Abstract
From the early Holocene onward, the Indian Subcontinent has accommodated a range of diverse human cultures and associated ecological adaptations and lifestyles. Around 10 kyrs ago, the Subcontinent has witnessed the development of later Mesolithic hunter-gatherers and their subsequent regional transitions to pastoralist (Neolithic) and agricultural (Chalcolithic) lifeways. The Holocene climate records reveal discrepancies in the timing and duration of climatic events, which can be attributed to a vast geographic isolation, the influence of height, elevation, and local climatic conditions. These changing climatic patterns including the development of a geographically variable monsoon directly impacted these various cultures including the Harappans and their contemporaries as well as younger Historical and Medieval empires across India, at various levels. In some regions, environmental changes led to uneven cultural transitions, geographic migrations, and the development of regionally-distinct material cultures along with establishment of sedentary life-ways. This paper attempts to present a review broadly correlating general climatic patterns throughout the Holocene period of India with regional cultural dynamics. All geomorphic-climatic zones of the Subcontinent showed strong inter-proxy coherence between 9 and 5 kyrs in response to increased precipitation. After this warming period ends, we see a moderate dry period as a result of a weakening monsoon and an overall tendency toward aridity throughout all zones (after 4 kyrs). The temporal variation of human habitation and respective adaptive responses suggest broad linkages to the varying climatic and physiographic features at a regional scale. Learning how this shaped human eco-dynamics in the past can help us expand our understanding of human history and implement lessons for the present as well as the future.
Background
Studies on various climatic proxies from the current interglacial interval have presented clear evidence of significant variability and abrupt changes. Climatic changes have long influenced human behavior in terms of settlement patterns and adaptation strategies. In recent decades, interdisciplinary studies have majorly focused upon gauging this complex relationship between large-scale climatic changes and changing cultural histories on a regional scale (Bevan et al., 2017; Crombé et al., 2015; Griffith, 2014; Helama et al., 2013; Manninen, 2014; Tallavaara and Seppä, 2012). The role of environmental change in the development of human populations and associated cultural/behavioral evolution has become an emerging topic of research interest with the escalating development of more accurate scientific data (Crema et al., 2016; Griffiths and Robinson, 2018). The Holocene epoch in particular, has witnessed a series of rapid climate change events on a global scale as depicted by the developing proxy approaches (Alley et al., 1997; Bond et al., 1997; Bustamante et al., 2016; Carroll et al., 2012; Cheng et al., 2009; Daley et al., 2011; DeMenocal et al., 2000; Dixit et al., 2014a; Giesecke et al., 2011; Liu et al., 2013; Morrill and Jacobsen, 2005; Poore et al., 2003; Prasad et al., 2009; Shuman, 2012; Zillén and Snowball, 2009). Indeed, a lot of studies have pointed to climate change as the main element responsible for the decline of various human civilizations and settlements or the long-term societal transformations. (e.g. Bhattacharyya et al., 2014; Brookfield, 2010; Brook et al., 2010; Brooks, 2006; Brzoska and Fröhlich, 2016; Cullen et al., 2000; DeMenocal et al., 2000; Haug et al., 2003; Riedel et al., 2021; Rosén and Hammarlund, 2007; Stanley et al., 2003; Weiss and Bradley, 2001). However, less light has been shed on the comparability of paleo-climatic and historical data to understand the role of environmental/ climatic conditions in the cultural modifications of human settlements (Davies and Watson, 2007; Kuzucuoğlu, 2007; Schulting, 2010; Ur, 2010). Human populations were and continue to be subjected to varying degrees of vulnerability based upon the ecosystems and have resulted in either socio-political collapse and fragmentation (Riede, 2009; Wicks and Mithen, 2014) or have opened different opportunities for cultural adaptations and resilience (D’Andrea et al., 2011; Manninen, 2014; Robinson, 2013). Settlements vary significantly within a geographic location and the impact of global climatic changes on local or regional scales are often assumed rather than scientifically demonstrated (Enzel et al., 1999; Sanders, 2003; Singh, 1971). Enhancement in the understanding of past environmental and population dynamics have helped us develop better apprehension for climate-human relationships. The concepts of adaptive response, vulnerability and resilience are now not only confined to modern communities but are also relevant for comprehending the cultural dynamics of the past (e.g. Adger et al., 2005, 2009; Janssen et al., 2007; Nelson et al., 2007; Redman, 2005).
This review aims to evaluate the contribution of climatic changes and their multifaceted relationships with the dynamic cultural history of the Indian Subcontinent throughout the Holocene (e.g. Mishra et al., 2020). The composite environmental diversity and specific geographic barriers separate the Indian Peninsula from the rest of the continent rendering it as a laboratory suitable for long-term socioecological studies (e.g. Kotlia et al., 2021). The time period of 11.7 kyrs BP to present, demarcated as the Holocene epoch, has witnessed a wide range of cultural diversification followed by extensive transitions within stratified civilizations including complex social structures (Anderson et al., 2007; Brooks, 2006; Foster et al., 2006; Richerson et al., 2001; Walker et al., 2012). We can evaluate the influence that climate played in the growth or collapse of human settlements by studying the dynamics of the various environmental variables and the accompanying human habitations. Using published studies on climatic conditions from the early Holocene to the present, the Indian Subcontinent was strategically divided into six zones in order to gain a regional geographic perspective. The zonation was carried out to reflect regional ecological and cultural diversity as well as potential links between archeological and historical data and paleoenvironmental contexts. With certainty, the main transformation processes under examination may be viewed as paradigmatic of this. These six zones include (1) Zone I: representing the Northern Himalaya and the nearby biogeographical regions, (2) Zone II: the North-western part that covers the Thar Desert, (3) Zone III: representing Western and Central India, (4) Zone IV: covering Eastern and North-east region, (5) Zone V: showing the Ganga plains demarcated by the alluvial plains of Northern India, and (6) Zone VI: including the Southern per extra-peninsular zone. The primary goal of this review article can be described through the following three objectives: (a) to evaluate and delineate short term as well as long term Holocene climatic variability through compilation of available high resolution multiple proxy based records throughout the Indian Subcontinent, (b) to broadly assimilate the archeological and historical data sets in order to build up a clear picture of past human habitation and establish periods of cultural discontinuities or transitions during the Holocene and (c) to produce a summarized research information about the challenges faced by the past societies and the actions as an outcome of the varying climatic and environmental conditions to determine the possible link among them. The high-resolution integration of climatic and cultural information also provides a suitable approach to address the testable hypotheses regarding abrupt versus gradual causal mechanism of cultural shifts. The paper primarily aims to increase awareness of the significance of environmental factors, such as climatic and demographic changes, on historical cultural transitions and declines, which are frequently ignored. It also assesses readiness for an uncertain future due to climate change in the Anthropocene.
