In a fascinating twist to the story of human evolution, scientists at the University of Tsukuba have revealed that chaos might have been our greatest teacher. Their new study, published in PLOS Complex Systems, shows that intensified environmental variability (EV)—the unpredictable shifts in climate and resources—can actually encourage cooperation, a hallmark of human society. This fresh insight, rooted in cutting-edge simulations of evolutionary game theory, may offer a new key to one of anthropology’s most enduring mysteries: why and how early humans became social beings.
A Riddle Born in the Middle Stone Age
For decades, scientists have puzzled over a pivotal moment in human history: the Middle Stone Age (MSA) in Africa, spanning roughly 300,000 to 30,000 years ago. This era, archaeologists argue, was when humans began to look and behave in ways that feel unmistakably modern. It gave rise to symbolic art, long-distance trade, complex tools, and—most significantly—elaborate social networks. But why did this evolutionary leap happen? And why in Africa?
One leading hypothesis has been the variability selection hypothesis (VSH). It proposes that the extreme environmental ups and downs that characterized ancient Africa didn’t just test early humans—they transformed them. As habitats dried up, greened again, and then collapsed once more, only the cognitively flexible—those who could adapt to rapid change—survived and thrived. According to the VSH, these pressures led to the development of advanced cognitive abilities such as planning, learning, and innovation.
But something was missing. While VSH explains individual adaptability, it largely overlooks a second, equally vital ingredient in the recipe of human success: cooperation. Without sociality—our tendency to work in groups, care for others, and build communities—cognition alone might not have carried us far. Enter the Tsukuba team’s study, which reimagines the VSH to encompass not only the rise of smart individuals, but also of social groups.
Simulating Prehistoric Struggles with Evolutionary Game Theory
To investigate how environmental chaos might encourage humans to cooperate, the researchers turned to a powerful conceptual tool: evolutionary game theory. This mathematical framework allows scientists to simulate interactions between agents—individuals, organisms, or even strategies—under specific rules. In this case, the rules are based on survival and resource sharing in changing environments.
The researchers created multiagent simulation models to mimic groups of early humans facing fluctuating environmental conditions. In these virtual worlds, some individuals chose to cooperate—sharing resources, information, or effort—while others acted selfishly. The twist was that the environmental backdrop was not stable. It swung between scarcity and abundance, simulating the volatile climate shifts that characterized Pleistocene Africa.
To better mirror real-world geography, the researchers built two kinds of models: one where environmental variability affected different regions differently (regional EV), and another where all regions experienced the same changes (universal EV). The distinction proved crucial.
A World in Flux Favors Friends, Not Foes
So, what did the digital experiments reveal? Quite remarkably, the simulations showed that regional environmental variability—where some areas suffer while others remain resource-rich—fosters cooperation. In times and places where resources became scarce, individuals who cooperated had better chances of survival, especially when interacting with groups in more prosperous areas. These cooperative links acted like survival lifelines, bridging gaps between suffering and abundance.
This result echoes an intuitive human truth: when times get tough, we often turn to one another. And in a world where neighboring communities may be faring better or worse at any given time, helping others could ensure help would come in return when the tables turned.
On the other hand, in simulations with universal variability, where all regions experienced the same fluctuations, the drive to cooperate diminished. Without the possibility of sharing across regions of differing fortune, cooperation provided less of a survival advantage. Everyone suffered equally—or flourished equally—making mutual aid less necessary.
These results suggest a powerful and elegant principle: interregional disparity under environmental pressure can be a crucible for social evolution. It’s not just hardship that breeds cooperation, but the unevenness of hardship.
Rewriting the Story of Human Social Evolution
The implications of the study ripple far beyond academic theory. For anthropologists, it offers a new lens to re-examine ancient archaeological evidence. Sites from the African MSA that show signs of long-distance interaction—like obsidian tools sourced from hundreds of kilometers away—may not just reflect technological prowess, but strategic social cooperation between groups in differently affected ecological zones.
The researchers’ model also provides a compelling extension to the variability selection hypothesis. Instead of focusing solely on how climate chaos honed the individual mind, the study suggests that such instability also fostered bonds between minds. In doing so, it proposes that social intelligence and communal strategies evolved hand-in-hand with individual adaptability.
This dual-evolution narrative may help explain not only why humans developed language, empathy, and cooperation, but why these traits exploded in complexity and prevalence during the Middle Stone Age. As groups began to navigate increasingly fragmented and unpredictable landscapes, their survival depended not just on strength or smarts—but on each other.
From Stone Age to Climate Change: Modern Echoes of Ancient Lessons
Beyond prehistory, the Tsukuba findings also cast a long shadow over the present—and the future. Today, we are once again living through an era of mounting environmental variability, driven by climate change. Droughts, floods, wildfires, and resource shortages are increasingly regional and uneven, often affecting vulnerable populations first and worst.
The evolutionary logic uncovered in this study offers both a warning and a beacon of hope. When crisis is uneven, cooperation becomes critical. The instinct to help others across borders, cultures, and resource divides is not just moral—it’s evolutionarily wise. It’s a strategy that our ancestors may have relied on, forged in the crucible of ancient African landscapes.
In fact, the study suggests that large-scale environmental instability doesn’t have to splinter society. It can, under the right conditions, foster resilience, trust, and mutual support. But only if systems exist that allow the fortunate to assist the less fortunate, just as ancient groups may have bartered, sheltered, or allied with struggling neighbors.
This dynamic resonates with the increasing emphasis on climate justice, where cooperation across nations, sectors, and income levels is seen as key to weathering the environmental storms ahead.
The Science of Survival is Also the Science of Belonging
What makes this study so compelling is not just its rigorous modeling or its anthropological insights. It’s the profound truth it uncovers: our greatest strength as a species may lie not in domination, but in connection.
From bands of hunter-gatherers seeking safety through alliances, to modern societies building international coalitions for climate action, the arc of our evolution seems to bend toward solidarity—especially in chaos. In essence, the very storms that threatened to break us may have instead taught us how to bind together.
Einstein once quipped, “In the middle of difficulty lies opportunity.” The researchers at the University of Tsukuba have taken that wisdom a step further. They’ve shown that in the middle of variability lies cooperation. And that lesson, forged in the wild swings of ancient climates, may be the secret engine behind the extraordinary social mind that defines us today.
Reference: Masaaki Inaba et al, Environmental variability promotes the evolution of cooperation among geographically dispersed groups on dynamic networks, PLOS Complex Systems (2025). DOI: 10.1371/journal.pcsy.0000038