Earth's climate has always been in a state of flux. Ever since our ancestors split from the primate evolutionary tree millions of years ago, the planet has faced dramatic swings between wet and dry periods, as well as long periods of freezing and melting glaciers. Clearly, early humans were able to survive these changes, and our very existence attests to their success. But a growing number of scientists believe that major climate changes may have also shaped some of humanity's defining characteristics.
Did Climate Change Influence Human Evolution?
Since the 1920s, suspicions have been raised that climate change may have played a key role in human evolution. At that time, scientists proposed that drier conditions may have led early humans to adopt bipedalism and adapt to life on the savannah. However, the limited availability of representative paleoclimate data has made this connection difficult to prove.
A new study published in Nature uses an unprecedented circulation model to simulate Earth's climate over the past two million years. Specifically, the study explores how long-term climate fluctuations caused by Earth's astronomical motions may have created the environmental conditions that spurred human evolution.
As our planet is pushed and pulled by other planets, its tilt and orbital shape change, altering its climate. For example, Earth's tilt oscillates with a 41,000-year cycle, affecting the intensity of seasons and rainfall in the tropics. Over periods lasting more than 100,000 years, Earth shifts from a less circular orbit to a more elliptical one. A more circular orbit brings more sunlight and longer summers, while a more elliptical orbit reduces sunlight, potentially leading to periods of glacial formation.
How Climate Change Affects Humans
1. Earth's climate has always been changing. Millions of years ago, human ancestors diverged and faced dramatic climate changes, including the freezing and melting of glaciers.
2. Climate change may have shaped some of humanity's defining characteristics. Major evolutionary leaps, such as larger brains and complex tool use, appear to coincide with major climate changes.
3. Theories about how climate may have triggered evolutionary advances include that major leaps were driven not by adaptation to specific habitat changes but by a series of frequent changes.
4. Two major events in human evolution are associated with periods of climatic instability. The extinction of Australopithecus afarensis and the emergence of humans occurred between 3 and 2.5 million years ago. 5. Africa's shift from wooded areas to open grasslands may have partially contributed to early humans' evolution from climbing to upright walking.
6. Rock cores show fluctuations between very wet and very dry periods every 20,000 years, and these fluctuations recur during two major periods of early human development.
7. The Great Rift Valley is home to many of the most important fossils in early human evolution, as they emerged and expanded as their preferred climate returned.
8. Neanderthals and Denisovans may have descended from earlier groups, such as Homo heidelbergensis, when they were isolated in small, habitable areas in the cold Northern Hemisphere.
9. Climate change may not have been as extreme in equatorial regions, and this process may not have played the same role.
10. Selection of variants is not a mechanism for creating new species, but rather a mechanism for the transmission of successful ones, allowing them to spread and cope with many different situations.
11. Geographic isolation is required for one species to become two. Homo sapiens is a species that can cope with many different things, and something in evolution must have done that. The simulations suggest that temperature and other planetary conditions influenced the migrations of early humans and may have contributed to the emergence of modern humans around 300,000 years ago. This result provides clear evidence that climate change played a role in shaping human evolution.
