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Climate Effects on Human Evolution
This article explores the hypothesis that key human adaptations evolved in response to environmental instability. This idea was developed during research conducted by Dr. Rick Potts of the Smithsonian’s Human Origins Program. Natural selection was not always a matter of ‘survival of the fittest’ but also survival of those most adaptable to changing surroundings.
Paleoanthropologists – scientists who study human evolution – have proposed a variety of ideas about how environmental conditions may have stimulated important developments in human origins. Diverse species have emerged over the course of human evolution, and a suite of adaptations have accumulated over time, including upright walking, the capacity to make tools, enlargement of the brain, prolonged maturation, the emergence of complex mental and social behavior, and dependence on technology to alter the surroundings.
The period of human evolution has coincided with environmental change, including cooling, drying, and wider climate fluctuations over time. How did environmental change shape the evolution of new adaptations, the origin and extinction of early hominin species, and the emergence of our species, Homo sapiens? (‘Hominin’ refers to any bipedal species closely related to humans – that is, on the human divide of the evolutionary tree since human and chimpanzee ancestors branched off from a common ancestor sometime between 6 and 8 million years ago.)
How do we know Earth’s climate has changed? How quickly and how much has climate changed? One important line of evidence is the record of oxygen isotopes through time. This record of δ18O, or oxygen stable isotopes, comes from measuring oxygen in the microscopic skeletons of foraminifera (forams, for short) that lived on the sea floor. This measure can be used as an indicator of changing temperature and glacial ice over time. There are two main trends: an overall decrease in temperature and a larger degree of climate fluctuation over time. The amount of variability in environmental conditions was greater in the later stages of human evolution than in the earlier stages.
Ten-million-year record of oxygen stable isotopes, measured in foraminifera recovered from deep-sea sediment cores, illustrates that global ocean temperature and glacial ice varied widely over the past 6 million years, the period of human evolution. The δ18O measurement is the ratio of the heavier 18O isotope and the lighter 16O, which is more easily evaporated from the ocean and sequestered in glacial ice on land. During the course of human evolution, the overall δ18O trend has been toward a cooler, glaciated world. However, the amplitude of oscillation also increased beginning around 6 Ma, and became even larger over the past 2.5 Ma. Evolution of the genus Homo and of the adaptations that typify H. sapiens were associated with the largest oscillations in global climate. Icons: (a) hominin origins, (b) habitual bipedality, (c) first stone toolmaking and eating meat/marrow from large animals, (d) onset of long-endurance mobility, (e) onset of rapid brain enlargement, (f) expansion of symbolic expression, innovation, and cultural diversity.
Organisms and Environmental Change
All organisms encounter some amount of environmental change. Some changes occur over a short time, and may be cyclical, such as daily or seasonal variations in the amount of temperature, light, and precipitation. On longer time scales, hominins experienced large-scale shifts in temperature and precipitation that, in turn, caused vast changes in vegetation – shifts from grasslands and shrub lands to woodlands and forests, and also from cold to warm climates. Hominin environments were also altered by tectonics – earthquakes and uplift, such as the rise in elevation of the Tibetan Plateau, which changed rainfall patterns in northern China and altered the topography of a wide region. Tectonic activity can change the location and size of lakes and rivers. Volcanic eruptions and forest fires also altered the availability of food, water, shelter, and other resources. Unlike seasonal or daily shifts, the effects of many of these changes lasted for many years, and were unexpected to hominins and other organisms, raising the level of instability and uncertainty in their survival conditions.
Many organisms have habitat preferences, such as particular types of vegetation (grassland versus forests), or preferred temperature and precipitation ranges. When there’s a change in an animal’s preferred habitat, they can either move and track their favored habitat or adapt by genetic change to the new habitat. Otherwise, they become extinct. Another possibility, though, is for the adaptability of a population to increase – that is, the potential to adjust to new and changing environments. The ability to adjust to a variety of different habitats and environments is a characteristic of humans.
Three possible outcomes of population evolution in environmental dynamics typical of the Plio-Pleistocene (left). The ability to move and track habitat change geographically (narrow lines) or to expand the degree of adaptive versatility is important for any lineage to persist. Extinction occurs if species populations have specific dietary/habitat adaptations (i.e., a narrow band of ‘adaptive versatility’; highlighted bands) and cannot relocate to a favored habitat. In the hypothetical situation (right band) where adaptive versatility expands, migration and dispersal may occur independently of the timing and direction of environmental change. The evolution of adaptive versatility is the impetus behind the variability selection idea, which is explored later in this article.
