Humans: The Species That Changed Earth

Image Credit: Adrian Kenyon

By Erle C. Ellis


Global climate change, widespread extinctions, and pervasive pollution are just a few of the many symptoms of the global environmental changes produced by human activities. There is a growing consensus that human societies have emerged as a “great force of nature” that is shifting Earth into a new epoch of geologic time, the Anthropocene 1, 2.  Why? Biology alone cannot explain this.

While Homo sapiens does have some distinctive biological traits, stone tools and control of fire are not among them; both were common to ancestral hominins long before sapiens emerged among them. The central question of the Anthropocene, why did behaviorally modern humans gain the unprecedented capacity to change an entire planet, cannot be answered by genetic changes in human behavior. To explain why human societies scaled up to become a global force capable of changing the Earth and why there are so many different forms of human societies and ecologies shaped by them, explanations must be sought beyond the theories of biology, chemistry or physics. Here I introduce a new evolutionary theory, sociocultural niche construction, aimed at explaining the origins of human capacity to transform the Earth 3. As will be seen, this theory also explains why behaviorally modern human societies came to transform ecology in so many different ways over the past 50,000 years as they expanded across the Earth.

Like most ecologists whose work involves humans, my research has focused primarily on the ecological consequences of human activities, not the causes.  In 2012, the editor of Ecological Monographs pushed me to go further, to explain why humans have reshaped more than three quarters of the terrestrial biosphere from their “natural” biome patterns shaped by climate, like tropical rainforests, grasslands, and deserts, into the urban, village, cropland, rangeland and seminatural anthropogenic biomes now common across Earth’s land (anthromes) 4.

My efforts to answer this question required a huge dose of social learning: a deep dive into evolutionary theory and a sustained effort at transdisciplinary synthesis across existing theories of niche construction, the Extended Evolutionary Synthesis, cultural evolution, ultrasociality, and social change. The resulting monograph presents a new theory, sociocultural niche construction, to explain the emergence of humanity as a global force that is reshaping ecology across the biosphere and weighs its implications for ecological science (anthroecology theory3.

In this article, I explain why cultural evolution is at the core of this theory, explaining why humans alone among multicellular species gained the capacity to change a planet and why different societies do this differently, as the result of sociocultural evolution (the social evolution of cultural systems), in which natural selection acts simultaneously on individuals, social groups, and societies causing long-term behavioral changes in them across human generational time (multi-level selection; explored in this blog post).

The first step in understanding why humans change environments is the recognition that all species do this to some degree.  This is known as ecosystem engineering in ecology. More importantly, these alterations can have evolutionary consequences. This is the theory of niche construction, in which organisms alter their environments, for example, by building nests or by producing chemicals that inhibit other species (allelopathy), producing an ecological inheritance– which may be beneficial, detrimental or neutral to their adaptive fitness, and/or to that of other species sharing their environment 5. Niche construction theory makes evolution a two-way street: organisms do not just adapt within environments that they cannot alter, they also alter their environments, producing ecological inheritances and altered environments that may require further adaptations.

In ecology, cultural inheritances are defined as heritable traits transmitted through social learning 6, 7. By bringing cultural inheritances together with ecological inheritances and other forms of genetic and nongenetic inheritances, we arrive at the Extended Evolutionary Synthesis (EES) mentioned above. The EES holds that the phenotypic traits of organisms are produced by a combined suite of inheritances, the “inclusive inheritance”, composed of cultural, ecological, genetic, epigenetic and parental inheritances that evolve together to produce evolutionary changes in phenotypes 7, 8.

The implications of coupling cultural and ecological inheritances together in a single evolutionary theory are profound even for nonhuman species. However, for behaviorally modern humans, in which cultural inheritances determine both the organization of societies and their strategies for utilizing and transforming their environments, the implications of the EES are absolutely transformative. Moreover, the EES also includes multiple modes by which inheritances may be transmitted, from the vertical transmission of genetic traits from parent to progeny to the horizontal transmission of inheritances across unrelated individuals within a single generation – common to both cultural traits and gene transfers across microbes – to the oblique inheritance of traits from older to younger generations, enabling horizontal transmission of inheritances across generations and their accumulation over the long-term.

