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Biogeographical Evidence for Paleogeography

Review Biogeographical Evidence for Paleogeography through official answer keys.

Official Question UPSC 2026

Consider the following assertion : In the Pleistocene period either the Yamuna once flowed into the Indus, or the Sutlej flowed into the Yamuna and one major tributary of either had shifted from the Ganga to the Indus or vice versa. Which of the following is/are the basis of the above assertion ? 1. The Nadi-Sukta of the Rigveda 2. The explorations of the Sutlej and the Yamuna by Robert Bruce Foote 3. The presence of the same species of dolphins in both the Indus and the Ganga river systems Select the answer using the code given below :

Solution & Analysis

Answer: D
The assertion describes the geomorphological theory of the Indo-Brahma (or Shiwalik) River—a massive ancient river that geologists believe was broken apart into the modern Indus and Ganga systems due to tectonic uplift (like the Delhi Ridge) during the Pleistocene epoch. Let's evaluate the evidentiary basis for this theory:
•Statement 1 is Incorrect: The Nadi-Sukta of the Rigveda indeed names these rivers, but it was composed during the Holocene (recent human history, approx. 1500 BCE). The Pleistocene epoch ended over 11,000 years ago. Therefore, a human text cannot serve as the primary scientific basis for a pre-human geological event.
•Statement 2 is Incorrect: Robert Bruce Foote is a legend in Indian archaeology, famous for discovering Paleolithic stone tools (handaxes) in 1863. He was not a paleogeographer focused on mapping the tectonic shifting of the northern Himalayan river systems.
•Statement 3 is Correct: This is a classic example of biogeography validating geology. Freshwater river dolphins physically cannot cross land or survive in the salty ocean. Finding two closely related, highly specialized species of blind river dolphins (Platanista gangetica minor and Platanista gangetica gangetica) in two river systems that are currently completely physically isolated proves that, at some point in geological history, the Indus and Ganga basins were a single, continuous, connected waterway.
Conclusion: Only biological distribution (Statement 3) provides the scientific basis for the assertion.

Quick Recall

Geologists theorize that during the Pleistocene epoch, the Yamuna and Sutlej rivers shifted, breaking apart a massive ancient river (the Indo-Brahma) into the separate Indus and Ganga basins. A primary basis for this paleogeographical assertion is biology: the presence of nearly identical species of blind river dolphins trapped in both isolated river systems proves they were once physically connected.

Concept Flow Mapping

Indo-Brahma River
Separated Into
Indus and Ganga Basins
River Separation
Occurred During
Pleistocene Epoch
Indus and Ganga Dolphins
Provide Proof Of
Past Geological Connection

Concept Question

What specific biological anomaly provides evidence for the geological theory that the Indus and Ganga river basins were once a single, connected river system?

Key Takeaway

The presence of closely related species of blind river dolphins in both completely separated basins.

Examiner's Trap

The examiner uses chronological mismatching to trick candidates. Option 1 sounds plausible because the Vedas mention these rivers, but the Vedas are only 3,500 years old, whereas the assertion is about the Pleistocene epoch (millions of years ago). Option 2 uses a famous name (Robert Bruce Foote), but he studied stone tools, not river mapping.

Core Insight

Freshwater river dolphins cannot survive in the salty ocean, nor can they walk across land. Therefore, if you find nearly identical species of blind freshwater dolphins in the Indus (Pakistan) and the Ganga (India)—which currently have zero physical connection—the only scientific conclusion is that millions of years ago, before tectonic forces created the Delhi Ridge water divide, those two rivers were a single, continuous waterway.

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