a human in a bedroom.
B (The "Source" Trap): This says the brain reacts differently to Artificial light vs Natural light.
Be careful—the prompt isn't attacking the source (lightbulb vs sun); it's attacking the Brightness.
If I brought the actual sun into your bedroom at midnight, it would arguably be just as bad as a lamp. The argument doesn't need the light to be "fake" to be a problem; it just needs to be "bright."
D (The "Better Days" Trap): This claims early mammals had "healthier" sleep.
This is irrelevant. Maybe early mammals slept terribly because of predators! We don't need their sleep to be better; we just need to know that their environment set the rules for our biology.
E (The "Intensity" Trap): This says artificial light is brighter than daylight.
Too extreme. The light doesn't have to be brighter than the literal sun to keep you awake. It just has to be bright enough to trigger your brain.
Bunuel
Biologist: Early mammals spent most of their time in environments with low light levels, such as forests and burrows. As a result, mammals today are naturally adapted to dim lighting. Hence, regular exposure to bright artificial light is likely to interfere with maintaining proper sleep cycles.
Which of the following is an assumption the biologist’s argument requires?
(A) Mammals that spend most of their time in brightly lit environments tend to have more irregular sleep cycles.
(B) Exposure to artificial light affects the brain differently than exposure to natural light.
(C) Modern mammals’ sleep cycles depend at least in part on environmental conditions to which mammals are naturally adapted.
(D) Early mammals had healthier sleep cycles than most mammals have today.
(E) Bright artificial light typically exceeds the intensity of daylight in natural environments.
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