I have done some research on South Asian prehistory and genetics. Here is the politically correct summary from AI:
The Comprehensive Trajectory of the Indus-Sarasvati Civilization: From Twin Origins to Post-Urban Collapse
The lifespan of the Harappan Civilization is defined by a distinct geographic shift: beginning as localized cultures in the west, merging into a unified urban superpower, and ultimately fracturing into a de-urbanized rural population. Analyzing this timeline reveals a dual-river origin story that explains the distinct territorial, genetic, and cultural legacies shared by modern Pakistan and India.
Phase 1: The Pre-Harappan Formative Phase (c. 7000 – 3300 BCE)
The Highland Roots and the Dual-River Valleys
The civilization did not arrive from a foreign land; it began as indigenous farming communities developing simultaneously along two separate, parallel river systems.
- The Western Frontier (Entirely Pakistan): The fundamental Neolithic foundation of the civilization was born exclusively within modern Pakistan. At Mehrgarh in the Balochistan foothills, hunter-gatherers transitioned to pastoral farming, mud-brick architecture, and early pottery as early as 7000 BCE. Populations gradually expanded along the mountain piedmont, eventually moving down into the fertile but volatile alluvial plains of the Indus River, giving rise to advanced metallurgical cultures like Kot Diji and Amri-Nal.
- The Eastern Frontier (Geographically Shared): Simultaneously and independently, a distinct parallel culture emerged along the Ghaggar-Hakra River system (often identified with the ancient Sarasvati). This perennial, monsoon-fed river ran from northwest India right across the modern border into eastern Pakistan's Cholistan Desert. Indigenous communities established the Hakra Ware Culture at early sites like Bhirrana and Kunal (Haryana, India), utilizing the river's gentler, more predictable flood cycles to build dense networks of farming villages long before any major eastward migration occurred.
Phase 2: The Early Harappan Regionalization Era (c. 3300 – 2600 BCE)
Fortification and the Mountain "Highway"
As populations outgrew their initial valleys, they expanded along the base of the northern mountains. This geographical path naturally led western groups to intercept the headwaters of the Ghaggar-Hakra system.
During this era, regional agricultural networks transformed into fortified, organized towns. Locations like
Early Harappa (Punjab, Pakistan) and
Kalibangan (Rajasthan, India)—which features the world's earliest discovered plowed agricultural field dating to c. 2900 BCE—began developing independent trade networks, early administrative structures, and rudimentary granaries.
Phase 3: The Mature Harappan Integration Era (c. 2600 – 1900 BCE)
The Great Economic Merger and Imperial Peak
Around 2600 BCE, expanding trade networks reached a tipping point, prompting the purely Pakistani Indus cultures and the geographically shared Hakra cultures to merge into a single, monolithic civilization. Operating much like a prehistoric trading bloc, these independent regional groups mutually adopted a universal "standard package" to maximize economic efficiency:
- Standardized Engineering: All major settlements stopped using local dimensions and adopted an identical, mathematically precise mud-brick baking ratio of 1:2:4 (thickness to width to length).
- Unified Metrology: Merchants across both river systems utilized the exact same system of highly accurate stone cubical weights.
- The Indus Script: A shared symbol system was invented, allowing merchants from different valleys to seamlessly read and verify commercial seals.
The Resulting Modern Territorial Claims
This integration created a Bronze Age superpower spanning 1.5 million square kilometers. However, the distribution of power during this peak era establishes the foundation for modern national claims:
- The Case for Pakistan (The Civic Heartland): Pakistan holds a definitive territorial claim as the custodian of the imperial core. The apex megacities and dominant capitals—Mohenjo-Daro and Harappa—sit entirely within its borders, housing 75% to 80% of the civilization's dense urban population. The entire empire was structurally and logistically anchored to the Indus River spine.
- The Case for India (Site Density and Genetics): Modern India contains a significantly higher total volume of discovered sites (over 900 out of roughly 1,400 total), serving as critical industrial outposts like Lothal (Gujarat) or sprawling frontier metropolises like Rakhigarhi (Haryana). Furthermore, ancient DNA (aDNA) confirms that the population was not a genetic monolith; eastern and southern sites maintained a higher percentage of Ancient Ancestral South Indian (AASI) lineage, making modern Indian populations the closest living genetic descendants of the Harappan people.
Phase 4: The Late Harappan Localisation Era (c. 1900 – 1300 BCE)
Climate Shift and De-Urbanization
The civilization did not suffer a sudden military conquest. Instead, shifting tectonic plates, weakened monsoons, and severe environmental degradation caused the core river basins—particularly the Ghaggar-Hakra system—to dry up completely, while the main Indus River suffered catastrophic, unpredictable flooding.
By
1900 BCE, the signature civic infrastructure collapsed. The sophisticated empire fractured into its rural aftermath:
- Standardized weights, advanced drainage networks, uniform brick ratios, and the written Indus script vanished.
- The population underwent a multi-generational, post-urban drift away from the dried-up core valleys. Survivors migrated eastward into the fertile Ganga-Yamuna Doab (establishing late pottery workshops like Alamgirpur and cemeteries like Sanauli in western Uttar Pradesh) and southward into the Saurashtra peninsula of Gujarat.
By 1300 BCE, the grand urban traditions were completely gone, leaving behind fragmented, rural populations whose descendants eventually integrated into the broader cultural fabric of the subcontinent.