Present climate
The monsoon is the dominant controlling factor of the present climatic conditions of the Indian Subcontinent (Figure 1). The monsoon regime is, in turn, controlled by multiple components such as solar activity, uneven surficial heating of land and water masses, orographic settings of the region, the extent of snow cover and circulation of the upper jet stream (Evans and Slaymaker, 2004; Meehl, 1994; Peyrillé and Lafore, 2007; Singhvi and Krishnan, 2014; Wu et al., 2012). In addition, factors such as carbon dioxide concentrations and El Nino-Southern Oscillation (ENSO) have also been linked to the gradual intensification of the monsoon during the Holocene (Liu et al., 2012; Prodhomme et al., 2014; Yamada et al., 2018). The high spatial variability reported for the rainfall distribution pattern of monsoon-prone regions are an outcome of these mentioned factors. The Indian summer monsoon (ISM), which is a part of the Asian monsoon system, brings in a seasonality pattern in the Indian Subcontinent, causing plenty of rainfall from the months of June to early September (Ashok et al., 2004; Rajeevan et al., 2010). As per the records of the India Meteorological Department, the Southwest monsoon accounts for 74% of annual precipitation (Singhvi and Krishnan, 2014). The northward migration of the Inter Tropical Convergence Zone (ITCZ) occurring during these months results in the in-flow of southwest surface winds landwards, which in turn accumulates ample moisture and brings in a large amount of rainfall. This precipitation maximum is achieved by latent heating of the Tibetan land mass and Bay of Bengal, resulting in deep convection over the northern part of the Bay of Bengal. This convection developed shifts northwest wards toward the Ganga plains causing gradient precipitation in Northwest India. The convective system gradually dissipates as it progresses along this path across the Indian Peninsula. Upon subsidence, the descending air warms and humidity falls, hence decreasing the probability of additional precipitation. The air mass over Arabia descends to offset convection over India, and the tropical easterly jet slows over southern India and the Arabian Sea, causing subsidence over Pakistan, Somalia, and Arabia. Hence, any further convection arising from the heated landmass is restricted by the subsiding air above it, preventing the air mass to rise beyond the lower troposphere. This phenomenon is associated with the characteristic drought experienced in and around the deserts surrounding the Western Arabian Sea (Fasullo and Webster, 2003). A very crucial facet of the South Asian summer monsoon is the seasonal variability faced by the Indian Subcontinent in terms of fluctuating rainfall. Normal precipitation gradient of the South Asian monsoon is manifested during the period of “active monsoon” whereas spells of weak precipitation are termed as “weak monsoon.” These intervals of “weak monsoon” interrupt the precipitation session of the “active monsoon” every 40 days (Yasunari, 1980). Spells of one to three “weak monsoons” club together to form a phase called “break monsoon,” which last approximately 2 weeks long. Annual precipitation is low when prolonged and frequent “break monsoon” phases are associated (Krishnan et al., 2000; Rajeevan et al., 2010; Ramamurthy, 1969). During winters however, the ITCZ retreats southwards of the equator, bringing the Indian Winter Monsoon (IWM). A large portion of Central Asia, including Siberian High and India, turns into a high-pressure zone because of fast cooling of the continental masses. The cold, dry winds originating at high northern latitudes travel over continental masses creating a drier monsoon. During its journey through the offshore, it absorbs moisture from the Bay of Bengal and entrenches itself over the southeastern coast of India and the east coast of Sri Lanka (Kumar et al., 2007; Misra and Bhardwaj, 2019; Sreekala et al., 2012; Zubair and Ropelewski, 2006). Statistically, IWM accounts for about 11% of the annual rainfall of India, with many zones of the southern peninsular tip receiving 30–60% of their annual rainfall from it (Rajeevan et al., 2012). The months from October to December are also associated with cyclones triggered by the Westerlies (Prasad and Enzel, 2006). The monsoon has varied immensely both in spatial as well as temporal aspects owing to its various controlling components, however it has never failed totally. Hence, it can be determined from the present climatic structure of Indian Subcontinent that monsoon precipitation is indeed a very crucial variable that affects the broader environment-society interface.
Schematic depiction of the major characteristics of winds in SW Asian controlling the present climatic conditions of Indian Subcontinent.
Holocene climatic variability
Geological archives such as sediments from lakes, dunes, depositions by rivers and lagoons, with approaches as sedimentological and geochemical records, isotope ratios, pollen, phytoliths, speleothems, peat deposits, tree rings, and so on can be used to reconstruct the pre-instrumental monsoonal records. The information acquired is generally qualitative since the signals maintained by these geological proxies are variations in response to climatic/ monsoonal or temperature fluctuation. In this current study, both marine and terrestrial records are deployed to understand the climatic variability of different zones (as mentioned earlier) of the Indian Subcontinent for the past 11.7 kyrs (Figure 2 and Table 1). This overview compiles the existing studies based on their chronological resolution, data interpretability, climate transitions, and geographical dispersion.
List of paleoclimatic studies discussed in the study classified into various zones.
(i) Zone I (Northern Himalaya and adjacent regions)
Locations of paleoclimatic studies discussed in the study coupled with geographic zones (one. Garhwal Himalaya (Phadtare, 2000); 2. Chandra Tal (Rawat et al., 2015); 3. Sutlej River (Bookhagen et al., 2006); 4. Kilang Sarai (Bohra and Kotlia, 2015); 5. Spiti Valley (Anoop et al., 2013a); 6. North Pulu Lake (Phartiyal et al., 2020); 7. Benital Lake (Bhushan et al., 2018); 8. Kumaun Himalaya (Kotlia et al., 2010); 9. Sainji Cave (Kotlia et al., 2015); 10. Kalakot (Kotlia et al., 2016); 11. Dharamjali Cave (Kotlia et al., 2018); 12. Badanital Lake (Kotlia and Joshi, 2013); 13. Tso Moriri (Mishra et al., 2015); 14. Deoria Tal (Niederman et al., 2021); 15. Manasbal Lake (Babeesh et al., 2019); 16. Mansar Lake (Das and Malik, 2010); 17. Tso Kar (Demske et al., 2009); 18. Chandratal Lake (Shamurailatpam et al., 2021); 19. Lunkaransar Lake (Swain et al., 1983); 20. Didwana (Singh et al., 1990); 21. Lunkaransar Lake (Enzel et al., 1999); 22. Wadhwana Lake (Prasad et al., 2014a); 23. Riwasa (Dixit et al., 2014a); 24. Kotla Dhar (Dixit et al., 2014b); 25. Karsandi (Dixit et al., 2018); 26. Banni Grassland (Makwana et al., 2021); 27. Luni, Sabarmati and Mahi River (Jain et al., 2004); 28. Rann of Kachch (Chatterjee and Ray, 2017); 29. Dholavira (Sengupta et al., 2020); 30. Didwana and Lunkaransar Lake (Bryson and Swain, 1981); 31. Nal Sarovar (Prasad et al., 1997); 32. Panch Pimpla, Sina River Basin (Behera et al., 2022); 33. Amjhera Swamp (Chauhan and Quamar, 2012); 34. Padauna Swamp (Chauhan et al., 2013); 35. Lonar Lake (Anoop et al., 2013b); 36. Lonar Lake (Prasad et al., 2014b); 37. Lonar Lake (Menzel et al., 2014); 38. Lonar Lake (Sarkar et al., 2015); 39. Lakadandh Swamp (Quamar and Bera, 2017); 40. Nonia Tal (Kumar et al., 2019); 41. Deosila Swamp (Dixit and Bera, 2011); 42. Mahandi River (Tripathi et al., 2014); 43. Ziro Lake (Ghosh et al., 2014); 44. PT Tso Lake (Mehrotra et al., 2019); 45. Dzüko Valley (Misra et al., 2020b); 46. Dzüdza River (Imsong et al., 2022); 47. Mawmluh Cave (Dutt et al., 2015); 48. Mawmluh Cave (Lechleitner et al., 2017); 49. Mawmluh Cave (Kathayat et al., 2018); 50. Mawmluh Cave (Kalpana et al., 2021); 51. Debaka Swamp (Dixit and Bera, 2012); 52. Chayagaon Swamp (Dixit and Bera, 2013); 53. Bay of Bengal (Rashid et al., 2011); 54. Godavari River (Ponton et al., 2012); 55. Rushikulya River (Ankit et al., 2017); 56. Kolleru Lake (Rao et al., 2020); 57. Chilika Lagoon (Amir et al., 2021); 58. Chilika Lagoon (Ahmad Shah et al., 2022); 59. Shilloi Lake (Ankit et al., 2022); 60. Jalesar Lake (Trivedi et al., 2013); 61. Lahuradewa Lake (Saxena et al., 2013); 62. Chaudhary-Ka-Tal (Saxena et al., 2015); 63. Kikar Tal (Saxena and Trivedi, 2017); 64. Baraila Tal (Misra et al., 2020a); 65. Nakta Lake (Quamar and Kar, 2020); 66. Palar River (Resmi et al., 2017); 67. Palar River (Resmi and Achyuthan, 2018); 68. Cauvery delta (Mohapatra et al., 2021); 69. TSpettai mangrove wetland (Srivastava and Farooqui, 2013); 70. Kukkal Lake (Rajmanickam et al., 2017); 71. Shatisagara Lake (Sandeep et al., 2017); 72. Ennamangalam Lake (Mishra et al., 2019)).
