#14 - The common ancestors of Australian land- and tree-

[wwarui]

Hi Dave,

September 1998 Logical Reasoning Section II
Question: 14

Please explain the stimulus and the answer to this question. I haven't been able to figure out the correct answer even after redoing it without a timer.
Thanks.

[Robert Carroll]

wwarui,

This is a Resolve the Paradox question. Since the common ancestors of land- and tree-dwelling kangaroos had prehensile tails and long opposable thumbs, and these traits are useful for tree-dwelling kangaroos, one would expect this not to change, especially if a kangaroo's environment didn't change. Land-dwelling kangaroos lost these traits, but tree-dwelling ones did too. Hence the paradox - it's obviously how the environment changed for land-dwelling kangaroos, but tree-dwelling kangaroos should have kept the same traits.

One thing we don't know about is the subsequent evolution of kangaroos. They had a common ancestor with prehensile tails and long opposable thumbs, but it's possible that after that, all kangaroos became land-dwelling, and then some went back to being tree-dwelling, but only after a long enough time to lose the traits that the ancestor had in the first place. This is exactly what D says. Modern land- and tree-dwelling kangaroos have a more recent common ancestor, a land-dwelling variety, that lost the traits that the older ancestor referred to in the stimulus had. So modern kangaroos developed from a land-dwelling kangaroo that would naturally lack the qualities that are useful only in the trees, and modern tree-dwelling kangaroos have not yet developed the different traits that would help them in their new environment.

Robert

[wwarui]

Hi Robert,

Thank you so very much.
The paradox is clear now.

[sgd2114]

Hi,

Thank you, the above is helpful for seeing why (D) is correct. I chose (A) - the answer choice states that modern tree-dwelling kangaroos climb down trees differently than the common ancestor. If that's the case, the change in climbing habits could explain why the modern tree-dwelling kangaroos lost the attributes of their ancestor.

Any clarification for why that thought process is incorrect would be great. Thank you!

[AthenaDalton]

Hi sgd,

Answer choice (A) doesn't directly resolve the question raised by the stimulus -- if tree-dwelling kangaroos used to have opposable thumbs and grasping tails which would have helped them descend trees quickly and easily, why did they lose those traits over time? From the perspective of an evolutionary scientist, it wouldn't make much sense if a tree-dwelling kangaroo who had the ability to quickly descend a tree face-first using its thumbs and tail would instead choose to slowly descend backwards. In other words, why would the kangaroos who once had this ability stop using it, thereby eliminating the trait altogether over thousands of years?

Answer choice (A) doesn't provide an answer to that question. It essentially re-states the basic facts of the paradox: tree-dwelling kangaroos used to have this useful trait, and now they don't. We're looking for a reason for why this useful trait disappeared. It's unlikely that a kangaroo with this useful trait would stop using it if it were available.

I hope this makes sense. Good luck studying!