A Brief Exploration of Climate in Early Human History
The relationship between climate and evolution in the Pleistocene epoch
Around 300,000 years ago, well within the Chibanian age (formerly known as the Middle Pleistocene), Homo sapiens first emerged from Africa. At the same time, Neanderthals were evolving in Eurasia and slowly replacing other species of archaic humans. The Brunhes-Matuyama Reversal, the latest reversal of Earth’s magnetic poles that occurred around 781,000 years ago, marked the beginning of the Chibanian.
Cyclical Climate and Ice Ages
Climate changes according to a variety of factors, some of which are Milankovitch cycles. Milankovitch cycles are cyclical changes concerning three conditions: the shape of Earth’s orbit around the sun, the angle the axis is tilted, and the direction in which the axis points.
Due to these cycles, periods of high glaciation known as ice ages generally occur every 41,000 years. However, a change in the approximate length of time between ice ages happened around 800,000 years ago, near the beginning of the Chibanian period. The time between cycles increased to 100,000 years. The reason for this shift may have been albedo, as the Pleistocene epoch saw a large amount of sea ice in the south pole.
The new time scale for ice ages matched Earth’s eccentricity patterns, in which Earth’s orbit varies between a slightly elliptical shape and a near-perfect circle in 100,000-year cycles. When it is elliptical, the relative length of each season is uneven, and about 23 percent more solar radiation reaches Earth at the point in its orbit when it is closest to the sun.
Earth’s orbit was near its most elliptical (eccentric) point 300,000 years ago. The heightened level of solar radiation caused changes in air circulation and rainfall, leading to a marked increase in climate variability in Africa.
Paleoclimate of Africa
Even before the change in ice age timing, there was still pronounced variation in the African climate. Cycles of aridity and humidity with a long-term trend towards a drier climate characterized conditions in Africa. Precipitation levels fluctuated considerably during the Pleistocene. From 2.8 million years ago (mya) and onwards, conditions became increasingly arid in step-like shifts.
Hominins before this period, such as Australopithecus Afarensis, well-known for the near-complete fossil of an individual nicknamed Lucy, were adapted to living in environments such as forests that required a steady amount of water. In the following period between 2.9–2.4 mya, adaptations to drier environments began to emerge. The first robust hominin appeared, with thick teeth and strong jaws to chew the hardy plant matter that evolved to protect against water loss in dry conditions.
Not long after, the first species in the Homo genus evolved. These early humans had smaller frames and larger brain-to-body ratios than previous hominins. Large brain size was associated with the use and modification of stones as tools. While the use of stone tools was not necessarily exclusive to the genus Homo, its growing prominence reflected an adaptation to an increasingly varied and arid climate.
Climate and Hominins Over the Last Million Years
While conditions were becoming more arid overall, they were not characterized by a simple gradual increase in aridity. Between 1.2–0.8 mya, dry periods became longer and more extreme, followed by shorter wet periods. It was during this period that a direct ancestor of modern humans, Homo erectus, emerged. Homo erectus is widely considered the first hominin to migrate beyond Africa.
The final phase of arid-adapted species turnover (that is, a period of selection for a specific taxon or feature) occurred between 1.2–0.6 mya. At the 0.8 mya mark, 100,000-year glacial cycles began to dominate the middle to upper Pleistocene. Cycles of heavy precipitation and aridity determined by glaciation periods continued, affecting hominins and influencing the location and size of their populations.
By 0.3 mya, Homo sapiens evolved in Africa, coexisting with a few other species in the same genus. Intelligence, tool use, and adaptability were crucial for early humans living in an unstable climate. Periods of high precipitation were favorable, causing a decrease in dust storms and increasing vegetation in East Africa and the southern Sahara. Extreme dry periods brought on by advanced glaciation caused distress for hominins, particularly Neanderthals and Denisovans that suffered severely low population sizes during such events. Homo sapiens experienced severe population bottlenecks as well, though they managed to survive, possibly due to having sustained a larger population size on average than Neanderthals and Denisovans.
Reflection
Over the past 3 million years, climatic changes resulting from numerous factors, including Milankovitch cycles and albedo, defined the history of hominins and other species living in Africa. Rigorous dry periods during wet-dry cycles created challenging conditions for hominins, driving evolutionary changes such as high intelligence and tool use. If it were not for the unique climate of the last few million years and the resilience of life, Earth as we know it may have looked radically different today.
