Date: Wednesday, 03 October 2018
Scientists 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, brain enlargement, prolonged maturation, the emergence of complex mental and social behavior, and dependence on technology.
Human evolution has coincided with environmental change, including cooling, drying, and wider climate fluctuations. 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? How do we know the 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, which comes from measuring oxygen in the microscopic skeletons of foraminifera that lived on the sea floor. It 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 earlier stages.
All organisms encounter environmental change. Some changes occur over a short time, and may be cyclical, such as daily or seasonal variations in 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. 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 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 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.
There are many ideas about the role of the environment in human evolution. Some views such as the savanna hypothesis assume that certain adaptations, including upright walking or tool-making, were associated with drier habitat and the spread of grasslands. 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.
A different hypothesis is that the key events in human evolution were shaped not by any single type of habitat or environmental trend but rather by environmental instability. 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 in a single type of environment.
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 to adapt to changing conditions. 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.
Ancient hominin remains have been found in a variety of different habitats, including wooded habitats and in diverse types of vegetation within a small geographic area. In general, the fossil animals represent several different habitats such as open floodplains, gallery forests, and dry bushlands. 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. Australopithecus afarensis’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 (e.g., Ardipithecus ramidus).
The first known stone tools date to around 2.6 million years ago. Making and using stone tools also conferred versatility in how hominin tool-makers interacted with and adjusted to their surroundings. Simple tool-making by stone-on-stone fracturing of rock conferred a selective advantage in that these hominin toolmakers possessed sharp flakes for cutting and hammer-stones that were useful in pounding and crushing foods. Basic stone tools 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 that would have widened their diet.
Although simple tool-making 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, Oldowan tool-making, involved carrying rock several kilometers and is 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.
As predicted by the variability selection hypothesis, hominins were found in a variety of habitats. A major signal of the ability to tolerate different environments was the dispersal of the genus early Homo beyond Africa into Asian environments. By 1.9 million years ago, the genus Homo is found in a variety of locations in Asia, including some relatively far north.
Environmental instability and variability may have been a factor not only in shaping adaptations but also in contributing to the extinction of some lineages. For example, at the Buia deposits (Eritrea), where numerous environmental shifts are recorded, sediments, stone artifacts, and animal faunal span most of the past million years.
The pattern of climatic turnover in the fauna and archeological sites in eastern Africa show 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 and had a less specialized diet or habitat.
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 tool-making hominins were able to cope with changing habitats in Africa, the Acheulean way of life disappeared from the region between 500,000 and 300,000 years ago, perhaps also a casualty of strong environmental uncertainty and changing circumstances.
By 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 during 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 closely. Grindstones were used to process plant foods. Other tools were used to make clothing, important for hominins in cold environments.
Trading between groups to obtain materials and strengthen 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 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 especially beneficial during times of environmental change and resource uncertainty.
Evidence of the human capacity for communication using symbols is apparent in the archeological record back at least 250,000 years ago, and probably older. The use of color, incised symbols, decorative objects and language are part of this capacity for communication. Language is an essential part of human communication. 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, the hallmarks of human language.
Symbolic communication may be linked with information storage, and preserved pigment pieces are among 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.
Neanderthal populations in Europe endured many environmental changes 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. 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, as well as broad social networks. 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 to Europe sometime around 46,000 years ago. Neanderthals usually did not exchange materials over so wide a distance as Homo Sapiens, and 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.
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 were able to cope with a range of climatic conditions, hot and cold environments, arid and moist ones, and with all kinds of varying vegetation. We use resources from a 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.
Humans today represent the one species that has survived from the diversity of hominin species. Despite their 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 are now extinct. The question is how well will our sources of resilience as a species succeed as our alterations of the landscape, atmosphere, and water interact with the tendency of Earth’s environment to shift 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.
Reference: Smithsonian - National Museum of Natural History