The Neolithic Revolution: Why Did It Happen Exactly When It Did?

The Neolithic Revolution – the shift from foraging and hunting to farming – is one of the most important events in human history. It began around 10,000 BCE in the Fertile Crescent, later emerging independently in several other regions and gradually spreading over most of the Earth’s land surface. It is fairly clear why it happened in that particular area, but today we will discuss the question of why exactly then.

Three factors can be identified as necessary for successful and productive agriculture:

• high average annual temperatures with long summers;
• a fairly stable and predictable climate;
• a relatively high concentration of carbon dioxide in the atmosphere.

To understand how all three factors came together at the same point in time, we need to talk about a key event linked to the Neolithic Revolution – the Last Glacial Maximum, which occurred roughly 23,000–18,000 years ago.

Generally speaking, there is nothing special about this particular glaciation – glaciations happen quite regularly and are mostly driven by Milankovitch cycles. These cycles are related to the Earth’s motion through space: over a period of about 100,000 years, the shape of the Earth’s orbit changes from nearly circular to more elongated; the tilt of the axis oscillates with a period of 41,000 years; and the precession of the axis (a change in its direction) has a period of about 26,000 years.

The mechanism that starts (or ends) an ice age is linked to the length of summer in the Northern Hemisphere. Unlike the Southern Hemisphere, the configuration of continents here allows vast amounts of snow and ice to accumulate on land. A smaller axial tilt smooths out seasonal temperature variations – which lowers the average summer temperature. Precession controls whether Northern Hemisphere summer occurs when the Earth is nearest the Sun or when it is farther away.

If the ice does not melt completely during summer, several positive feedbacks are triggered. First, white ice has a high albedo, reducing the amount of solar energy absorbed. Second, a cold ocean absorbs more carbon dioxide – its atmospheric concentration drops, the atmosphere becomes more transparent to infrared radiation, and the greenhouse effect weakens. Third, increased aridity causes deserts and steppes to expand, while cloud cover decreases – further cooling the planet.

Interestingly, the 100,000‑year cycle is the weakest (providing only about 0.1% variation in insolation), yet in the last 800,000 years it has dominated glacial‑interglacial cycles. According to one hypothesis, this is because the ice sheets became so massive that they cannot respond to shorter cycles; they only break down when the conditions from all three cycles coincide.

The entire history of Homo sapiens falls within this long glacial period. Let us examine its climate chronology in more detail. It is fairly well known thanks to the study of oxygen‑18 (¹⁸O) isotopes in sediment and ice cores. The abundance of this stable isotope on Earth is about 0.2% of all oxygen. It is also present in water in about the same proportion. However, water made with the lighter isotope (¹⁶O) evaporates more easily, while water made with the heavier isotope (H₂¹⁸O) is comparatively more likely to fall as precipitation. By measuring the ratio of heavy to light oxygen in ancient ice, we can reconstruct past temperatures – the basis of the oxygen isotope stage classification.

From this, we know that roughly 150,000 to 115,000 years ago the Eemian interglacial occurred – a warm period when global temperatures were a couple of degrees warmer than even today, and sea level was 4–10 metres higher due to the melting of Greenland and part of Antarctica. By that time, members of the genus Homo had already left Africa twice. The first wave happened about 1.8 million years ago, when Homo erectus spread across southern Eurasia; relict populations survived until about 100,000 years ago (i.e., they coexisted with modern humans). The second wave occurred around 500,000 years ago, when Homo heidelbergensis widely colonised Eurasia, probably evolving into Neanderthals in the west and Denisovans in the east. During the Eemian interglacial, Homo sapiens mostly stayed in Africa, although their remains have also been found in the Middle East.

Then a cooling began, reaching a maximum around 80,000–60,000 years ago, when atmospheric CO₂ levels fell to about 200 ppm and sea level dropped by 60–80 metres. Climate change very likely pushed modern humans out of Africa; they rapidly spread across Eurasia and even reached Australia and the Americas for the first time.

Later, the Milankovitch cycles produced a moderate warming (60,000–30,000 years ago), which put the climate system into an extremely unstable state. Normally (including in our own era) the AMOC (Atlantic Meridional Overturning Circulation) operates, carrying warm water from the tropics to the North Atlantic and warming Europe. During that same period, Dansgaard–Oeschger cycles occurred.

When the warm current flows north, it cools and sinks, drawing more warm water from the south. The surrounding ice sheets start to melt rapidly, releasing fresh water. This lighter fresh water does not sink easily, eventually stopping the circulation and causing abrupt cooling. The melting of the ice halts, salinity recovers, and the cycle repeats. Such events could cause rapid temperature changes of up to 10°C per decade.

Finally, the last glacial period (30,000–11,000 years ago) arrived. After it ended, a strong warming took place, and from about 12,000 years ago onwards we have a combination of fairly stable climate, high carbon dioxide levels, and high average temperatures. All three conditions together created sufficient circumstances for the start of the Neolithic Revolution – though they did not directly cause it. Before that, the Neolithic Revolution as a widespread transition to farming could not have happened; after that, it arose independently in various regions.