Adaptation to Change
There are many ideas about the role of the environment in human evolution. Some views assume that certain adaptations, such as upright walking or tool-making, were associated with drier habitat and the spread of grasslands, an idea often known as the savanna hypothesis. According to this long-held view, many important human adaptations arose in the African savanna or were influenced by the environmental pressure of an expanding dry grassland.
If key human adaptations evolved in response to selection pressure by a specific environment, we would expect those adaptations to be especially suited to that habitat. Hominin fossils would be found in those environments and not present in diverse types of habitat.
The Variability Selection Hypothesis
A different hypothesis is that the key events in human evolution were shaped not by any single type of habitat (e.g., grassland) or environmental trend (e.g., drying) but rather by environmental instability. This idea, developed by Dr. Rick Potts of the Human Origins Program, is called variability selection. This hypothesis calls attention to the variability observed in all environmental records and to the fact that the genus Homo was not limited to a single type of environment. Over the course of human evolution, human ancestors increased their ability to cope with changing habitats rather than specializing on a single type of environment. How did hominins evolve the ability to respond to shifting surroundings and new environmental conditions?
Over time (from left to right), new adaptations may evolve during periods of (A) relatively stable environment; (B) directional or progressive change, such as from wet to dry; or (C) highly variable habitat, as predicted by the variability selection hypothesis.
One way organisms can cope with environmental fluctuation is through genetic adaptation, where several alleles, or different versions of genes, are present in the population at different frequencies. As conditions change, natural selection favors one allele or genetic variant over another. Genes that can facilitate a range of different forms under different environments (phenotypic plasticity) can also help an organism adapt to changing conditions.
Another response to environmental change is to evolve structures and behaviors that can be used to cope with different environments. The selection of these structures and behaviors as a result of environmental instability is known as variability selection. This hypothesis differs from those based on consistent environmental trends. Environmental change in an overall direction leads to specializations for those specific conditions. But if the environment becomes highly variable, specializations for particular environments would be less advantageous than structures and behaviors that enable coping with changing and unpredictable conditions. Variability selection refers to the benefits conferred by variations in behavior that help organisms survive change. To test the variability selection hypothesis, and to compare it with habitat-specific hypotheses, Potts examined the hominin fossil record and the records of environmental change during the time of human evolution.
If environmental instability was the key factor favoring human adaptations, new adaptations would be expected to occur during periods of increased environmental variability, and these adaptations would have improved the ability of early human ancestors to deal with habitat change and environmental diversity.
Overall, the hominin fossil record and the environmental record show that hominins evolved during an environmentally variable time. Higher variability occurred as changes in seasonality produced large-scale environmental fluctuations over periods that often lasted tens of thousands of years. The variability selection hypothesis implies that human traits evolved over time because they enabled human ancestors to adjust to environmental uncertainty and change. The hypothesis addresses the matter of how, exactly, adaptability can evolve over time.
Ancient Hominins Were Found in Diverse Habitats
Ancient hominin remains have been found in a variety of different habitats. While some hominins, such as Orrorin tugenensis and Ardipithecus ramidus have been found in wooded habitats, others such as Sahelanthropus tchadensis were found associated with diverse types of vegetation within a small geographic area. Reconstructions of the ancient habitat of Ardipithecus ramidus at two different Ethiopian sites suggest that this species occupied both wooded areas (the Aramis site) and wooded grasslands in which grazing animals predominated (the Gona site). Australopithecus anamensis has been found at Kanapoi and Allia Bay, Kenya, in association with another type of mosaic – an open savanna with low trees and shrubs, but with both grasslands and gallery forests nearby.
At Kanapoi, research by Dr. Jonathan Wynn on paleosols and pedogenic carbonates demonstrates the presence of these varied habitats at the time when Australopithecus anamensis inhabited the area. Other members of Au. anamensis at Allia Bay encountered a different environment. The fossil animals represent several different habitats including open floodplains, gallery forests, and dry bushlands. Isotopic studies done by Dr. Margaret Schoeninger and her colleagues indicate that most of the Allia Bay vegetation consisted of woody plants such as trees and shrubs (known as C3 vegetation). Australopithecus anamensis at Allia Bay was thus associated with a mosaic environment, including woodlands near the ancestral Omo River and open savanna further away.
