The common ancestors of Australian land- and tree-dwelling kangaroos

D. This statement follows the argument made in the passage that land dwelling kangaroo eventually lost these attributes. Now if modern tree-dwelling kangaroo are descendent of land-dwelling kangaroo, it is obvious that they too won’t have these attributes. Hence, D is the answer.

Hey experts,

KarishmaB GMATNinja AjiteshArun DmitryFarber ExpertsGlobal5

Doubt - I am unable to understand why Option D is correct and why Option A is incorrect. My understanding - Option D goes against fact given in passage that land and tree dwelling kangaroos descended from a common ancestor. We cannot contradict fact hence i rejected this. Option A gives us a reason that why modern kangaroos lack those features i.e. they must back down slowly and opposable thumb where used to descend quickly.

Please help me understand flaws in my reasoning.

There were one type of kangaroos long long ago. They had grasping tails and long opposable thumbs which are great for tree dwelling.
From them, split two different types - land dwelling & tree dwelling long long ago.
Then, land dwelling ones lost these two features long ago because they were useless to them.

The odd thing is that the current tree dwellers also do not have these features (though these features would have been useful to them). They would have retained the useful features.
Why don't they have these features?

Option (D) tells us that the modern tree dwelling kangaroos ascended from the land dwellers. So it seems that the old tree dwellers became distinct and the current tree dwellers have evolved from land dwellers only (after the land dwellers lost the two features).
Note that option (D) is not in conflict with the premises. The common ancestors of all kangaroos are the ancient species. They split into two but only land dwellers survived. From the land dwellers, the modern tree dwellers evolved later.
waytowharton

(A) Modern tree-dwelling kangaroos must back down tree trunks slowly and carefully, but the common ancestors of modern tree-and land-dwelling kangaroos used their opposable thumbs to descend trees quickly headfirst.

This is the impact of not having the two features (grasping tails & long opposable thumbs). The modern tree dwellers must back down slowly.
It doesn't explain WHY they don't have the two features.

(B) Modern tree-dwelling kangaroos are smaller than most modern land-dwelling kangaroos but larger than their common ancestors.

Irrelevant.

(C) Modern tree-dwelling kangaroos’ tails cannot grasp branches, but they are somewhat longer and more flexible than those of modern landdwelling kangaroos.

Irrelevant. We need to know why their tails are not grasping tails.

(E) Modern tree-dwelling kangaroos have smaller and weaker hind legs than modern landdwelling kangaroos, and they move more slowly on land than do modern land-dwelling kangaroos.

Irrelevant.

Answer (D)