Zone I is designated to Northern Himalayas and the nearby high-altitude landscapes. The region is primarily influenced by the Southwest monsoon and the winter westerlies. Kotlia et al. (2016) carried out isotopic analysis (δ13C and δ18O) of a 20 cm long calcite stalagmite from Kalakot in northwest Himalayas. The study recorded an episode of high precipitation from 12.2 to 9.5 cal kyrs BP marking the transition from Pleistocene to Holocene. Similar transition into the Holocene with increased monsoon phase spanning from 12 to 8.3 cal kyrs BP have been reported in a prior multi-proxy study on sedimentary sequence by Kotlia et al. (2010). Demske et al. (2009) found an increased ISM (Indian Summer Monsoon) phase from the Tso Kar Lake in Ladhakh, Himalaya, between 10.9 and 9.2 cal kyrs BP. Additionally, other paleoclimatic research from this region documented similar circumstances of enhanced monsoon roughly on the same historical scale (Bookhagen et al., 2006; Ghosh et al., 2020; Mishra et al., 2015; Rawat et al., 2015). Bhushan et al. (2018) suggested multiple phases of an enhanced monsoon and of varying magnitude from the sedimentological and geochemical proxies of the Benital lake sequence in the Central Himalaya occurring at 10–9.6 cal kyrs BP, 9.5–9.2 cal kyrs BP, and 8.6–5.8 cal kyrs BP. However, the period from 8.3 to 6 cal kyrs BP has been documented as a prominent cold excursion in warm early Holocene by number of studies (Kotlia et al., 2010, 2016). Das and Malik (2010) carried out biogeochemical and amino acids analysis of a 30 m core retrieved from Mansar Lake, Central Himalaya, which suggested a warm and humid climatic regime during 7.58 cal kyrs BP, followed by a switch to aridification at ~5.6 cal kyrs BP. Preliminary paleoclimatic records available from a pollen study and magnetic susceptibility data of peat profile from the Garhwal Himalaya (Phadtare, 2000) states that cold and warm phases occurred from the period of 7.8 cal kyrs BP. The warming of climate since 7.8 cal kyrs BP indicates strengthening of the summer monsoon, which reaches its maximum peak at about 5 cal kyrs BP. Environmental reconstruction from the Spiti Paleolake sediments by Anoop et al. (2013a) indicated a weaker monsoon with cooler conditions from the period of 7.6 to 6.8 cal kyrs BP, followed by a phase of an enhanced monsoon from ~6.8 to 6 cal kyrs BP, hence contrasting the forgoing record. The biogeochemical analysis of sediments retrieved from Chandratal Lake by Shamurailatpam et al. (2021) documented a period of an intensified monsoon with a wet climate during ~6.3–5.8 cal kyrs BP. From the multi-proxy analysis of the sedimentary sequence in Kumaun Himalaya, Kotlia et al. (2010) also documented a phase of monsoon weakening at 5–4.5 cal kyrs BP. Similar records of arid event at 4.2 cal kyrs BP have been reported by several studies in Central Himalaya indicating this drought pulse as a prominent signature of global climatic event (Bohra and Kotlia, 2015; Das and Malik, 2010; Kotlia et al., 2015). Kotlia et al. (2018) produced high resolution climatic records between ~4.0 and 1.9 cal kyrs BP from isotopic analysis of stalagmites from Dharamjali Cave in Kumaun Himalaya. A gradual decline in precipitation is observed from ~ 3.7 to 3.0 cal kyrs BP with spells of drought centered at ~3.4, ~3.2, and ~3.0 cal kyrs BP. Climatic amelioration from ~2.9 to 2.7 cal kyrs BP is followed by further decline in precipitation at ~2.7-2.4, ~2.4–2.3 cal kyrs BP and at 2.1 cal kyrs BP. Ali et al. (2019) extracted a 13 m sediment core from Penzi-la Pass, Zanskar Valley in the NW Himalaya and carried out geochemical and sedimentological analysis on it. The data suggested a weak phase of ISM with a cold and dry climate between ~5 and 4 cal kyrs BP which ameliorated at ~4.5–3.4 cal kyrs BP. This phase was followed by a weak to moderate monsoon during ~3.5 to 2 cal kyrs BP. Abrupt dry spells were encountered at ~3.3, 2.6, 1.7, and 0.4 cal kyrs BP with gradual increase in moisture availability. Kotlia and Joshi (2013) suggested similar interpretations from the geochemical study of sediment core reterived from Badanital Lake (Garhwal Himalaya). The study reported a cold/ dry phase from ca. 5.1 to 3.5 cal kyrs BP followed by a moist phase from ca. 3.5 to 1.8 cal kyrs BP. Further arid conditions were observed at 1.8–0.9 cal kyrs BP and 0.44–0.16 cal kyrs BP followed by modern warming from 0.16 cal kyrs BP onward. In a multi-proxy approach by Phartiyal et al. (2020) from North Pulu Lake, an intermitted warm and wet period was suggested during 5.4–4.8 cal kyrs BP followed by a cold and dry phase until 4.4 cal kyrs BP. This phase was marked as a transition to a warmer phase between 4.4 and 2 cal kyrs BP and the period of ~2–1.7 cal kyrs BP was colder followed by warmer spells during 1.2–1.1, 1.1–0.9, 0.9–0.6, and 0.6–0.4 cal kyrs BP, respectively. Phadtare (2000) also reported a gradual decrease in monsoon strength from the period of ~4.5 to 3.5 cal kyrs BP. Further, an unstable trend of strengthening occurred until ~1.5 cal kyrs BP, followed by a weakening monsoon that intensified at 0.8 cal kyrs BP. Niederman et al. (2021) also reported episodes of dry events at 4.8, 4.2, and 3.1 cal kyrs BP based on multi-proxy analysis of sediment core retrieved from Deoria Tal. Contradicting this, Bhushan et al. (2018) reported periods of enhanced monsoon at 5–4.2, 3.5–2.4, and 1.8–1 cal kyrs BP, respectively with relatively cold and dry periods occurring at 5.8–3.7 and 2.1–0.69 cal kyrs BP. Similar amelioration in climatic conditions at 3.7–2.1 cal kyrs BP have been reported by Shamurailatpam et al. (2021). Babeesh et al. (2019) suggested 3.3–2.5 cal kyrs BP and ~1.8–1.3 cal kyrs BP as periods of intense precipitation based on multi-proxy study of sediment core retrieved from Manasbal lake. The study further stated the periods of 3.4–3.3 cal kyrs BP and 2.5–1.8 cal kyrs BP as relatively dry and cold phases.