A key prediction of the EES is that when environments vary rapidly, within the span of a single generation, this tends to favor adaptive traits that are inherited horizontally and obliquely, like cultural traits (or horizontal gene flow among microbes) over traits transmitted vertically (like genetic traits in multicellular organisms passed from parent to offspring), because this produces greater phenotypic plasticity in adapting to environmental changes within a single generation.  As a result, the EES predicts that cultural traits (and other forms of horizontally transmitted traits – like bacterial genes) will tend to be selected for in highly variable environments.

Given that humans, even those living in hunter gatherer societies, are among the most potent of all ecosystem engineers, their environments tend to change rapidly, within a single generation, through use of fire to clear vegetation, by massive hunting and foraging pressures, and the propagation, management and dispersal of species 9. Human ecosystem engineering behaviors also tend to be socially learned and socially enacted (produced by cooperation within and among social groups), highlighting the adaptive importance of cultural traits and their interplay with ecological inheritances in defining the evolutionary processes of behaviorally modern human societies and their socially engineered environments.

While there are many social species, the sociality of behaviorally modern humans is exceptional. Our capacities for social learning and the accumulation and evolution of cultural inheritances are unrivalled, and have produced societies structured largely by socially learned social relationships that include dependence on non-kin individuals for survival, marking our species as the most social of all, Earth’s first ultrasocial animal 10. To survive and reproduce within behaviorally modern human societies, it is necessary to socially learn the requisite behaviors for interacting with both kin and non-kin individuals and within and across social groups and societies, such that human adaptive fitness is a function of culture, not of biology 11. Individual, group and societal behaviors vary profoundly both within and among societies, including strategies for ecosystem engineering, exchange of food and other resources among kin and non-kin, forms of social organization, and even the modes of social transmission of culture (e.g. languages, technologies, ritual practices, artistic expressions, etc.).

In behaviorally modern human societies, direct interactions with the environment to gain sustenance and other necessities by foraging, farming or even shopping at the store, may be optional; sustenance and other necessities may be gained through complex social relationships among unrelated and unknown individuals – like when you go to the grocery store to get food without ever setting foot on a farm. The human niche, the way that humans as a species utilize and transform environments to survive and to reproduce, is thus largely sociocultural, constructed and enacted within, across, and by social groups and societies. Long-term changes in the construction of the behaviorally modern human niche, the structure and functioning of human societies and their transformation of environments, is the product of evolution by natural selection acting on the individual and groups via social modes of sociocultural niche construction.

This perspective is not entirely new either. Charles Darwin noted that human cultural traits appear to evolve much more rapidly than biologically-determined traits 12. This is clearly one of the main reasons why human societies have evolved so many diverse and complex cultural forms and why they have changed so much over the mere 50,000 years since behaviorally modern humans first spread across the Earth. Hunter gatherer societies, some of which successfully sustain themselves today even in the face of pressures from larger scale societies, are generally composed of small bands of mostly related individuals and rely on remarkably complex sociocultural toolkits including social hunting, traps and projectiles, resource sharing, niche broadening (expanding the range of utilized species when preferred species are driven extinct), food processing, the clearing of vegetation using fire to increase success in hunting and foraging and even the propagation of favored species- the first stages of domestication.

Agricultural societies have built on these complex sociocultural strategies to develop even more novel and transformative ecosystem engineering regimes, from domesticated species, tillage, and irrigation to manuring and the marketplace. They have also developed larger and larger social groupings with more and more complex and unequal social organization requiring increasingly diverse and specialized social roles, from urban dwelling craftsmen to traders, taxmen, the utilization of ever more complex tools and technologies enabling even greater modification of environments, and the harnessing of domestic livestock and water power to supplement human energy. Industrial societies have scaled up even more, developing massive and rapidly growing populations, global trade in food and other resources, the use of fossilized biomass and even non-biomass forms of energy to supplement and eliminate human energy in ecosystem engineering, food and resource sharing, and even communications.