Two different types of environment – dense woodlands and open bushland – occurred in the same areas of East Africa during the period of human evolution. Climate fluctuation altered the proportion of these habitats, and thus led to repeated changes only in population density and variable conditions of natural selection.
Two Legs, Long Arms; Moving Around in Diverse Habitats
By about 4 million years ago, the genus Australopithecus had evolved a skeletal form that enabled adjustment to changes in moisture and vegetation. The best current example of adaptability in Australopithecus is apparent in the skeleton known as Lucy, which represents Au. afarensis. Lucy’s 3.18-million-year-old skeleton has a humanlike hip bone and knee joints coupled with long apelike arms, longer grasping fingers than in humans, and flexible feet for walking or climbing. This combination of features, which appears to have characterized Australopithecus for nearly 2 million years and possibly older hominins, afforded an ability to move around in diverse habitats by changing the degree of reliance on terrestrial walking and arboreal climbing. This flexibility may also have characterized earlier hominins such as Ardipithecus ramidus.
Stone Toolmaking: Gaining Access to Diverse Foods
The first known stone tools date to around 2.6 million years ago. Making and using stone tools also conferred versatility in how hominin toolmakers interacted with and adjusted to their surroundings.
Simple toolmaking by stone-on-stone fracturing of rock conferred a selective advantage in that these hominin toolmakers possessed sharp flakes for cutting and hammerstones that were useful in pounding and crushing foods. Basic stone tools thus greatly enhanced the functions of teeth in a way that allowed access to an enormous variety of foods. These foods included meat from large animals, which was sliced from carcasses using sharp edges of flakes. Bones were broken open using stones to access the marrow inside. Other tools could be used to grind plants or to sharpen sticks to dig for tubers. Tool use would have made it easier for hominins to obtain food from a variety of different sources. Tool use would have widened the diet of hominins. Meat, in particular, is a food that was obtainable in equivalent ways, with similar nutritional value, in virtually any type of habitat that early humans encountered.
Although making simple toolmaking may have developed originally in one type of environment, the carrying of stone tools over considerable distances – and becoming reliant on stone technology – may have arisen due to the benefits of altering the diet as environments changed. The oldest known stone technology – called Oldowan toolmaking – involved carrying rock over several kilometers and is found associated with a variety of ancient habitats. Redistributing stone and other resources, such as parts of animal carcasses, by transporting them may have helped hominins cope with variable habitats.
The Expanding World of Early Homo
As predicted by the variability selection hypothesis, hominins were not found solely in one kind of habitat, but rather in a variety. A major signal of the ability to tolerate different environments was the dispersal of the genus early Homo beyond Africa into Asian environments. After 1.9 million years ago, the genus Homo is found in a variety of locations in Asia, including some that are relatively far north.
Early evidence of the diversity of Homo erectus environments in Asia includes the following sites:
- Dmanisi, Republic of Georgia, 1.85 to 1.78 Million years ago. This site has grasslands surrounded by mountains with forests. Hominins had access to lava as a raw material for tools.
- Yuanmou, China, 1.7 Million years ago. This site, located near an ancient lake, had a mixture of habitats with grasslands, bushlands and forests.
- Nihewan Basin, China, 1.66 Million years ago. The Nihewan sites were also near a lake. Hominin toolmakers experienced many changes in vegetation over time, with habitats ranging from forests to grasslands. This region may have been much more arid than others, and temperatures changed seasonally between warm and cold.
- Java, 1.66 Ma: Hominins here encountered grasslands, rivers and marine coastal environments in a tropical latitude setting.
In these locations, hominin groups encountered distinctly different environments, different plants and animals and foods, and different climatic conditions – a very wide range of temperature and strong variations in aridity and monsoonal rains.
Hominins Persisted Through Environmental Change
Environmental instability may have been a factor not only in shaping adaptations but also in contributing to the extinction of some lineages. Environmental variability associated with the extinction of large mammal species has been proposed for the southern Kenya region. Sediments, stone artifacts, and animal faunal at the site of Olorgesailie span most of the past 1.2 million years. Numerous environmental shifts are recorded in the Olorgesailie deposits. The ancient lake level and its chemistry, for example, changed frequently, and sometimes the lake dried up, leaving small wetlands and streams as the main source of water in the basin. Volcanic eruptions also blanketed the landscape in ash, killing off grass and reshaping the properties of the ecosystem.