(ii) Zone II (North-western part including Thar Desert)
Numerous records on significant climatic variations are available from the Thar Desert and its margins which constrains Zone II. This zone receives the most gradient precipitation from the Southwest Monsoon along with winter rainfall due to the mid-latitude westerlies and the Holocene records established from here can be helpful to understand the regional variation of monsoonal precipitation. Initial work includes pollen analysis of lacustrine sediments of Lunkaransar by Singh (1971). However, the study lacked a stable chronometric framework for the region. Bryson and Swain (1981) and Swain et al. (1983) established a more uniform chronology of these lake sediments and reconstructed the climate based upon a precipitation-sensitive pollen study. Their reconstruction suggested intensified precipitation from ~11.5 cal kyrs BP which peaked at ~6.3 cal kyrs BP. In a similar palynology study by Singh et al. (1990), a period of high precipitation was suggested during ~10.4–8.6 cal kyrs BP. Enzel et al. (1999) proposed similar climatic variations from geochemical analysis of sediments collected from Lunkaransar. According to the study, a phase of wet conditions prevailed from 10 to 4.8 cal kyrs BP with amelioration at ~6.3–4.8 cal kyrs BP. Prasad et al. (1997) used organic geochemical information of sediments collected from Nal Sarovar for paleoclimatic reconstruction. Based on this study, an arid phase existed from ~7.8 to 7.2 cal kyrs BP and was followed by a period of climatic amelioration from ~7.2 to 6.1 cal kyrs BP. The period from ~6.1 to 5.4 cal kyrs BP was marked as dry climate due to reduction in lake levels. In a multi-proxy approach from Wudhwana Lake in mainland Gujarat, Prasad et al. (2014a) documented a wet phase from ~7.5 to 6.7 cal kyrs BP followed by onset of a dry phase at around 5.5 cal kyrs BP. Similar wet phase was established during ~7.5 cal kyrs BP with intensified monsoon followed by a dry phase from 7.5 to 5 cal kyrs BP by Dixit et al. (2018) in an isotopic analysis of Karsandi sediments from northern Rajasthan. Further paleohydrologic studies carried out in this zone suggest higher freshwater levels in the lakes of Rajasthan and enhanced fluvial activity in the southern Thar during the early to Mid-Holocene, that is, ~8–5 cal kyrs BP (Jain et al., 2004; Thomas et al., 2007). Chatterjee and Ray (2017) suggest a regional decreasing trend of monsoon strength since ~7 cal kyrs, from geological and geochemical analysis on samples from the Great Rann of Kachchh. Prasad et al. (1997) also reported the period from 5.4 cal kyrs BP onward, a phase of gradual drying conditions prevailed with major dry spells occurring at 4.2, 3.3, and 1 cal kyrs BP, respectively. Enzel et al. (1999) also suggested similar interpretation from geochemical analysis of sediments implying the period of 4.8 cal kyrs BP to present as a gradual drying phase. A spell of extreme dry phase was encountered by Prasad et al. (2014a) at ~4.2 cal kyrs BP, followed by strengthening of the monsoon from ~3.5 cal yrs BP until the present with another pulse of dry phase from ~3.2 to 2.7 cal kyrs BP. Dixit et al. (2018) also reported intensified dry conditions from 4.4 to 3.2 cal kyrs BP as indicated by the complete drying of the Karsandi playa lake, followed by aeolian deposition at its location. Makwana et al. (2021) suggested arid phase occurring during 4.8–4.4 cal kyrs BP, 3.3–3 cal kyrs BP and at 2.4 cal kyrs BP from the geochemical and mineral magnetic proxies-based analysis of sediments from Banni grassland, Rann of Kachchh. Whereas phases of humid climatic conditions have been reported by this study during >4.8 cal kyrs BP, 4–3 cal kyrs BP, 1.9–1.4 cal kyrs BP and 0.9–0.5 cal kyrs BP. Other studies have also suggested a similar trend of intensifying aridity in this region from ~5 kyrs onward with a major drought event occurring at ~4.1 cal kyrs BP (Dixit et al., 2014a, 2014b; Staubwasser et al., 2003). Short spells of wet phase have been reported by Chatterjee and Ray (2017) during 5.1–4.7 cal kyrs BP, 4.1–2.6 cal kyrs BP and the Medieval Warm Period (MWP) accompanied by gradual decreasing trend of monsoon as mentioned earlier. Prominent signatures of such aridification have also been reported by Sengupta et al. (2020) from the isotopic analysis of gastropods collected from Dolvaria in Kutch, Gujarat. According to the study, intensity of monsoon was comparatively higher during the Mid-Holocene phase (δ18O and δ13C analysis of T.palustris dating to an age of 4.7 cal kyrs BP) than the Late-Holocene (T.palustris age ~4.3 cal kyrs BP) or present. Hence, from ~5 cal kyrs BP onward to present, the region appears to experience gradual aridification with the lowering of lake levels and reduction in fluvial activity (Madella and Fuller, 2006).
(iii) Zone III (Western and Central India)
The low-pressure system (LPS) developed near the Bay of Bengal crosses the central part of India and establishes the core monsoon zone (CMZ). This region as well as the Western margin of the Indian Subcontinent are considered as Zone III. In order to comprehend the ISM variability in CMZ, Kumar et al. (2019) used a multi-proxy method on a 1.54 m core collected from Nonia Tal in Madhya Pradesh. They discovered that the ISM was enhanced between 11.4 and 9.5 cal kyrs BP. After analyzing lipid biomarkers on sediments from Lonar Lake, Sarkar et al. (2015) postulated that the early Holocene (10.1–6 cal kyrs BP) experienced a period of enhanced monsoon activity. A gradual shift toward drier climatic conditions was recorded beginning around 6 cal kyrs BP and peaked around 5.1 cal kyrs BP. The 4.8–4 cal kyrs BP period was marked as a transition phase to complete an arid climate which prevailed throughout the Late-Holocene after 4 cal kyrs BP, indicated by the permanent presence of a saline lake. Behera et al. (2022) suggested gradual trend of intensifying aridification from 7.5 cal kyrs BP onward through the geochemical and sedimentological analysis of a fluvial sequence in the Sina River Basin. In a multi-proxy approach by Anoop et al. (2013b), a 10 m long core was retrieved from Lonar Lake to document the paleoclimatic conditions of the Late-Holocene marking the period of 4.6–3.8 cal kyrs BP as the onset of a weak summer monsoon. Similar studies carried out on sediments from Lonar Lake suggest an enhanced monsoon during the early Holocene which is followed by two prolonged spells of drought at 4.6 and 3.9 cal kyrs BP (Menzel et al., 2014; Prasad et al., 2014b). Kumar et al. (2019) also implied a gradual weakening of ISM from 9.5 cal kyrs to 2 cal kyrs BP. However, an abrupt increase in ISM precipitation was witnessed from 2 to 1.4 cal kyrs BP indicated by an expansion of C3 plants. Above mentioned studies such as Anoop et al. (2013b), Prasad et al. (2014b) and Menzel et al. (2014) suggested otherwise demarcating the period of ~2–0.5 cal kyrs BP as drought phase. In a contrast, palynological studies suggested humid climatic conditions in the Mid-Late Holocene while dry and cold climatic conditions prevailed during the early Holocene (Chauhan and Quamar, 2012; Chauhan et al., 2013; Quamar and Bera, 2017).
