In the rainforests of west Africa, the woodlands of Brazil and the beaches of Thailand, archaeologists have unearthed some truly remarkable stone tools.
It's not the workmanship that makes them special. If anything, a casual observer might struggle to even identify them as ancient tools. It's not their antiquity that's exceptional either: they're only about the same age as the Egyptian pyramids.
What makes these tools noteworthy is that the hands that held them weren't human.
These stone tools were wielded by chimpanzees, capuchins and macaques. The sites where they have been unearthed are the basis of a brand new field of science: primate archaeology.
The tools are crude. A chimpanzee or monkey stone hammer is hardly a work of art to rival the beauty of an ancient human hand axe. But that's not the point. These primates have developed a culture that makes routine use of a stone-based technology. That means they have entered the Stone Age.
A few decades ago, biologists thought humans were the only species that made extensive use of tools.
Orang-utans, bonobos and gorillas have been seen using plant tools but never stone tools
Not any more. We now know that many mammals, birds, fish and even insects use objects in their environment as tools to make their lives easier.
Many primates use tools too. In 2014, for instance, there were reports of a wild gorilla using a twig to fish ants out of a nest. But primates don't, as a rule, turn stones into tools.
"Orang-utans, bonobos and gorillas have been seen using plant tools but never stone tools," says Michael Haslam at the University of Oxford in the UK, and leader of the Primate Archaeology (Primarch) project.
Exactly why stone tools are so rarely used by great apes is a mystery, he says.
There aren't enough rocks around for that tradition to be aped by others in the group
But it might have to do with the fact that stones are not readily available to species that spend much of their time in and around trees. "Plants are ubiquitous in primate habitats but stones are not," says Haslam.
This means that even if a particularly clever great ape does begin using stone tools, there aren't enough rocks around for that tradition to be aped by others in the group, and passed down through the generations.
However, the chimpanzees of west Africa do seem to have managed to pass their stone-based technology – which they use to crack open nuts – down many generations. We know this because of a landmark study in primate archaeology that was published in 2007.
"Normal", human archaeology relies on the idea that we can recognise human behaviours in the artefacts we leave behind. Tiny scuffs and marks that might be dismissed by the casual observer carry a wealth of information to the trained observer.
They had been used in a cruder way, as pounding tools for cracking open tough nuts
"Primate archaeologists" led by Christophe Boesch at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, set about applying these principles to chimpanzee tools.
In the rainforests of Ivory Coast they excavated an area of the forest floor down to a depth of about 1m, unearthing a 4300-year-old horizon rich in stone artefacts.
Some of those stone artefacts have been worked with a level of precision possessed by humans alone.
But others had marks that suggested they had been used in a cruder way, as pounding tools for cracking open tough nuts: just as chimpanzees in the region use stone tools today.
Boesch and his colleagues had previously studied modern chimpanzee stone tool culture in the region. This research revealed that the chimpanzees have an idiosyncratic way of choosing and using their tools.
For instance, chimpanzees will often deliberately opt for particularly large and heavy stone hammers, between 1kg and 9kg, while humans prefer to use stones that weigh 1kg or less. Many of the 4300-year-old stone tools weighed more than 1kg, suggesting they were used by chimpanzees.
The chimpanzee Stone Age might have begun very early indeed
Chimpanzees also use their stone tools to crack open certain types of nuts that humans don't eat. Starch residues on some of the ancient tools came from these nuts.
Together, these findings led to an obvious conclusion: chimpanzees have been using stone tools in the rainforests of Ivory Coast for at least 4300 years.
The chimpanzee Stone Age began at least that early, and maybe even earlier, says Boesch. However, "it is very hard to predict where you would find soil layers that would be old enough to look at earlier periods."
In theory, the chimpanzee Stone Age might have begun very early indeed.
Chimpanzees are our closest living relative. The fact that they, like us, can use stone tools might imply that the common ancestor of chimpanzees and humans was the first to develop a stone based technology.
This west African chimpanzee Stone Age is completely distinct from the human Stone Age
Haslam says this is unlikely. If that was the case we might reasonably expect all chimpanzees to use stone tools, but only a small number of communities in west Africa do so.
