in the lower Danube Valley were burned and abandoned, while the Cucuteni-Tripolye culture showed an increase in fortifications, meanwhile moving eastwards towards the Dniepr. Steppe herders, archaic Proto-Indo-European speakers, spread into the lower Danube valley about 4200–4000 BCE, either causing or taking advantage of the collapse of Old Europe. The Yamna horizon was an adaptation to a climate change which occurred between 3500 and 3000 BCE, in which the steppes became drier and cooler. Herds needed to be moved frequently to feed them sufficiently, and the use of wagons and horse-back riding made this possible, leading to "a new, more mobile form of pastoralism". It was accompanied by new social rules and institutions, to regulate the local migrations in the steppes, creating a new social awareness of a distinct culture, and of "cultural Others" who did not participate in these new institutions. In the second century BCE widespread aridization led to water shortages and ecological changes in both the Eurasian steppes and south Asia. At the steppes, humidization led to a change of vegetation, triggering "higher mobility and transition to the nomadic cattle breeding". Water shortage also had a strong impact in south Asia: The Indus Valley civilisation was localised, that is, urban centers disappeared and were replaced by local cultures, due to a climatic change that is also signalled for the neighbouring areas of the Middle East. many scholars believe that drought and a decline in trade with Egypt and Mesopotamia caused the collapse of the Indus Civilisation. The Ghaggar-Hakra system was rain-fed, and water-supply depended on the monsoons. The Indus valley climate grew significantly cooler and drier from about 1800 BCE, linked to a general weakening of the monsoon at that time. The Indian monsoon declined and aridity increased, with the Ghaggar-Hakra retracting its reach towards the foothills of the Himalaya, leading to erratic and less extensive floods that made inundation agriculture less sustainable. Aridification reduced the water supply enough to cause the civilisation's demise, and to scatter its population eastward. Indigenous Aryanism Indian nationalistic opponents of the Indo-Aryan migration question it, and instead promote Indigenous Aryanism, claiming that speakers of Indo-Iranian languages (sometimes called Aryan languages) are "indigenous" to the Indian subcontinent. Indigenous Aryanism has no support in contemporary mainstream scholarship, as it is contradicted by a broad range of research on Indo-European migrations. See also Early Indians List of ancient Indo-Aryan peoples and tribes Indo-Aryan peoples