Q.97 Which one of the following rooted tree topologies best describes the primate
phylogeny?
(A) I
(B) II
(C) III
(D) IV

The best‐supported rooted tree topology for the given primate phylogeny is Option III.​​

Introduction

Choosing the correct rooted tree topology for primate phylogeny requires understanding how humans, chimpanzees, bonobos, gorillas and gibbons are related evolutionarily. Molecular and morphological studies consistently show that humans group most closely with the genus Pan (chimpanzee and bonobo), while gibbons such as Hoolock hoolock form the sister clade to all great apes and humans.​​

Known primate relationships used to solve the question

• Gibbons (family Hylobatidae, including Hoolock) diverged first, forming the sister group to the great apes and humans (family Hominidae).​​

• Within Hominidae, gorillas diverged earlier, and a later split produced the Homo lineage and the genus Pan (chimpanzees and bonobos).​​

• Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are sister species within Pan.​

So the correct rooted topology, reading from root (oldest) to tips (most recent), must be:

$$(((Homo,(Pantroglodytes,Panpaniscus)),Gorilla),Hoolock)$$

This structure is exactly what Option III depicts.

Option I explained

In Option I, the outermost branch (earliest divergence) is not Hoolock hoolock; instead, Hoolock appears nested inside the main clade with Homo, Gorilla and Pan. This would imply that gibbons diverged after some great apes, contradicting strong molecular evidence that gibbons (Hylobatidae) split from the great ape–human line 18–12 million years ago, before any divergence within Hominidae.​​

Moreover, in Option I the internal branching does not clearly show a clade containing Homo plus Pan as sister to Gorilla. This conflicts with multiple genomic analyses that support a ((human, chimpanzee–bonobo) gorilla) arrangement. Therefore, Option I is incorrect for the true primate phylogeny.​

Option II explained

In Option II, Hoolock hoolock is placed as the most recent branch, implying that gibbons diverged last among the taxa shown. This reverses the known order of divergences, because gibbons are actually the earliest branching hominoids relative to gorillas, chimpanzees, bonobos and humans.​​

Additionally, the grouping inside the tree does not clearly separate a Homo–Pan clade from Gorilla, again disagreeing with well‐supported hominid relationships. Thus, Option II does not match the accepted rooted tree topology for these primates.​

Option III explained (correct answer)

Option III shows the following branching pattern when read from the root upward:

1. The root splits first into Hoolock hoolock on one branch and a clade containing Gorilla, Pan paniscus, Pan troglodytes and Homo sapiens on the other.​

2. Within this latter clade, Gorilla gorilla diverges earlier, leaving a more recent clade with Pan paniscus, Pan troglodytes and Homo sapiens.

3. Finally, Pan paniscus and Pan troglodytes form a sister pair, and this Pan clade is sister to Homo sapiens.

This matches the established hominoid phylogeny: gibbons as sister to Hominidae; within Hominidae the branching order is gorilla, then the split between Homo and Pan; and within Pan the bonobo–chimpanzee sister relationship. For this reason, Option III best describes the primate phylogeny.​​

Option IV explained

Option IV places Gorilla gorilla and the two Pan species together in a tight clade, while Homo sapiens is depicted as branching earlier, outside this African ape group. That topology suggests that humans are more distantly related to chimpanzees and bonobos than gorillas are, implying a ((Pan, Gorilla) Homo) arrangement.​

However, genomic studies consistently indicate that humans share a more recent common ancestor with chimpanzees and bonobos than either does with gorillas, giving a ((Homo, Pan) Gorilla) pattern instead. Furthermore, Option IV again fails to position Hoolock hoolock as the earliest hominoid branch, contrary to gibbon systematics. Thus Option IV is also incorrect.​

Short exam‑style answer

• Correct rooted tree topology that best describes the given primate phylogeny is Option III.

• It correctly shows Hoolock hoolock (gibbon) as sister to all great apes and humans, Gorilla gorilla as an earlier diverging African ape, and a terminal clade in which Homo sapiens is sister to the genus Pan (with Pan troglodytes and Pan paniscus as sister species).​​