Rearrange the parts of the sentence in the correct logical order so that it reads as a meaningful statement: The existence of P can be traced back to Q mammals on the earth R at least the Triassic time.

Introduction / Context:
This jumbled sentence question tests your ability to rearrange labelled parts so that a scientifically meaningful and grammatically correct statement emerges. The sentence deals with the prehistoric origin of mammals and references the Triassic period, which is an era in geological time.

Given Data / Assumptions:
The fixed opening is The existence of, followed by three labelled parts: P can be traced back to, Q mammals on the earth, and R at least the Triassic time. Your task is to order P, Q, and R so that the sentence clearly states how long mammals have existed on earth.

Concept / Approach:
A natural English sentence here would be The existence of mammals on the earth can be traced back to at least the Triassic time. In this structure, mammals on the earth immediately follows the phrase the existence of, because that phrase needs a noun phrase to complete it. The verb phrase can be traced back to follows next, and the time expression at least the Triassic time comes at the end.

Step-by-Step Solution:
Step 1: Attach a suitable noun phrase to The existence of. Only Q mammals on the earth functions as a noun phrase and completes this beginning.Step 2: Place the verb phrase that comments on this noun phrase. P can be traced back to clearly performs this role.Step 3: Reserve the time phrase R at least the Triassic time for the end of the sentence, since time expressions usually come after the main verb phrase.Step 4: Combine the parts in the order Q P R to form The existence of mammals on the earth can be traced back to at least the Triassic time.Step 5: Check for both grammatical correctness and logical scientific sense.

Verification / Alternative check:
If you try a different order such as P Q R, you get The existence of can be traced back to mammals on the earth at least the Triassic time, which is grammatically awkward and places mammals after the verb phrase instead of as the main noun. Other combinations like Q R P or R P Q break normal English word order and make the statement confusing or ungrammatical.

Why Other Options Are Wrong:
QRP produces The existence of mammals on the earth at least the Triassic time can be traced back to, which misplaces the time expression and leaves the preposition to stranded. PRQ begins with can be traced back to, leaving the subject incomplete. PQR and RPQ similarly disrupt the natural flow from subject to verb to time phrase. Only QPR preserves a clear subject phrase, followed by the verb phrase, and finally the time reference.

Common Pitfalls:
Students sometimes rush and pick P first because can be traced back to looks like the main action of the sentence. Others do not recognise that the existence of must be immediately followed by a noun phrase. Always check which fragment can complete the initial phrase and which parts are adjuncts that can be placed later.

Final Answer:
The correct rearrangement is QPR, giving The existence of mammals on the earth can be traced back to at least the Triassic time.