In the cladogram on p. 270 of your lab manual, what does point B represent?

Question

BlackTom AI · Accepted Answer

We don’t have access to the lab manual’s p. 270 cladogram image, so we can’t see exactly where point B sits on that specific diagram. Without the visual, we can still reason about what each option would mean in a typical primate cladogram and why each would or wouldn’t be plausible depending on B’s position.

Option A: the ancestor of gorillas and humans
- If point B is the node at which the human lineage branches away from the lineage leading to gorillas, this would represent the most recent common ancestor of humans and gorillas (the last common ancestor of Hominini and Gorillini). In many cladograms of great apes, such a node could plausibly be labeled as the ancestor of both humans and gorillas. This interpretation is consistent with a node that sits at the junction of the human and gorilla lines before they diverge into their respective lineages.
- Why this could be correct (in a typical layout): it reflects a shared ancestry for humans and gorillas prior to the split that yielded modern humans and modern gorillas.
- Why it might be wrong: if the diagram has B placed at a deeper or shallower node (for example, at the root of all apes, or at the point where humans alone diverge from all other apes), this option would not match B’s actual position.

Option B: the ancestor of humans only
- This would be the most recent common ancestor exclusive to humans, implying that no other species share that node. In most standard great-ape cladograms, there isn’t a node that exclusively represents the human lineage without sharing with other apes at some prior divergence; humans typically branch off from a node that also relates to other great apes. If B is at a node that sits before the human lineage separates from other apes, labeling it as the human-only ancestor would be incorrect.
- Why this could be wrong: it conflicts with the usual pattern of branching where lineages split from shared ancestors, not a terminal, human-only origin at a higher-level ancestor node.
- Why it might be correct: only if B is drawn at a terminal position directly on the human lineage with no side branches immediately above it (i.e., a node unique to humans in that specific diagram), which would be unusual for standard cladograms.

Option C: the ancestor of gorillas and baboons
- Gorillas are great apes, while baboons are Old World monkeys. Their most recent common ancestor would be far back in primate evolution, at a level far above the split between apes and monkeys. If B labeled this ancestor, it would imply a node much more ancient than typical great-ape clades and would generally be located toward the base of the entire primate tree, not at a node that directly separates humans and gorillas.
- Why this is unlikely: most cladograms separate apes (including humans and gorillas) from monkeys earlier, so a node specifically uniting gorillas and baboons without humans would be an unusual placement unless the diagram is intended to show a very broad primitive primate common ancestor.
- Why it might be correct: only if the diagram’s labeling placed B at a node that corresponds to the last common ancestor of all catarrhines (Old World monkeys and apes), which would then indeed include both baboons and gorillas along with humans as descendants. In that case, this option could reflect a more basal node, not a direct gorilla-human split.

Option D: the ancestor of baboons, gorillas, and humans
- This would be a node representing the last common ancestor of the three groups: baboons (monkeys), gorillas (great apes), and humans (great apes). In most primate phylogenies, the last common ancestor of these three groups lies very near the base of Catarrhini, before the split between monkeys and apes. If B is at a node that unites these three lineages, it would be a very basal node.
- Why this could be correct: if the diagram’s B sits at the root or a very basal node that precedes the split between monkeys and apes, this option would accurately describe that ancestor.
- Why it might be wrong: if B is shown on a more derived part of the tree (e.g., on the branch leading to humans or the branch uniting humans and gorillas only), labeling it as the ancestor of all three groups would misidentify the node.

Summary guidance for choosing among these options in your specific diagram:
- Determine exactly where point B sits: is it at a junction that unites humans and gorillas, is it on the human-only branch, is it a deeper root that would include monkeys, or is it at the base that precedes both monkeys and apes? The position dictates which description is accurate.
- If B is at the node that two modern great apes (humans and gorillas) share before diverging, Option A is a plausible match.
- If B appears on a branch unique to humans with no other lineages converging at that node, Option B would be suspect.
- If B sits at a broad, ancient node that includes both apes and monkeys (but not necessarily humans alone), Options C or D might apply depending on whether that node is the most recent common ancestor of gorillas and baboons alone, or of all three groups altogether.

Without the exact image, we can’t confirm which choice the diagram labels as point B. Examine the diagram’s topology and locate the immediate descendants and sister groups at B to map it to one of these ancestor descriptions accurately.

In the cladogram on p. 270 of your lab manual, what does point B represent?单项选择题

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