(iv) Zone IV (Eastern and Northeastern region)
Zone IV comprises parts of eastern as well as northeastern India with ~75% of the annual precipitation contributed by the ISM. Climatic reconstruction from fatty acids indices on speleothem collected from Krem Mawmluh byKalpana et al. (2021) indicate a period of intense monsoon activity from early to Mid-Holocene (~10 to 6.5 cal kyrs BP). Previous research from this location by Dutt et al. (2015) offered agreeable interpretations. Imsong et al. (2022) inferred a comparable episode of strong precipitation throughout the early Holocene (until 9 cal kyrs BP), followed by moderate precipitation between 7.5 and 3.5 cal kyrs BP, from sedimentological and stratigraphic studies of the fluvial sequence of the Dzüdza River. Sharma and Chauhan (1994) described the time period from 11 to 4 cal kyrs BP as being warm and humid based on palynological research from the Mirik Lake. A sediment core from Kolleru Lake in eastern India was subjected to a multi-proxy study by Rao et al. (2020), the results of which indicated robust precipitation up till 8 cal kyrs BP. Following this, precipitation gradually decreased from 8 to 4.9 cal kyrs BP. A study by Tripathi et al. (2014) previously reported similar warm and humid conditions from 8 to 4.3 cal kyrs BP based on palynological study of sediment sequence of Mahanadi River. The biogeochemical research carried out by Amir et al. (2021) on a core obtained from Chilika Lagoon revealed a general deteriorating trend of monsoon rainfall from early to Mid-Holocene (10.4 to 4.9 cal kyrs BP), followed by a strong drought period at 4 cal kyrs BP. Ankit et al. (2017) carried out sedimentological and geochemical analysis on a 1.6 m long sediment core retrieved from Rushikulya River and suggested a period of enhanced monsoon precipitation during 6.8–3.1 cal kyrs BP. This perception was refuted by a number of palynological studies on sedimentary sequences from Assam (Deosila, Dabaka, Chhayaggaon swamp, and Ziro Lake, respectively) that claimed the time between 11 and 6.8 cal kyrs BP was a cold and dry period (e.g., Dixit and Bera, 2011, 2012, 2013; Ghosh et al., 2014). Through a geochemical and isotopic investigation of sediment core from Ahmad Shah et al. (2022) have reported on the signatures of 8.2 and 4.2 cal kyrs BP dry episodes. The ISM weakened significantly after ~5 cal kyrs BP as implied by Rashid et al. (2011) from stable isotope analysis on sediments collected from the coast in the Bay of Bengal. Similar drier climatic conditions were established by Rao et al. (2020) thereafter from 4.9 cal kyrs BP representing a weakening in the monsoon. Stable isotopes analysis of speleothems from Mawmluh Cave in Meghalaya records a dramatic decrease of the ISM at ~4.2 cal kyrs BP (Kathayat et al., 2018; Lechleitner et al., 2017). Mehrotra et al. (2019) suggested cold-humid conditions at 4.6 cal kyrs BP followed by cold-dry period from isotope and geochemical analysis of sediment profile from Pankang Teng Tso Lake in Arunachal Pradesh. Ponton et al. (2012) carried out climatic reconstruction from a sediment core retrieved from the mouth of the Godavari River. The study suggested a gradual increase in aridification from ~4 to 1.7 cal kyrs BP, followed by the intermittent persistence of these arid climate conditions to the present. Records from Ankit et al. (2017) that go beyond 3.1 cal kyrs BP to the present day indicate similar decrease of the monsoon. To determine the paleoclimatic conditions of the Late-Holocene, Misra et al. (2020b) performed palynology and compound specific lipid analysis on a sediment profile from the Dzüko Valley in northeastern India. Dry climate conditions prevailed from 3.1 to 2.3 cal kyrs BP, and then abruptly changed to a wet environment from 2.3 to 1 cal kyrs BP. From 1 cal kyr BP onward, a decrease in precipitation was seen to the present. Amir et al. (2021) observes a similar increase in monsoon precipitation after 1.1 cal kyrs BP, followed by a slow decline toward present. Most recently, Ankit et al. (2022) investigated a lacustrine context dated to within the last 2000 years at Shilloi Lake in Nagaland and associated geochemical and sedimentological data revealed a weakening in precipitation since ~1.9 cal kyrs BP.
(v) Zone V (Indo-Gangetic plains)
This zone includes the Gangetic plains of northern India and covers a major portion of the northern alluvial plains. Holocene climatic reconstruction based on a multi-proxy approach by Trivedi et al. (2013) from Jalesar Lake in the Central Ganga plains suggests a period of increasing ISM precipitation during 13–5.2 cal kyrs BP. Misra et al. (2020a) investigated the vegetation transition of Baraila Tal in the Central Ganga plains using a multi-proxy method and proposed an event of rising humidity about 12–9 cal kyrs BP, representing colder climatic conditions. Saxena et al. (2015) conducted a multi-proxy research on Chaudhary-ka-Tal in Raebareli District, documenting a warm and fairly humid environment from 8.4 to 6.4 cal kyrs BP. The SW monsoon then underwent a phase of intensification between 6.4 and 3.1 cal kyrs BP. On the other hand, a phytolithic analysis of sediments from Lahuradewa Lake in the Ganga lowlands by Saxena et al. (2013) indicates a period of arid weather between 10.3 and 9.2 cal kyrs BP. By citing evidence of a drying lake and the lack of aquatic pollens, Misra et al. (2020a) also indicated a period of diminishing precipitation. This research also postulated a period of ISM strengthening beginning 7–5 cal kyrs BP. The time between 5.2 and 4.7 cal kyrs BP was portrayed by Trivedi et al. (2013) as a stage of moderate monsoon precipitation. Similarly, Quamar and Kar (2020) suggested the period from 8.5 cal kyrs BP to present as warm and relatively more humid from the palynological study of sediment profile from Naktu Lake. Following this was a phase of active SW monsoon between 4.7 and 3.2 cal kyrs BP. The period from 3.2 to 1.2 cal kyrs BP is marked as period of relatively less humidity and followed by the weakening of the SW monsoon from 1.2 cal kyrs BP onward. Similar dry episodes between 5.3 and 4.1 cal kyrs BP, 1.6 and 1.2 cal kyrs BP, and 0.9 and 0.7 cal kyrs BP were also noted in the phytolithic investigation by Saxena et al. (2013). Saxena et al. (2015) noted a gradual weakening of the ISM from 3.1 cal kyrs BP onward to the present. Numerous additional paleoclimatic research from this zone point to a similar pattern of heightened monsoon throughout the early to Mid-Holocene, followed by a slow drop in the monsoon intensity toward the end of the Holocene (Saxena and Trivedi, 2017; Srivastava et al., 2015).
