Rethinking Homo erectus: How Cooperation May Have Powered Our Ancestors’ Survival
- Aug 25, 2025
- 3 min read
When Homo erectus first appeared nearly two million years ago, they were unlike anything the world had seen before. Taller and longer-limbed than their predecessors, with body proportions closer to modern humans, they may have stood over 1.5 metres tall and weighed around 50 kilograms or more. Their brains were significantly larger than those of earlier hominins such as Australopithecus, though still smaller than ours.
They were the first of our ancestors to look recognisably human - walking fully upright, making more advanced stone tools, and eventually spreading out of Africa into Asia and Europe. Fossil evidence shows Homo erectus endured for more than a million years, making them one of the most successful species in human evolution.
They brought with them something radically new to human evolution: bigger bodies, bigger brains – and a problem. For the females of the species, the biological costs of pregnancy, breastfeeding and raising young suddenly skyrocketed.
A new paper by anthropologist Leslie Aiello revisits this ancient dilemma, first laid out in a landmark 2002 study she co-authored, and comes to a striking conclusion: survival for Homo erectus mothers was only possible through a fundamental reshaping of how early humans lived, ate and supported each other.
The cost of being bigger
Compared with their smaller-bodied australopithecine ancestors, Homo erectus females faced a steep rise in energy demands. Pregnancy and nursing scale up with body size - more tissue to grow, more milk to produce, more calories to burn.
Aiello and her colleague Cathy Key once calculated that if Homo erectus mothers had followed the same slow reproductive schedule as chimpanzees - with long gaps between births and years of breastfeeding - the energy bill per child would have been crippling.
The solution? Something much closer to the modern human pattern: shorter breastfeeding, shorter intervals between children, and more offspring over a lifetime. That would spread the energy costs more efficiently, but it created a new challenge: dependent infants arriving in quick succession, all needing food and care long before they could fend for themselves.
A society built on sharing
That impossible equation could only be solved, Aiello argues, by cooperation. Mothers needed support: older children, males, and especially grandmothers contributing food and care.
This kind of “cooperative breeding” - a radical departure from ape society, where mothers shoulder the burden alone - may have been the turning point that pushed our lineage onto a new evolutionary path. Food had to be shared, labour divided, risks pooled.
And the food itself was different. Archaeological evidence points to Homo erectus increasingly relying on higher-quality, riskier resources – hunted meat, processed plants, perhaps even early cooking. Hunting and gathering, with its high rewards and high risks, demanded collaboration and intergenerational sharing.
New science, old conclusion
Since 2002, advances in the science of energetics - especially precise measurements of energy expenditure using “doubly labelled water” - have refined the numbers.
Surprisingly, Aiello notes, her team’s original assumptions about energy budgets were off in opposite directions: they underestimated overall energy needs, but overestimated the costs of reproduction. The errors cancelled each other out.
The broad conclusion remains intact: for Homo erectus, adopting a more human-like reproductive rhythm was not just advantageous – it was essential.
Why it mattered
By about 1.9 million years ago, Homo erectus had spread out of Africa, endured for more than a million years, and laid the foundations for later humans. Aiello suggests that their survival hinged on cooperation, food sharing and the ability to raise more children despite ballooning energy costs.
The stakes were existential. Other primates hit a “grey ceiling”: once brains grow too big, reproduction slows so much that populations risk collapse. Homo erectus broke through by bending biology with culture - leaning on one another to make it work.
The first step toward us
The lesson, Aiello concludes, is that our humanity – our unusually social, cooperative lives – may have begun not as a luxury but as a necessity for survival. Although Homo erectus did not transform directly into Homo sapiens (our current form of 'human'), they were a crucial ancestor in the evolutionary chain.
Bigger brains and bodies forced Homo erectus mothers into a corner. The only way out was together. From those first steps, cooperation became the bedrock of human success: food sharing evolved into complex social networks, intergenerational care laid the groundwork for grandparents and extended families, and the ability to support bigger brains opened the door to language, culture and technology.
What began as a survival strategy in the Pleistocene set us on the path to the societies, cities and global communities we inhabit today.
Comments