Across societies, sociocultural evolution has been accelerated by both the ratchet effect and by runaway processes of sociocultural niche construction. In the ratchet effect, multiple cultural traits may become aggregated into a single complex cultural trait, such as the “recipe” to produce a bow and arrow, and this complex trait may then be transmitted horizontally as a single trait from one society to another. Even greater bursts of sustained evolutionary change – sociocultural regime shifts – can also occur through runaway sociocultural niche construction, in which the social and environmental changes produced by cultural + ecological traits, such as the cultivation of soil leading to long-term fertility loss, must be adapted to by even more transformative cultural and ecological traits, such as the harvesting and utilization of manures to maintain soil fertility, locking societies into a continuous cycle of increasingly transformative change in their sociocultural niche.

Human ultrasociality changed the Earth. While evolution is never linear or progressive, there are some remarkable general long-term trends in human social change (see Table 3 in Ellis, 2015). Over the past 50,000 years, the potential scale of individual human societies has grown from a few dozen individuals to several hundred million. The potential productivity of a single square kilometer of land has been increased through cooperative ecosystem engineering from sustaining less than ten individuals to sustaining more than one thousand. Energy use per individual has expanded by a factor of more than 20 times through use of non-biomass energy, mostly from fossil fuels. The flow of materials, energy, biota and information across human societies has become essentially global. And human individuals now live nearly twice as long on average as they did in the Paleolithic.

A number of theories have been advanced to explain why human societies gained these unprecedented scales and capacities, from the competitive advantages of large scale cooperation in warfare 13, to the increasing economies of scale in dense populations 14. No doubt the many evolutionary patterns of human sociocultural niche construction have emerged in response to many different pressures – and even at random – but multilevel selection acting on human cultural inheritances is the ultimate shaper of both the great diversity and the unprecedented scales of the human sociocultural niche.

Sociocultural niche construction theory is still at an early stage of development.  It is critical to remember that like biological evolution, sociocultural evolution is a process, not a destiny. The future remains fully open to surprise – the large scale societies of today could quickly go the way of the dinosaurs. Given current trends in environmental disruption and growing social inequality, such outcomes seem increasingly plausible. Nevertheless, as contemporary societies advance in their ability to understand the ultimate causes, not just the consequences, of their transformation of the Earth, this knowledge has the potential to enable societies to seek and implement social strategies aimed towards sustaining both themselves and nonhuman species more successfully and to make progress toward more desirable futures.

The call to recognize the Anthropocene as a new epoch of geologic time confronts us with the need to understand and better guide the dynamics of human societies as a global force reshaping the Earth System.  Long-term changes in human social organization, cooperative ecosystem engineering, exchange relationships, and energy systems are now tightly coupled with long-term changes in the Earth System that are altering ecology across our planet in profound and possibly permanent ways. While it is possible that for most people on Earth, times have never been better, the opposite is true for most other species.

In an increasingly anthropogenic biosphere it is essential to shift the paradigm. Humanity has emerged as a global sociocultural force. We and all other species now live on a used planet reshaped by generations of our ancestors. On the one hand, human societies are polluting air, land and sea, changing Earth’s climate and expanding into the habitats of other species, driving them to extinction. But the opposite is also true. Societies have managed to reduce their pollution, restore habitats, conserve species, and may yet implement the massive shift in energy systems that could circumvent catastrophic climate change. It is time to go beyond the idea that somehow a “balance of nature” will pull humanity back towards some safe harbor and move forward to embrace the sociocultural tools and the “cultures of nature” in which human societies become better both for humans and for all the rest of Earth’s species that must now live together with us on a used planet.


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