(vi) Zone VI (Southern India and Sri Lanka)
This zone demarcates an area up to the southern tip of Peninsular India which is characterized by moisture from both the Southwest Monsoon (SWM) as well as the Northeast Monsoon (NEM), although contribution of the northeast monsoon to the annual rainfall is significantly higher. Studies on sediment cores from this zone provide the following inference about the paleoclimatic variations. Sandeep et al. (2017) reconstructed the variation in the ISM from Shantisagara Lake in this zone through a biogeochemical approach. The study suggested intensified precipitation from 10 to 8.6 cal kyrs BP followed by a weakening of the ISM during the Mid-Holocene (8.6–4.5 cal kyrs BP). Similar evidences of episodes of aridifications at 8 cal kyrs BP and 3.2 cal kyrs have been reported by Rajmanickam et al. (2017) based on geochemical analysis of sediment core retrieved from Kukkal Lake. Evidence for a gradual decrease in ISM precipitation was obtained for the period between 4.5 and 3.3 cal kyrs BP by Sandeep et al. (2017) as well. Resmi and Achyuthan (2018) suggest the period from ~10 to 3.5 cal kyrs BP as high precipitation phase from the geomorphic analysis of paleochannel of Palar River Basin with a short phase of weakened monsoon during 4.8–3.5 cal kyrs BP. The phase from ~3.3 cal kyrs BP to the present was marked by a stable ISM with an increase in precipitation. Various other studies from this zone have shown agreeable tendencies of climatic changes, such as the geochemical and sedimentological investigation of a sediment profile from Ennamangalam Lake by Mishra et al. (2019), which shows a period of dry climate from 4.8 to 3.1 cal kyrs BP followed by a period of high precipitation from 1.6 cal kyrs BP to the present. Pollen records from a 20 m core recovered from Porayar in the Cauvery delta were investigated by Mohapatra et al. (2021) and showed that there was a high NEM between 5.1 and 2.5 cal kyrs BP. Resmi et al. (2017) proposed a period of enhanced northeast monsoon (NEM) during the Mid-Late Holocene (3.5 cal kyrs BP) near the Palar River in Tamil Nadu, followed by a period of weakening NEM during the Late-Holocene. From the aforementioned pollen analysis, Mohapatra et al. (2021) also proposed a period of dry weather that lasted from 2.5 cal kyrs BP to the present. On the contrary, Sandeep et al. (2017) and Mishra et al. (2019) propose a period of increased monsoon from 3.1 cal kyrs BP to the present. A palynological investigation conducted by Srivastava and Farooqui (2013) on a sediment core taken from the TSpettai mangrove wetland suggests a period of warm and humid conditions between 3.6 and 3.2 cal kyrs BP. At 1.3 cal kyrs BP, these gradually changed to become dry and arid environments, which got more worse. The earlier stated work by Resmi and Achyuthan (2018), which revealed a steady aridification and weakening of the monsoon from 3.5 cal kyrs BP to the present, is also consistent with this explanation.
Cultural dynamics across India during the past 11.7 kyrs
The last 11,700 years have witnessed cultural transitions in India broadly in sync with global transitions: Mesolithic to Neolithic (beginning of the end of the Stone Age), Neolithic to Chalcolithic (transition from Stone Age to Bronze Age) and Chalcolithic to Early Historic (transition from Bronze Age to Iron Age) (Misra, 2001). What makes the Holocene cultural transitions in India so unique are three key attributes: (i) the transition from hunting-gathering to domestication and farming is chronologically and culturally uneven and independent across the Subcontinent; (ii) reduced long-distance mobility and increased sedentism across the Subcontinent from the terminal Mesolithic/early Neolithic onward lead to regionalization of cultures and independent transitional pathways; and (iii) from 1500 B.C. (~3.45 cal kyrs BP) onward, that is, the beginning of the Iron Multiple cultures and economic lifestyles have chronologically, geographically, and socially intertwined; this includes hunter-gatherers with a Mesolithic lifestyle, pastoralists and select farming communities with a Neolithic lifestyle and full-fledged farmers with a Chalcolithic lifestyle. All of these various transitional milestones and evidences are broadly outlined below (Figure 3).
(i) Early Holocene cultural transitions
Sites of archeological significance and evidence of several cultures that lived on the Indian Subcontinent throughout the Holocene period (1. Bhimbetka; 2. Adamgarh; 3. Tarsang; 4. Kanewal; 5. Langhnaj; 6. Tilwar; 7. Bagor; 8. Sarai Nahar Rai; 9. Chopani-Mando; 10. Mahadaha; 11. Lekhania; 12. Koldihwa; 13. Mahagara; 14. Lahuradewa; 15. Mehgarh; 16. Jalilpur; 17. Gumla; 18. Rehman Dheri; 19. Kot Diji; 20. Amri; 21. Balakot; 22. Kalibangan; 23. Utnur; 24. Inamgaon; 25. Budihal; 26. Watgal; 27. Nagarajupalle; 28. Sanganakallu; 29. Hallur; 30. Chirand; 31. Daojali Hading; 32. Golabai sasan.
From a cultural perspective, the Early Holocene is marked by a broad transition from the Mesolithic to the Neolithic phase, although hunter-gatherer lifeways have continued into modern day in many parts of the Subcontinent. Technologically, the Mesolithic is recognized primarily by the presence of “microliths,” small stone tools which first appear in Africa at about 65 cal kyrs BP (Ambrose, 2008). In the Indian Subcontinent, the earliest dated microlithic assemblages are about 48–45 cal kyrs BP (see Chauhan, 2020 and references therein) and chronologically overlap with terminal Middle Paleolithic and Upper Paleolithic technologies at places. Microlithic technology starts before the Mesolithic phase proper and the techno-complex in general includes a wide range of small tools of different geometric and non-geometric shapes (e.g. points, microblades, flakes, thumbnail scrapers) made of lithics (including backed specimens) and used along with antler, shells, ivory, wood, and animal bones. These materials were often combined to manufacture composite tools such as arrows, knives, harpoons, sickles, and so forth (Misra, 2001). This gradually led to the development of more sophisticated hunting and subsistence methods including fishing, trapping and digging for underground resources in order to advance into newer ecological niches. In fact, Mesolithic and Neolithic stone tool technology continued to be used well into the Chalcolithic phase and into the Iron Age and associated subsistence strategies continue into modern day. Archeological investigations of key Mesolithic sites indicate animal protein to be the primary source of their diet making hunting a vital mode of survival. For example, the evidence gathered from rockshelter and open-air sites such as Bhimbetka, Adamgarh, Tarsang, Kanewal, Langhnaj, Tilwar, Bagor, Sarai Nahar Rai, Chopani-Mando, Mahadaha, and Lekhania (among many others) suggest a prominent Mesolithic stage throughout India. Many of these identified Mesolithic sites have also preserved evidence of diverse herbivores suggesting rich faunal diversity. In addition to meat, it is also well known that Mesolithic hunter-gatherers also relied on gathering/ collecting of edible roots, seeds, nuts, fruits, honey and grasses, fishing and stock-rearing to attain their economic essentials. Another significant attribute of the Mesolithic stage in India is symbolic behavior, primarily in the form of rock art, followed by personal ornamentation, human burials, and other sophisticated behaviors. Skeletal remains of cattle and sheep/goat obtained from the sites of Chopani-Mando, Sarai-Nahar-Rai and Mahadaha in the Ganga plains provide convincing evidence of domestication that became the hallmark of the subsequent Neolithic and Chalcolithic phases.
(ii) Mid-Holocene cultural transitions
The Neolithic stage began emerging at around 5.9 cal kyrs BP (6000 BC) while Mesolithic practices were reported to be overlapping with subsequent cultural phases including social interactions (e.g. Morrison and Junker, 2002). The Neolithic stage of human social and cultural development is globally marked by the domestication of plants and animals and the establishment of sedentism. In the Subcontinent, clear evidences of this stage are reported from eastern India, northeastern India, southern India and Kashmir Valley and the Swat region (Singh, 2008). This pattern of increasing sedentism reflects reduced or ceased mobility and trade between regional cultures across India until Historical times when kingdoms emerged along with associated geographic/territorial expansions. The gradual shift from hunting-gathering to domestication of animals and plants is evident from the co-existing skeletal remains of wild and domesticated varieties of animals such as goat, sheep, cattle and dog. For instance, the remnants of wild cattle recovered from Koldihwa and Mahagara sites showed reduction in successive phases followed by smaller size of the animal bones recovered (Sharma et al., 1980). Remains of domesticated rice were also recovered from the Neolithic layers of Koldihwa. Very early dates of crop exploitation from Lahuradewa (Fuller, 2006; Tewari et al., 2006) have suggested possible indigenous origins of agriculture instead of it being introduced to India; however, more research is required across India to verify this. The records of seeds and grains show the production of crops such as wild and cultivated rice, goose-foot, Artemis, and flatsedge, as well as the gathering of goose-foot, job’s tear, and catchfly. The Neolithic phase is also marked by the earliest evidences of handmade pottery associated with food storage and cooking. The main stone tools associated with the Neolithic stage are polished adzes/axes/celts - chisel-ended lithics which imply extensive deforestation for purposes of farming, livestock as well as fuel for regular fire use. The recovery of bones of humped cattle, numerous food grains and other evidences from Mehrgarh in Pakistan indicate an independent/indigenous establishment of sedentary cultural practices including the evolution of the regional Chalcolithic stage in the northern part of the Subcontinent. In other parts of South Asia (e.g. Budihal, Watgal, Utnur, Sanganakallu, Hallur, and Nagarajupalle), the Neolithic stage appears to be variably contemporary with the Indus Valley/Harappan Civilization and younger cultures. One of the key sites in northeastern India include Daojali Hading and those in eastern India is Golabai Sasan (see Mishra et al., 2020).