An example of a hillside of sediments in the Olorgesailie region. The hillside, which represents about 10,000 years of time with a volcanic ash at its base dated around 1 million years ago, shows evidence of strong environmental shifts. Inset: Layers of sediments show the fluctuation between dry and wet environments and a time when volcanic ash covered the ancient landscape.
Dr. Rick Potts studied the pattern of climatic turnover in the fauna and the occurrence of archeological sites at Olorgesailie and another site in southern Kenya, and found that several large mammal species that had previously dominated the fauna of this region went extinct between about 700,000 and 300,000 years ago, during a period of repeated environmental instability. These species were replaced by modern relatives, which tended to be smaller in body size and not as specialized in diet or habitat.
For example, the zebra Equus oldowayensis had large and tall teeth specialized for eating grass. Its last known appearance in the fossil record of southern Kenya is between 780,000 and 600,000 years ago; it was replaced by Equus grevyi, which can graze (feed on grass) as well as browse (feed on leaves and other high-growing vegetation). The fossil baboon Theropithecus oswaldi, which weighed over 58 kg (over 127.6 pounds), lived on the ground exclusively; it had very large teeth and consumed grass. It also went extinct between 780,000 and 600,000 years ago. Its extant relative, Papio anubis, is omnivorous and moves easily on the ground and in trees. Two other large-bodied animals that specialized in eating grass, the elephant Elephas recki and the ancient pig Metridiochoerus, were also replaced by related species that were smaller and had more versatile diets (Loxodonta africana and Phacochoerus aethiopicus). The aquatic specialist Hippopotamus gorgops was replaced by the living hippopotamus, which is capable of traversing long distances between water bodies.
The replacement of the specialized species by closely related animals that possessed more flexible adaptations during a time of wide fluctuation in climate was a key piece of initial evidence that led to the variability selection hypothesis. Although Acheulean toolmaking hominins were able to cope with changing habitats throughout much of the Olorgesailie record, the Acheulean way of life disappeared from the region sometime between 500,000 and 300,000 years ago, perhaps also a casualty of strong environmental uncertainty and changing circumstances.
Encephalization and Adaptability
Brain enlargement during human evolution has been dramatic. During the first four million years of human evolution, brain size increased very slowly. Encephalization, or the evolutionary enlargement of the brain relative to body size, was especially pronounced over the past 800,000 years, coinciding with the period of strongest climate fluctuation worldwide. Larger brains allowed hominins to process and store information, to plan ahead, and to solve abstract problems. A large brain able to produce versatile solutions to new and diverse survival challenges was, according to the variability selection hypothesis, favored with an increase in the range of environments hominins confronted over time and space.
New Tools for Many Different Purposes
After 400,000 years ago, hominins found new ways of coping with the environment by creating a variety of different tools. In some parts of Africa, a shift occurred in which a technology dominated by large cutting tools was replaced by smaller, more diverse toolkits. Technological innovations began to appear in the Middle Stone Age in Africa, with some early examples dating prior to 280,000 years ago. Some of the new tools provided ways for hominins to access food in new ways. Points were hafted, or attached to handles such as spear or arrow shafts, and were later used as part of projectile weapons, which allowed hominins to hunt fast and dangerous prey without approaching as closely. Barbed points were used to spear fish. Barbed points made from bone were found at the site of Katanda, in the Democratic Republic of the Congo, along with the remains of huge catfish. Grindstones were used to process plant foods. Other tools were used to make clothing which would have been important for hominins in cold environments.
Regional Exchange and Social Networks
Over the past 300,000 years or so, the direct ancestors of living humans developed the capacity to create new and diverse tools. Archeological discoveries show that wider social networks began to arise, enabling the transfer of stone material over long distances. Symbolic artifacts connoting complex language and the ability to plan are also evident in the archeological record of the Middle Stone Age of Africa. These findings indicate an improved capacity to adjust to new environments. Most of the past 350,000 years in East Africa was a time of strong climate oscillation. The timeline at the bottom of the image is 280,000 to 40,000 years ago (right to left). This figure is based on an analysis by archeologists Sally McBrearty and Alison Brooks.
