The hypothetical sudden and complete disappearance of humanity would trigger an instantaneous, large-scale environmental shift across the planet. This scenario would immediately cease the profound anthropogenic pressures that have shaped global ecosystems for centuries, including industrial pollution and habitat destruction. The world would not become empty, but rather a vast, abandoned environment where natural succession would begin to reclaim human infrastructure. Within this new context, competition for dominance would begin among surviving animal species, driven by their ability to exploit available resources and adapt to a rapidly changing, wilder world.
Establishing the Criteria for Dominance
To determine what species might succeed humans, it is necessary to move beyond an anthropocentric definition of “dominance,” which typically focuses on technological intelligence or apex predation. A more accurate ecological and evolutionary measure of dominance relies on four key criteria for success in a post-human world. The first is a widespread geographic distribution, which provides a species with a built-in advantage by ensuring survival across diverse biomes and multiple local extinction events. This geographic spread is often paired with a high biomass or population density, indicating a species’ success in converting available resources into living matter and sustaining large numbers.
Next, behavioral flexibility and the ability to adapt to new niches are crucial, allowing a species to exploit the decaying remnants of human civilization and the rapidly shifting natural environment. This adaptability is often linked to the final criterion: a high reproductive speed or short generation time. Species that reproduce quickly can undergo rapid evolutionary change, allowing their populations to genetically adapt to new selective pressures much faster than long-lived species. By focusing on these metrics—distribution, biomass, flexibility, and reproductive speed—the most likely contenders for future planetary success can be identified.
The Immediate Beneficiaries
In the short term, over the course of decades to a few centuries, the most successful species would be those already thriving in the “ruin ecosystem” of abandoned human settlements. These immediate beneficiaries are the generalists and scavengers that have become commensal with human life. Species like the brown rat (Rattus norvegicus) and the common house mouse (Mus musculus) are prime examples, possessing a global distribution and a diet that allows them to subsist on the food stores and refuse left behind.
Feral populations of previously domesticated animals, such as cats (Felis catus) and dogs (Canis familiaris), would initially experience an immense population boom by exploiting the sudden lack of human-imposed controls and the abundance of livestock or abandoned pets. Similarly, highly adaptable urban wildlife, including raccoons (Procyon lotor), pigeons (Columba livia), and coyotes (Canis latrans), would quickly expand their territories. These species are uniquely positioned because they are accustomed to the built environment, using buildings for shelter and navigating complex, human-made landscapes. Their initial success is tied directly to their ability to scavenge within the existing infrastructure before it completely decays.
Long-Term Evolutionary Success
Over vast geological timescales, the true inheritors of the planet would be species with the potential for significant evolutionary development, likely leading to advanced cognitive function and complex social structures. This distinction separates the short-term scavengers from the long-term architects of a new ecological order. Among the strongest candidates are the corvids, the family of birds that includes crows, ravens, and jays. These birds are already known for remarkable intelligence, problem-solving skills, and the capacity for tool use, demonstrating a cognitive flexibility that could be selected for in a new world.
Non-human primates, particularly those with complex social systems like chimpanzees (Pan troglodytes) and baboons (Papio genus), also possess the necessary groundwork for future complex dominance. They already exhibit tool use, intricate communication, and social learning, all of which are traits that facilitate rapid adaptation to novel environments, including the use of abandoned human objects. As human structures crumble and the environment reverts, the selective pressure to exploit these new, complex resources could drive the evolution of greater dexterity and intelligence.
The marine realm offers another potential lineage for long-term intelligence-driven dominance, particularly with the cephalopods, such as octopuses. Octopuses exhibit extraordinary problem-solving abilities, including opening complex enclosures and using objects for defense, suggesting a path toward highly flexible behavior. While the development of a terrestrial, fire-based civilization is unlikely for an aquatic species, their increasing intelligence and adaptability could lead to a form of oceanic dominance that reshapes the world’s largest biome.
The Overlooked Contenders
While the focus often rests on intelligent animals, dominance can also be measured by sheer biomass and the control of fundamental ecological processes. In this context, the most significant contenders are groups that operate at the base of the food web. Insects, such as ants and termites, already possess staggering population numbers and a collective biomass that dwarfs that of all mammals combined. These social insects control massive territories and exhibit highly organized, cooperative behavior, effectively dominating their local environments through sheer numbers and coordinated effort.
Beyond the animal kingdom, the often-overlooked kingdom of fungi and the ubiquitous microbes represent another form of ecological dominance. Fungi are nature’s primary decomposers, and their total biomass on Earth is estimated to exceed that of all animals. In a post-human world, the planet would be littered with the dead biomass of humans, their livestock, and their wooden and organic structures. The fungi would effectively rule the nutrient cycle, breaking down this immense organic legacy and controlling the flow of energy and matter through the ecosystem.