(iii) Later Holocene cultural transitions
The middle Holocene witnessed the establishment of the Harappan culture in different stages of technological advancement (pre-Harappan phase followed by Early Harappan phase leading to the Mature Harappan phase before eventually shifting toward the Late Harappan phase) from ~7 to 3 cal kyrs BP. Some of the major Harappan sites include Jalilpur in Punjab, Gumla and Rehman Dheri in Gomal valley, Kot Diji and Amri in Sind, Balakot and Kalibangan in Rajasthan. The Mature Harappan phase (2800–1900 BC or 4.75–3.85 cal kyrs BP) represents the first real urbanization in the Indian Subcontinent, according to evidence reported from Harappa, Mohenjodaro, Chanhudaro, Amri, Kalibangan, and Banawali to name a few. The Harappan Civilization is considered to be one of the greatest Bronze Age civilizations in the world due to its technological advancement and cultural standardization (e.g. town planning, architecture, script, agricultural practices, weights and trade). However, numerous contemporary cultures of various stages existed in different parts of the Subcontinent without any links or interactions with the Harappan populations. Most of the contemporary and post-Harappan Chalcolithic sites discovered in India are located in central (key site includes Inamgaon in Maharastra), eastern and southern India. It is crucial to point out that these cultural transitions did not occur evenly and comprehensively across the Subcontinent. The Late Harappan phase (1900–1300 BC or 3.85–3.25 cal kyrs BP) is marked by the introduction of Ochre Colored Pottery (OCP) and the existence of the Cemetery H culture. The Late Harappan phase also comprises a gradual decline in settlement size, economy, material richness and trade and the disappearance of the use of its script and pottery quality. Multiple urban centers gradually collapsed and fragmented into smaller population groups that shifted from their larger urban locations. Further cultural transitions during the Late-Holocene are marked by the beginning of the Iron Age (1500–800 BC or 3.05–2.35 cal kyrs BP) and the Early Historical Period. The cultural traits associated with the Iron Age include the transition from bronze to iron technology which is associated with multiple cultures across time and space and recognized primarily on pottery styles: Black-and-Red Ware, Painted Gray Ware and regional Megalithic cultures (Allchin et al., 1982). Other than domestication and rearing of animals, the manufacture of ornaments and utilitarian objects and the development of other various craftsmanship were witnessed.
Discussions
As evident from the above studies, a clear variability occurs both in climatic as well as in cultural contexts during the entire Holocene epoch in the Indian Subcontinent (Figure 4). Intense wet conditions of the Early Holocene came to an end with the cooling event at ~8.2 cal kyrs BP (Feng and Hu, 2008; Prasad et al., 2009; Walker et al., 2012). Sea surface temperatures drop due to meltwater input into the North Atlantic from the collapsing of the North American ice margin might have resulted in the weakening of the monsoon system (Ellison et al., 2006). Another such climatically-abrupt event occurred at ~4.2 cal kyrs BP (Berkelhammer et al., 2012; Kathayat et al., 2017; Sandeep et al., 2017; Sinha et al., 2015; Staubwasser and Weiss, 2006; Walker et al., 2012). The end of Mid-Holocene marked by this global cooling and drying event. The primary causal factor for the “4.2 cal kyrs BP” event is reduction in solar activity which in turn resulted in atmospheric cooling of Northern Hemisphere. This might have led to the weakening of the thermohaline overturn (Bond et al., 2001). These so called “mega-drought” events were extremely intense in nature and lasted over many decades and occurred persistently. As a consequence, several cultures and numerous regional population groups across the Indian Subcontinent may have been indirectly affected. The increasing frequency of El-Nino events coupled with the lowering of sea surface water temperatures of the North Atlantic might have affected the intensity of the Asian monsoon system (Barron and Anderson, 2011; Walker et al., 2012). The Late-Holocene is also characterized by centennial scale events of the Medieval Warm Period (MWP) occurring between 1.2 and 0.9 cal kyrs BP, followed by the Little Ice Age (LIA) occurring from 1.8 to 1.3 cal kyrs BP. The availability of resources in terms of food, water, lithic raw materials and so on were vital factors in influencing cultural activities and human settlement patterns (Davies, 2005; Oguchi et al., 2008; Wilkinson, 2000). Several transitions were initiated during different cultural phases of the Indian Subcontinent which appear to be adaptive measures taken in order to cope with the changing climatic conditions. For instance, cultural responses to extreme climatic events are well documented within the Harappan Civilization and can be compared with the cultural evolution different zones accordingly. This will in turn enhance the understanding of the linkage of subsistence pattern of ancient human settlements with the changing Holocene climate.
Paleoclimate variability data for each zone in the Indian Subcontinents, together with key cultural events (The advent of significant habitant events such as sedentism, livestock rearing, and crop cultivation occurred at different times in distinct zones, as shown by the relevant legends on the age scale).
Pre-Early Harappan phase or the Early Holocene (~11.7 to 5 cal kyrs BP)
The early Holocene was an interval of intensified monsoon as depicted by documentation of various paleoclimatic records. On the banks of the Bolan River, close to Mehrgarh, archeological evidence of the earliest human habitation of the Indus basin has been discovered. Mehrgarh offers crucial evidence for the transition to a subsistence economy based on established agriculture and the domestication of wild animals from one that was based on hunting, gathering, and pastoralism. Agriculture may have facilitated the development of stratified civilizations, permanent communities, and sedentary lifestyles. Archeological evidence reveals that the production of cereals, mainly wheat, and the building of mud-brick storehouses expanded rapidly in the Mehrgarh region throughout the succeeding period (7–6 cal kyrs BP). Thus, the development of agriculture largely consisted of carrying on the highly productive pattern of producing wheat and barley as well as domesticating cattle, sheep, and goats that first appeared in the Mehrgarh region. Around 5.5–5 cal kyr BP, this phase of agricultural growth most likely contributed to the development of the Harappan civilization in the Indus valley. Several significant rivers, including the Indus, were flowing vigorously at the period. Between 10 and 7 cal kyrs BP, the Ganga-Brahmaputra River system supplied twice as much silt to the Bengal basin as it does now. Early Holocene peat formation in Tibet and higher sediment discharge by the Ganga-Brahmaputra rivers are evidence of greater precipitation in response to a SW monsoon over the Indian Subcontinent that was stronger than it is today. As human habitation grew in the surrounding areas, efficient agricultural methods and deforestation occurred. According to the existing archeological data, people must have employed stone tools to cut dense forest during this time period and create new settlements (Figure 4). Following the formation and growth of settlements, many forms of subsistence are made possible (especially development of agriculture methods). A shift from a regularly-mobile hunting-gathering lifestyle to a sedentary lifestyle with agriculture and pastoralism can also be deduced as an effect of a gradual environmental shift. This can be considered to be a global phenomenon related to changes in global climates and/or sea level changes during the Pleistocene-Holocene transition (e.g. Abell, 2000; Richerson et al., 2001; Yanko-Hombach and Kislov, 2018).