Trading between groups to obtain materials and to cement alliances is a hallmark of modern human behavior. Larger brains and symbolic ability facilitated more complex social interactions. By 130,000 years ago, hominins were exchanging materials over distances of over 300 km. The social bonds that were forged by exchanging materials between groups may have been critical for survival during times of environmental change when one group relied on the resources or territories of a distant group. Modern foragers use social ties to mitigate the effects of famines and droughts. The exchange of gifts maintains relationships between groups, which may be called upon when one group needs to live at another’s camp or waterhole, a capacity that proved especially beneficial during times of environmental change and resource uncertainty.
Communication and Symbols
Engraved ocher plaque from Blombos Cave, Republic of South Africa; about 77,000–75,000 years old
Evidence of the human capacity for communication using symbols is apparent in the archeological record back to at least 250,000 years old, and probably older. The use of color, incised symbols, decorative objects, and language are part of this capacity for communication. Symbolic communication may be linked with information storage. Language is an essential part of modern human communication. Language makes it possible to convey complex ideas to others. Communication of ideas and circumstances via language would have made survival in a changing world much easier. However, there is no fossil evidence for words and grammar that are the direct hallmarks of human language.
Preserved pieces of pigment are one of the earliest forms of symbolic communication. Ocher and manganese can be used to color objects and skin. Other symbolic objects such as jewelry, personal adornments, and art convey information about the owner’s social status, group membership, age or sex. Paintings and drawings were also used to represent the natural world. Use of symbols is ultimately connected to the human ability to plan, record information, and imagine.
Neanderthals Endured Climatic Oscillations, Too!
Neanderthal populations (Homo neanderthalensis) in Europe endured many environmental changes, including large shifts in climate between glacial and interglacial conditions, while living in a habitat that was colder overall than settings where most other hominin species lived. Some of the environmental shifts they endured involved rapid swings between cold and warm climate.
The Neanderthals were able to adjust their behavior to fit the circumstances. During cold, glacial periods, they focused on hunting reindeer, which are cold-adapted animals. During warmer, interglacial periods, they hunted red deer. During extreme cold periods, they shifted their range southwards toward warmer environments.
Neanderthals and modern humans had different ways of dealing with environmental fluctuation and the survival challenges it posed. Modern humans, Homo sapiens, had specialized tools to extract a variety of dietary resources. They also had broad social networks as shown by the exchange of goods over a long distance. They used symbols as a means of communicating and storing information. Neanderthals did not make tools that were as specialized as those of modern humans who moved from Africa into Europe sometime around 46,000 years ago. The Neanderthals usually did not exchange materials over so wide a distance as Homo sapiens. They occasionally produced symbolic artifacts. Despite many climatic fluctuations, modern humans were able to expand their range over Europe and Asia, and into new areas such as Australia and the Americas. Neanderthals went extinct. This evidence suggests that adaptability to varying environments was one of the key differences between these two evolutionary cousins.
During the time when Neanderthals evolved in Europe, global climate fluctuated dramatically between warm and cold. The highlighted area on the right side of the graph represents the last 200,000 years.
Overall, the evidence shows that hominins were able to adapt to changing environments to different degrees. The genus Homo, to which our species belongs, had the capacity to adjust to a variety of environmental conditions, and Homo sapiens is especially able to cope with a broad range of climatic conditions, hot and cold environments, arid and moist ones, and with all kinds of varying vegetation. We use resources from a vast variety of plants and animals and use many specialized tools. We have many social contacts and means of exchanging resources and information to help us survive in a constantly changing world.
The idea that the major adaptations in our evolutionary history arose in response to environmental variability and shifting selection pressures (variability selection) leads to a new understanding of human evolution. The figure above illustrates how the emergence of human characteristics from 6 million years ago to present conferred benefits that improved the ability of our ancestors to survive unpredictable and novel environments. Ma = million years ago; ka = thousand year ago.
Humans today represent the one species that has survived from the diversity of hominin species. Despite their very close relationship with our species, and despite the fact that all of them possessed some combination of features that characterize humans today, these earlier species and their ways of life are now extinct. The question ahead is how well our sources of resilience as a species will succeed as our alterations of the landscape, atmosphere, and water interact with the tendency of Earth’s environment to shift all on its own. This is an ‘experiment’ just now unfolding, one that has never occurred before. The intensity of environmental change seems likely to create entirely new survival challenges for the lone hominin species on the planet, and many other organisms as well.