Use of tools, a monkey-made phenomenon?

The earliest stone tools were assumed to have been intentionally made by our pre-human ancestors but maybe there's a different explanation.
08 November 2016

Interview with 

Dr Tomos Proffitt, The University of Oxford


Capuchin stone on stone percussion at Serra da Capivara National Park, Brazil. A hammerstone breaking as it is impacted against a embedded quartzite cobble.


Matthew's ostrich eggs came from hominid sites - areas that have been identified Capuchin Monkey Stone Toolsas places of early human activity either through the discovery of human remains or through other artifacts considered to be uniquely human. One such artifact is a particular type of stone tool that has been found in sites as old as 3.3 million years and used as a marker for human activity. But, in a paper in Nature last month Oxford University's Tomos Proffitt speculated that these "tools" may have a different story to tell. He brought some examples in to show Chris Smith...

Tomos - Okay. So what we have here are stone flakes and they're made from various raw materials that we find in East Africa. So we have some lava, some basalt flakes, and we have some quartzite flake, which is this incredibly pretty looking white, translucent material.

Chris - By flakes you're just saying thin slivers of rock?

Tomos - Thin slivers of stone, basically, that have been taken off a larger core, a larger piece of stone. And the way that hominins produced these flakes, and this is one of the characteristic tools we see in the archeological record and the material record of hominins, is through a process called knapping, and they produce these flakes in a controlled fashion through a very specific mechanism called "conchoidal fracture." We do not see this conchoidal fracture repeated or evident in the natural world and we definitely don't see it often in the primate world. Even though primates use stone tools, they use them for nut cracking and for various percussive behaviours. The way that these types of flakes are produced is not the same process that you see with the nut cracker.

Chris - So, basically, these are of a particular shape or configuration that nature just wouldn't do that?

Tomos - Absolutely. You very rarely get a sharp cutting edged flake produced naturally by nature. What we find even in the very earliest archeological assemblages are many, many flakes in the same place so it's a very hominin behaviour.

Chris - So what have been doing with your monkeys?

Tomos - Iin Northeast Brazil we have a number of groups of Capuchins that live in Serra da Capivara National Park.

Chris - These are just monkeys?

Tomos - These are just monkeys. These are New World monkeys, they're Capuchin monkeys and they're one of the three tool using primates that we know of. And what we have with the Capuchins is they use stone tools for a much more varied range of behaviours than any other primate and they use them for nut cracking the same as other primates. They use them for digging - the ground is relatively hard in this part of Brazil so they will use them to loosen up the ground to dig for spiders and for roots. And they use them for sexual displays, so the females will throw them at the males.

Chris - Gosh! That's some valentines gift isn't it?

Tomos - Absolutely, yes.

Chris - Get hit by a rock. How does that work?

Tomos - Well, it's just a way of getting attention I think. I think that's all it is. But they also do this very strange behaviour which we've called "stone on stone percussion" and it's where a Capuchin will pick up a hammer stone and repeatedly impact another quartz cobalt.

Chris - Just bash them together?

Tomos - Basically the quartz cobalt is imbedded in a conglomerate, it's embedded in a cliff face and the Capuchins will climb up onto the cliff face with their hammer stone and they will repeatedly hit this embedded stone.

Chris - Now are you telling me that they do this interesting behaviour and go and smash stones together because you are going to go in the direction that sometimes the stones break?

Tomos - Absolutely! So the hammer stone that they're using, the hammer stone that they're holding with their hands will, relatively frequently, break, they will fracture. And because it's quartzite, because it's a raw material which is very amenable to this type of conchoidal fracture, entirely unintentionally what these Capuchins are doing, they are producing many flakes. The stone artifacts that they are leaving on the ground, the primate archaeological record, about 30% of that assemblage are flakes. It isn't the same number of flakes that you see by hominins but they do have the exact same characteristics. 

Chris - Is one possibility then that actually, when we say we find these flakes at important sites where early human relatives were active that, in fact, we're inferring that this is evidence of early human activity when, in fact, it could have been simple early primates like your Capuchin monkeys knocking stones together and making these flakes?

Tomos - So there's quite an important distinction here I think. The early archaeological record is far more complex than anything we see produced by the Capuchins. The hominins are making flakes in ways that are more complex and a much more varied way. They're also producing a lot more flakes than the Capuchins will ever produce. And we know the hominins are often using these flakes for butchering and cutting organic materials like plants or wood.

So, we know the hominins are producing the flake as a tool.  Whereas, with the Capuchins, they're producing a flake unintentionally and they're never using it, they never have any intention of ever using a sharp cutting edge.

Chris - But could we nonetheless be mislead when we find these flakes into thinking that was evidence that humans and our ancestors, our human relatives were active in a site and, in fact, they weren't, it was these monkeys?

Tomos - So yes. If I found these materials in correctly dated sediments in East Africa (2-3 million years ago)  without knowing that primates do this behaviour, I would automatically assume that these were artifacts and the only known individual, the only known species that make these types of artifacts are hominins. But I think what's really important and really interesting about this material is it gives us an insight into the processes that may have led to the emergence of stone tools technology, not that we have reclassify.

Chris - Well I was going to say what does this do to our thinking - what are the implications off the back of the paper you've published on this?

Tomos - What we have here is another primate that gives a window into what type of behaviour may have led to the production of flakes, which may have given the correct environmental produced sharp cutting edges and put then in the environment for them to use.

Chris - Is another important interpretation or deduction from this that that the ability to make these things - we've always ascribed that to a highly intelligent or a developing intellect -  yet you're showing here that with these beautiful stones that something with actually a very small brain is capable, nonetheless, of producing something that is quite useful.

Tomos - Yeah, absolutely. The fraction mechanic (conchoidal fracture) has long been seen as a watershed moment in hominin evolution. The understanding of this fraction mechanic opened up the world of stone tool technology to hominins. And now we see that, actually, conchoidal fracture isn't necessarily unique to hominins now. It's now unique to primates, modern monkeys are producing this type of material and, exactly, they don't have a particularly large brain. They also have incredibly, compared to hominin hands, relatively unadvanced hands.  So they are hands that are hands that have advanced for arboreal tree climbing basically. They haven't evolved for tool use and they haven't evolved for the same range of motions as human hands have. So we have questions about how complex hand morphology has to be to produce archaeological material that we would classify as uniquely human.


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