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Difficulty: HardFunction of Detail in Context

Read the following excerpt from an essay on evolutionary anthropology:

For decades, paleoanthropologists maintained that the structural remodeling of the hominin postcranial skeleton was driven almost exclusively by selection for energy-efficient bipedal walking. Under this orthodox paradigm, features such as elongated lower limbs and an arched foot were treated as comprehensive adaptations for low-speed terrestrial locomotion. However, recent biomechanical modeling demonstrates that walking at moderate velocities requires minimal dynamic stabilization of the head and trunk, leaving several derived anatomical structures functionally unaccounted for. Researchers Bramble and Lieberman argue instead that sustained endurance running exerted a decisive selective pressure on hominin morphology. In support of this hypothesis, they point to the prominent cranial attachment site for the nuchal ligament—an elastic band of connective tissue that prevents pitching of the head during running but remains functionally inactive during walking. Crucially, fossil crania show that this attachment site is entirely absent in Australopithecus afarensis and only makes its evolutionary appearance with early Homo erectus, roughly two million years after the inception of habitual bipedalism.

In the context of the argument, the author's reference to the absence of the nuchal ligament attachment site in Australopithecus afarensis serves primarily to:

  1. A
    identify a biomechanical deficiency that prevented early hominins from engaging in habitual upright walking
  2. B
    report the empirical finding that fossil crania of early hominins lack the skeletal ridge associated with elastic neck tissue
  3. demonstrate that specialized cranial stabilization adaptations coincided with the emergence of the genus Homo rather than the initial evolution of bipedal locomotionAnswer
  4. D
    concede that orthodox paleoanthropological models adequately explain the cranial morphology of early hominin species
  5. E
    suggest that Australopithecus must have relied on alternative physiological mechanisms to maintain head stability during sustained running

Answer

The choice stating that the reference serves to demonstrate that specialized cranial stabilization adaptations coincided with the emergence of the genus Homo rather than the initial evolution of bipedal locomotion.
The author mentions the absence of the nuchal ligament attachment site in Australopithecus afarensis to demonstrate that this running-specific stabilization feature did not emerge alongside the original advent of bipedal walking, but instead appeared millions of years later with the genus Homo, thereby supporting the endurance running hypothesis over orthodox walking-centric models.

Step-by-Step Solution

1
Analyze the structural role of the referenced detail in its immediate argumentative context.
The detail shows that the nuchal ligament attachment appears in Homo erectus but is absent in Australopithecus afarensis, occurring roughly two million years after upright walking first evolved.
Determining the rhetorical function requires locating the detail within the passage's logical progression.
2
Connect the detail to the author's central thesis regarding endurance running.
Because the ligament is needed for running but inactive in walking, its temporal appearance in Homo rather than early bipedal hominins supports running as a distinct selective pressure.
Details in LSAT Reading Comprehension are deployed to substantiate, illustrate, or qualify specific argumentative claims.
3
Differentiate the structural function of the claim from a simple restatement of its factual content.
The author uses the fossil timeline to prove that the adaptation was linked to the genus Homo and running, rather than the earlier advent of bipedalism.
Eliminates distractor options that describe what the text says instead of why the author included it.

Key Concept

Function of Detail in Context
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Function of Detail in Context

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