These maps are useful in shedding light on the prehistory of India.
The Harappans
The Gangetic plain is predominantly lin the temperate zone which has the highest population density in India and the most agriculturally oriented economy.
This region had agriculture very early on, probably no later than 4000 BCE and possibly sooner, initially using Fertile Crescent Neolithic crops, via the Caucuses and Iran in its formative era whose migrating farmers account for a significant share of the genetic roots of of the Indus Valley Civilization (a.k.a. the Harappan civilization a.k.a. the Meluhha) and later adopting rice as a crop as well via Austroasiatic migrants from Southeast Asia who gave rise to the Munda languages of India, ca. 2000 BCE.
We know from well dated residues on Harappan pots, by the way, that curry was a Harappan invention that predated the Indo-Aryans.
The prevailing view is that Harappan society was united politically, perhaps in a federation of city-states, and was largely free of war or fortifications, apart from some trade outposts on its frontiers, until it collapsed. It has relatively modern plumbing for the Bronze Age and its cities had less obviously palatial complexes that one might associate with a more hierarchal society dominated by local kings who simply enriched themselves.
Harappan society had its own script, and the majority view is that this was a proto-script used for accounting and trade purposes, similar to the Vinca script in the Neolithic Balkans and the earliest Sumerian cuneiform inscriptions, but not a full fledged written language.
Harappans had trade connections to Sumeria, where they had a historically attested trade colony, and based on those records, know that the Harappans called themselves the Meluhha.
Meluḫḫa or Melukhkha (Sumerian: 𒈨𒈛𒄩𒆠 Me-luḫ-ḫaKI) is the Sumerian name of a prominent trading partner of Sumer during the Middle Bronze Age. . . . most scholars associate it with the Indus Valley Civilisation. . . . Sumerian texts repeatedly refer to three important centers with which they traded: Magan, Dilmun, and Meluhha. The Sumerian location of Magan is now accepted to be the area currently encompassing the United Arab Emirates and Oman. Dilmun was a Persian Gulf civilization which traded with Mesopotamian civilizations. The current scholarly consensus is that Dilmun encompassed Bahrain, Failaka Island and the adjacent coast of Eastern Arabia in the Persian Gulf.
The Harappans also had trade (and possibly a sphere of influence) in the adjacent region of Central Asia known as the BMAC (for Bactria–Margiana Archaeological Complex a.k.a. Oxus culture).
The Ghaggar-Hakra river f.k.a. the Saraswati River, which features prominently in the Rig Vedas, once coursed through what is now arid Indian steppe along the dashed orange route shown in the map above. And, its now mostly dry bed is the home of many Harappan ruins.
But, the Saraswati River dried up around 2000 BCE, as part of a major climate event which made the region more arid that stretched, at a minimum from Egypt to India, and caused the Middle Bronze Age Late Harappan society to collapse.
Dravidian languages, Sanskrit or Paramunda languages could not have been candidates for the [language of the] Indus Valley Civilization which flourished from 2600 BC to 1900 BC in the North-West of India and Pakistan.
If anything, the evidence that none of these three historical linguistic hypotheses can be supported by the evidence is stronger now than it was twenty-five years ago.
Like the Paleo-European languages (of which the only survivor is Basque), the Harappan language can probably never be fully reconstructed, even if we can glean some knowledge of it from inscriptions in Harappan script and substrate influences in Sanskrit and related languages, and areal influences on the Munda languages.
The Indo-Aryans
The Indo-Aryans, who had wheeled chariots and domesticated horses, and were used to a more arid climate, rushed in to fill the political vacuum. They may have had some farming, or may have ruled subject societies of farmers, but were ancestrally herders.
The Indo-Aryans arrived from Central Asia, ca. 2000 BCE - 1500 BCE, bringing with them Sanskrit, that later diversified into the various Indo-European languages of South Asia. The language of the Indo-Aryans extinguished the Harappan language as anything but a substrate influence on Sanskrit.
Another branch of the same people at about the same time gave rise to the oldest Indo-Iranians and the Mittani Kingdom in Northeast Mesopotamia in mostly what is now Iran.
We know that Indo-Aryans were the conquerors and not the conquered, because Indo-Aryan genes are more common in higher castes in India, and because these genes have origins (confirmed with ancient DNA) from outside the Indian sub-continent.
The religion of the Indo-Aryan migrants and the Harappan religion mutually influenced each other to produce the early Vedic religion, that would eventually give rise to modern Hinduism. But, given the differences between the Vedic religion and the religions that emerged in other places that the Indo-Europeans conquered, and the Rig Vedic references to things like the Saraswati river societies that were gone or almost gone by the time that the Indo-Aryans arrived, we know that the Indo-Aryan religious tradition and the Harappan religious tradition both influenced the fused religion that emerged from their fused culture.
Genetic evidence tells us that more than one wave of Indo-Aryan migration affected the area of India where Indo-Aryan languages are now spoken.
The Dravidians
The monsoon driven tropical region (in a medium blue on the map), that makes up most of Southern India, didn't adopt agriculture until the South Indian Neolithic revolution, around 2500 BCE, had significant reliance upon crops domesticated in the African Sahel, and even then, wasn't as optimal for agriculture. The Fertile Crescent package of crops wasn't naturally suited to this climate region and it took many centuries for these crops under the close guidance of early farmers to adapt to this tropical monsoon driven climate.
The Dravidian language family probably has its roots in the South Indian Neolithic revolution, probably from one of the South Asian hunter-gatherer population that was one of the first to adopt agriculture, possibly with some linguistic influence from the Africans who brought the Sahel African crops that made this Neolithic Revolution possible.
While the Harappan society had trading posts at the fringe of what was Dravidian India at the time, mostly along the northwest coast of the Deccan peninsula, the Harappans almost surely did not speak a Dravidian language and had only thin trade ties with Dravidian society.
The first wave of Indo-Aryan migration reached the Dravidian society, leaving traces in genetic admixture found in lower amounts in almost everyone in India, even Dravidians. In this initial wave, the fused Vedic religion took hold (although the preference for vegetarianism found among the formerly Harappan regions did not), and all but a small core of this region probably experienced language shift with their local Dravidian dialects going extinct. The Dravidian society on the eve of the Indo-Aryan arrival was not as technologically advanced as the Harappan one, but also may not have been in as advanced a state of collapse, as it was in an area that the climate event that impacted the Harappans may not have affected so strongly.
But, the Indo-Aryans were spread thin, were in an eco-region less familiar to them, and less completely dominated the Dravidian society. The core region, probably within the region where Telugu is now spoken, that held out expanded and reconquered almost all of the former Dravidian territory, bringing its sole surviving Dravidian dialect with it (which is why the Dravidian language family seems much younger than one that stretches back to the South Indian Neolithic). But this reconquest never ended up replacing the Vedic religion that had replaced or absorbed its own religion (which may not have been as well-developed and arose in an illiterate society). No later big wave of Indo-Aryan migration followed this reconquest.
The range of the Dravidian languages was probably wider than it is today, possibly extending to the fringes of the Indus River Valley civilization, as indicated by toponyms in these regions.
But, the North Dravidian languages (Brahui in what is now Pakistan) and the Kurukh-Malto languages of Northeast India), were probably not part of the original Dravidian language range and were the product of much later colonizations (probably around 1000 CE in the case of Brahui, where it was spoken as a result of an elite driven language shift, similar to the one that occurred at about the same time in what is now Hungary, rather than a mass migration of Dravidians to the region). Oral traditions among the Krukh-Malto peoples, at least, assign their origins to Dravidian homelands further south.