Some 10,000 years ago, agriculture arose independently in several areas across the world. The causes of the agricultural transition are still debated, but it is likely that the unprecedented warmth and stability of the Holocene climate made agriculture possible and that the climate instability of the Pleistocene made it impossible. As the weather became more predictable, people began to more intensively manage wild plants and began to store the grain they collected. Because of the unpredictability of harvests, people cultivated more crops than they thought they would need, and in most years, there was a surplus of food, leading to larger and more concentrated populations. By 5,000 years ago, small-scale agriculture had led to large-scale state-managed economies and total dependence on agriculture. Competition between city-states gave us full-blown ultrasocial economies and hierarchical, repressive states. Other consequences of agriculture for individuals include a marked decline in physical health, a reduction in brain size, and a loss of individual autonomy.