When did humans start social knowledge accumulation?

A key aspect of humans' evolutionary success is the fact that we don't have to learn how to do things from scratch. Our societies have developed various ways—from formal education to YouTube videos—to convey what others have learned. This makes learning how to do things far easier than learning by doing, and it gives us more space to experiment; we can learn to build new things or handle tasks more efficiently, then pass information on how to do so on to others.

Some of our closer relatives, like chimps and bonobos, learn from their fellow species-members. They don't seem to engage in this iterative process of improvement—they don't, in technical terms, have a cumulative culture where new technologies are built on past knowledge. So, when did humans develop this ability?

Based on a new analysis of stone toolmaking, two researchers are arguing that the ability is relatively recent, dating to just 600,000 years ago. That's roughly the same time our ancestors and the Neanderthals went their separate ways.

Accumulating culture

It's pretty obvious that a lot of our technology builds on past efforts. If you're reading this on a mobile platform, then you're benefitting from the fact that smartphones were derived from personal computers and that software required working hardware to happen. But for millions of years, human technology lacked the sort of clear building blocks that would help us identify when an archeological artifact is derived from earlier work. So, how do you go about studying the origin of cumulative culture?

Jonathan Paige and Charles Perreault, the researchers behind the new study, took a pretty straightforward approach. To start with, they focused on stone tools since these are the only things that are well-preserved across our species' history. In many cases, the styles of tools remained constant for hundreds of thousands of years. This gives us enough examples that we've been able to figure out how these tools were manufactured, in many cases learning to make them ourselves.

Their argument in the paper they've just published is that the sophistication of these tools provides a measure of when cultural accumulation started. "As new knapping techniques are discovered, the frontiers of the possible design space expand," they argue. "These more complex technologies are also more difficult to discover, master, and teach."

The question then becomes one of when humans made the key shift: from simply teaching the next generation to make the same sort of tools to using that knowledge as a foundation to build something new. Paige and Perreault argue that it's a matter of how complex it is to make the tool: "Generations of improvements, modifications, and lucky errors can generate technologies and know-how well beyond what a single naive individual could invent independently within their lifetime."

Measuring complexity

Their reasoning leaves the issue of how to measure that complexity and how to determine when it crosses a threshold that a "single, naive individual" couldn't develop on their own.

Paige and Perreault focus on what they term a "procedural unit," which they define as "discrete, mutually exclusive manufacturing steps that can be chained together." So, the more procedural units it takes to make a too, the more complex the manufacturing process is. The earliest stone tools only involve three of the 33 total procedural units the researchers identified. The most sophisticated, from within the past 10,000 years, required 19.

So, where is the cutoff that requires cumulative culture? The researchers use a number of examples: the toolmaking done by our primate relatives, asking unskilled humans to make a stone tool, and random chipping of rocks. At most, these used six procedural units, so that was considered the cutoff—anything beyond six means that cumulative culture was needed.

Looking at actual tools, they find three distinct periods. The first runs from the start of tool manufacture about 3.3 million years ago to roughly 1.8 million years ago. It's characterized by easy-to-make tools that require four procedural units or less. From 1.8 million years ago to about 600,000, tool manufacturing sits right about at the cutoff, requiring between four and seven procedural units. It's only at 600,000 years that complexity clearly exceeds the baseline, with nearly every tool being well about the complexity cutoff.

Paige and Perreault note that 600,000 years ago is similar to the timing of the split between Neanderthals and the ancestors of modern humans. At the time, the tools produced by these two branches of humanity seem to have been roughly equal in sophistication.

Acceleration

What does this tell us? The researchers acknowledge that stone tools are undoubtedly one of a number of manufactured technologies, and the rest aren't preserved, so there may be aspects of cumulative culture that don't show up in the archeological record. So, the long period with only low-complexity tools is consistent with a lack of cumulative culture but isn't definitive evidence of it.

The other thing that Paige and Perreault consider is the long periods of apparent stasis in technological development. They suggest two explanations for this. The first is that humans may have needed to evolve new mental capacities before they could make and use sophisticated tools. So, the growth of sophistication may have required both technological and biological evolution, which slowed the process down considerably. The alternative is that the stasis just represents a particularly challenging technological problem that took a lot of experimentation to solve.

Regardless of the precise details, one thing that's very clear is that, after extended periods of stasis, the sophistication of manufacturing went up dramatically at about the same time that modern humans came along. So, whatever the barriers to cumulative culture existed in the past, it's clear that the brakes were off by 100,000 years ago.

PNAS, 2024. DOI: 10.1073/pnas.2319175121  (About DOIs).