Mature – Late Harappan phase or the Mid-Holocene (5 – 3.5 cal kyrs BP)
It appears to us thus that the arid phase in the Indian Subcontinent started from 5 to 4 cal kyrs BP coinciding with a stepwise weakening of the SW monsoon, leading to desiccation of major river channels and a drop in the lake levels. A gradual transition to fragmented rural subgroups from an urbanized society is well-documented, along with increasing abandonment of many settlements and shifting to new water resources across the region that will eventually demarcates the Harappan domain. Similarly, the settlements from Baluchi appear to have shifted eastwards to the headwaters of the Saraswati (Khonde et al., 2017; Possehl, 1997). People had to migrate to the east and toward the Ganga plain due to adaptations required by a drastic decrease in rainfall in the northwest and the failure of agriculture. An increase in the quantity of archeological sites in the Ganga-Yamuna doab (interfluve) and throughout the Ganga Valley attests to this. Around 5 cal kyrs BP, people also moved southward to the Kachchh area. Climate change is anticipated to drive people to relocate closer to water resources when conditions become harsh. Not only did the extreme climatic events have an impact on human civilizations and cultures but also the gradual climatic shift showed to have control over the techno-cultural evolution of various habitations (Giosan et al., 2012; Madella and Fuller, 2006; Vollweiler et al., 2006). The increasing temperatures and a shift toward arid climatic conditions might have caused hunting-gathering unsuitable for many populations, making village farming and sedentism more viable modes of survival and subsistence. Since the beginning of agriculture however, double cropping (annually) was probably not an option for all regions of India and it appears that the climatic/monsoon variability over the Holocene may have led to intermittent diversifications in subsistence behaviors (e.g. Singh et al., 2018). Indeed, the planting of winter crops and the development of rainwater harvesting techniques are clear indicators of the influence of changing climate on adaptive strategies. In some regions of India, it seems that the impact of the monsoon on past cultures was indirect and associated vegetation change also played a major role. For instance, Riedel et al. (2021) demonstrate from their work near the Lonar Crater in Maharashtra that the weakening of the ISM led to the origin and spread of savannah environments during the Mid-Holocene, which in turn encouraged the introduction of agriculture and increased sedentism in peninsular India. It should be highlighted here that the Holocene interaction seen in some regions between hunter-gatherers, pastoralists and farmers was geographically uneven across the Subcontinent. Hence, climate change has not only resulted in the demise of certain cultures but have also led to shifts in technologies and the development of different adaptative strategies.
Post Harappan phase or later Holocene (3.5 cal kyrs BP onward)
This period is represented by the interannual precipitation variability which is supposed to have impacted the decline of the Harappan Civilization, marked by the gradual but prominent abandonment of established settlements. While the Harappan Civilization in the northwestern zone declined due to various factors, contemporary populations with different economic lifestyles (Mesolithic, Neolithic and Chalcolithic) continued to flourish in other parts of India. In certain circumstances, civilizations-built ponds, dams, and other rain-harvesting constructions as a response to monsoon failures. For instance, the emperor Akbar dug the Kukar Talao in Nagaur, central India, as a result of the mega-drought in ~1.8 cal kyrs BP. In other cases, people moved eastward toward the Ganga plain, where there was enough rainfall to support the weight of the fresh inflow of people. Massive famines and widespread population displacement occurred in India as a result of the pulse of persistent dry periods that occurred during the LIA (1.8–1.3 cal kyrs BP). Eventually this period also witnessed an upsurge in trade in central India due to the creation of new routes as a result of the clearance of forests and the development of new craftsmen skills (Behera et al., 2022). This adaptive resilience to changing ecologies and climates at the pan-Indian level may have been due to the high cultural diversity, a factor recently highlighted by Burke et al. (2021) at a global level.
However, the currently available environmental data from the Holocene is not adequate or consistent enough to establish discrete causal factors for cultural change across India. While the above proposed hypotheses based on this board review can be considered and tested, the primary limitation at the moment is that the Holocene-focused environmental research has been generally done independent of the archeological evidence. Likewise, very few archeological sites have been studied from paleoenvironmental perspectives with various associated proxies being targeted. In addition, most of the integrated environmental-archeological studies have been biased toward the Harappan evidence (e.g. Chatterjee and Ray, 2017, 2018; Giosan et al., 2012; Madella and Fuller, 2006) and other Holocene cultures across the Subcontinent have been neglected. Individual climatic studies done till date are relevant mostly for their immediate regions and should not be considered to be valid for archeological records outside those regions. The current distribution of published Holocene studies also shows major geographic gaps which need to be critically targeted in the future: (a) the vast region of western Madhya Pradesh between the Thar Desert and the Narmada Basin; (b) a large part of western India south of Gujarat; (c) western part of the Ganga Plains; (d) most of Maharashtra and Telangana in the center of peninsular India; and (e) the entire region between Nepal and the Bay of Bengal. Hence, caution is required when attempting to make environmental-archeological correlations at a pan-Indian level; independent regional perspectives may be comparatively more reliable and meaningful instead.
Conclusions
Initially, the Holocene was considered to be climatically stable but the detailed documentation of past climatic/environmental conditions suggests otherwise, with several cooling events occurring persistently, roughly at an interval of every 1.5 ± 0.5 kyrs (Misra et al., 2019). These climatic changes seem to be associated with subpolar and subtropical surface ocean circulations. The preliminary classifications of Holocene climate data revealed an unequal distribution of records across several geomorphic-climatic zones of the Subcontinent. The significance of determining the sensitivity of diverse marine and terrestrial records with regard to their geomorphic-climatic settings is further highlighted by this study. Additionally, some of the records contain evidence of major world events like the MWP and LIA, demonstrated through various proxy-based approaches. Overall, the Holocene Climate Optima, which occurred between 9 and 5 cal kyrs BP, coincided with the monsoon strength maxima in all zones. The warm and wet phase, however, was interrupted by a sudden drought event known as the 8.2 cal kyrs BP event, which was followed by signs of climatic improvement. The monsoon gradually diminished and the climatic regime resembled that of today about roughly 5 cal kyrs BP. The phase also reveals a significant aridification event that took place at 4.2 cal kyrs BP, which has been connected to the demise of numerous civilizations. Based on current evidence as outlined in this review, the only major event that is linked to climate change is the end of the Harappan Civilization. At the moment, the available Holocene climatic data is too geographically and chronologically incomplete to hypothesize about associated impacts on other regional cultures across the Indian Subcontinent. While some behavioral shifts and technological transitions must have been linked with global climate change – such as the intensification of the monsoon – it is advisable that the regional archeological records are viewed independently, thus decoupling the impact of regional environmental events from their pan-Indian counterparts.
Placing these Holocene paleoclimatic records within the context of available archeological documentation, the responses of specific complex societies to persistent climatic changes have been evaluated. For instance, geographically and annually diverse monsoon patterns may have affected the nature of cultural, technological and subsistence evolution through the Holocene. Furthermore, the lack of key transitional technologies and behaviors in some locations might be linked to Holocene environmental dynamics and monsoon variations across time, as well as social contacts through migration and trade. The demise of key civilizations, gradual migrations of some populations and the adaptation to new subsistence modes reflect and highlight the prominent human-environment interfaces at both long- and short-term levels. Although modern societies and advanced technology exhibit marked resilience to such inter-annual as well as decadal droughts, the cultural responses to such multidecadal events can only be understood by integrating detailed paleoclimatic and archeological records.
