Scientists know from the study of tree rings, layers of ice in glaciers, ocean sediments, coral reefs and sedimentary rocks that climate change has been occurring on Earth on a regular basis for at least the last 5 or 6 million years — without any help from humanity.

Over the last million years, for example, the surface temperature of Earth has risen and fallen by approximately 3 to 8 degrees Celsius in ice-age cycles roughly every 100,000 years. As recently as 20,000 years ago, the sea level was at least 120 meters lower than today because so much water was frozen in ice sheets.

As explained by the Australian Academy of Science, these global changes can be attributed to slight wobbles in Earth's orbit that have altered when and where sunlight falls on our planet's surface, variations in the sun's output of solar energy and even volcanic eruptions that have generated billions of sunlight-reflecting particles, effectively cooling the climate.

Climate Change as an Agent of Natural Selection

In parallel with these climatic gyrations, the human race has also been evolving. According to the Smithsonian's Human Origins Program, the emergence and survival of today's Homo sapiens species — an evolution that included the genera Ardipithecus, Australopithecus afarensis, Homo habilis, Homo erectus and Homo neanderthalensis — supports the theory that human adaptations have occurred largely in response to environmental instability and that individuals who survived were those best able to adapt to these changing environmental conditions.

The adaptations most central to this human evolution include the ability to walk upright, the capacity to make tools, the enlargement of the brain, the emergence of complex mental and social behavior, and dependence on technology to alter surroundings.

How Did Climate Change Affect Early Humans?

Researchers have observed that most organisms on the planet live in habitats that experience moderate daily or seasonal variations in temperature, light and precipitation. Over time, these organisms typically develop characteristics and behaviors well suited to living and surviving in this preferred habitat. If the cyclical variations become too extreme, however, an organism will either have to move its habitat or adapt to the new habitat through genetic change. Otherwise, it will go extinct.

Humans have proven to be extremely adaptable to changes in environmental conditions and habitat. Over the last 5 million years, for example, large-scale shifts in temperature and precipitation have turned grasslands and shrublands into woodlands and forests and changed cold climates into warm ones. Tectonic events have also altered the landscape, creating changes in rainfall patterns or in the size or location of lakes and rivers.

These changes forced hominins — any of the aforementioned bipedal species closely related to humans — to rethink the rules of engagement for securing food, clothing and shelter in their now-altered habitat.

Climate Instability Motivates Survival Skills

Human Origins Program Researcher Rick Potts has developed an idea called variability selection that hypothesizes that key events in human evolution were shaped not by habitat alone — some people think, for example, that walking upright evolved from the need to travel long distances in drier climates — but rather by environmental instability. Human ancestors may have responded to increased variability in their environment and habitat by developing coping mechanisms or characteristics that enabled them to survive.

Potts has examined this theory in the context of fossil records of early humans. He's found that ancient hominins lived in a variety of habitats on multiple continents. The genus Ardipithecus ramidus, for instance, has been found in wooded habitats, while Australopithecus anamensis, the notional beginning of upright-walking hominins, have been found in open savannas with low trees and shrubs adjacent to grasslands and forests.

A further indication of early humans' ability to tolerate different environments was the dispersal of the genus Homo erectus beyond Africa into Asia. Hominins have been found in geographically and environmentally diverse habitats in sites ranging from Hadar in Ethiopia and Turkana in Kenya to Dmanisi in the Republic of Georgia, Java and the Nihewan Basin in China. In these locations, the groups encountered a variety of plants, animals and foods in diverse climatic conditions, including harsh monsoonal rains and extreme aridity.

How Did Early Humans Adapt to Their Environment?

The genus Australopithecus anamensis (aka "Lucy") features a skeleton with a human-like hip bone, knee joints, long ape-like arms, longer grasping fingers than humans and flexible feet for walking or climbing. This configuration gave it the ability to move around in diverse habitats, alternating as needed between walking and tree climbing.

The first known stone tools, which date to about 3.3 million years ago, indicate hominins' ability to interact with and adjust to their environment. The earliest tools, which featured stone-on-stone pounding and crushing, enabled hominins to access a wider variety of foods than they could eat by simply using their teeth. This diversity of stone tools allowed them to include both animal meat and plants in their diets. Paleoanthropologists have found evidence that early humans could carry these tools from habitat to habitat as the climate changed.

But the evolution of tools (and humans) didn't stop there. Beginning about 400,000 years ago, hominins began developing smaller, more diverse tools that allowed them to access food sources in new and effective ways. By creating sharper-edged rocks and attaching them to spears or arrows, for example, hominins were able to create projectile-type weapons, which allowed them to spear fish and hunt prey from a distance.

Adding Brains to the Outfit

Another key adaptation of Homo sapiens, according to the Human Origins Program team, has been encephalization — the evolutionary enlargement of the brain relative to body size. Over the first 4 million years of human evolution, this trend was modest at best. But over the last 800,000 years, during the periods of greatest climatic fluctuations, encephalization became much more pronounced.

A larger brain allows humans to be more thoughtful about how they manage their lives. For example, it allows them to process and store information, plan ahead and solve more abstract problems. It can also help them generate more versatile solutions to new and diverse survival challenges.

Closely related to these problem-solving skills were increasingly sophisticated social interactions among groups of early humans. They might have used these social alliances for various tasks, such as trading resources during prosperous times or sharing them to mitigate the effects of periodic famines or droughts.

Surviving by Diversifying

Of course, not all hominins survived. Some fell victim to their inability to adapt to significant and persistent climate changes. Homo neanderthalensis, for example, endured dramatic shifts in climate between glacial and interglacial conditions, including rapid swings between cold and warm climates. Their approach to dealing with this environmental instability, however, proved ineffective.

While modern humans have developed more specialized tools to satisfy their diverse dietary needs, Neanderthals' tool sets (and, by extension, their diets) appear to be far more limited. While humans were busy using their growing brains to develop social networks and exchange goods with other groups over long distances, Neanderthals maintained more limited connections with outside groups. Eventually, modern humans were able to expand their presence to include Europe, Asia, Australia and the Americas, while Neanderthals went extinct.

How Will Future Climate Change Affect Human Evolution?

Today, humans are dealing increasingly with the very real effects of climate change, from rising sea levels, more frequent wildfires and more severe storms to periods of uncharacteristically hot and cold weather. In many ways, they're dealing with the same challenges faced by their hominin ancestors: radically altered landscapes, unpredictable environmental conditions and ways of life that may no longer even exist.

Against this backdrop, our brave new world will continue to test how flexible and adaptable the human species truly is and perhaps guide us toward the beginnings of an inevitable evolution to Homo futuris.

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