It's more logical to imagine that the chimpanzees of west Africa have developed a stone tool tradition in the time since they split from central and east African chimp communities. Haslam says this happened between 500,000 and 1 million years ago.
It now seems that this west African chimpanzee Stone Age is completely distinct from the human Stone Age.
There have been reports going back a few centuries that bearded capuchin monkeys (Sapajus libidinosus) in Brazil use stone tools. A study in 2004 confirmed that they do.
Even the macaques are separated from us by around 25 million years
So do long-tailed macaque monkeys (Macaca fascicularis aurea) in Thailand, according to a study published in May 2015.
Neither species sits particularly near humans in the primate evolutionary tree.
"Capuchins are New World monkeys, separated from us by some 35 million years of evolution," says Haslam. "Even the macaques are separated from us by around 25 million years."
In other words, the Stone Age primates are so widely scattered across the evolutionary tree that they must have each come up with the technology independently. "We have multiple inventions of the same behaviour," says Haslam.
His team is now applying the same sorts of techniques that Boesch used to study the chimpanzee archaeological record in Ivory Coast to investigate the history of tool use by the macaques and capuchins. This research, conducted as part of the Primarch project, is not yet published.
"We have now recovered buried stone tools from activity areas of all the stone-tool-using primates," says Haslam.
That means there is a deep history of stone tool use in at least three primates other than humans
The capuchins, like the chimpanzees, use their stone tools to crack open nuts, and also to dig for tubers.
"Every time someone has gone to look at wild groups of bearded capuchins in their natural habitat, they've found stone tool use," says Haslam. "This species may be the only one other than humans that has ubiquitous stone artefacts."
By contrast, the macaques live on islands and use their stones to crack open shellfish.
Both species of monkey have apparently passed the tradition down the generations. That means there is a deep history of stone tool use in at least three primates other than humans.
Chimpanzee and monkey stone tools look very primitive. But then again, our ancestors' stone tools were just as primitive too, long ago.
In May 2015, archaeologists working in Kenya published details of the earliest stone tools ever made by members of our lineage.
Within about 700,000 years of those Lomekwian stone tools, human technology had moved on
These "Lomekwian" stone tools were recovered from 3.3-million-year-old deposits. According to the team that found them, they were produced using techniques similar to those used by stone-wielding chimps and monkeys.
This means studying primates that use stone tools could tell us about the nature of early human behaviour. However, drawing conclusions won't be easy: early humans are very different from chimpanzees and monkeys.
Within about 700,000 years of those Lomekwian stone tools, human technology had moved on. First came "Oldowan" tools, including stones that had been deliberately modified to make a sharp edge by "flaking" off small pieces. A million years later, Acheulean hand axes with carefully-shaped cutting edges begin appearing in the archaeological record.
Why did our ancestors learn to make such sophisticated stone tools, and so long ago, while chimps and monkeys never got beyond a Lomekwian-style technology?
You might think it would be down to evolutionary advances in the anatomy of our hands, perhaps allowing for finer manipulation of objects.
The problem probably lies in their brains
In fact, a July 2015 study by Sergio Almécija of George Washington University in Washington, DC suggests that, if anything, human hands have changed less over the last few million years than chimpanzee hands.
"The ancestor of humans and chimps had hand length proportions more similar, but not equal, to humans than to chimpanzees," says Almécija. "In terms of [digit] length proportions, humans are in fact more primitive than chimps."
If it's not chimpanzees' and monkeys' hands that are holding them back, the problem probably lies in their brains, says Almécija.
"It seems plausible that the ability to create stone tools requires some additional cognitive abilities: not just recognising what would be a useful tool, but also creating it," says Alexandra Rosati at Harvard University in Cambridge, Massachusetts.
Cooking increases the energy available from food relative to a raw diet
Humans' larger brains, and our resulting greater smarts, may be what allowed us to make ever better tools. But it's difficult to say exactly why our ancestors' brains began to swell.
One idea, suggested by primatologist Richard Wrangham, also at Harvard, is that our growing brains were fuelled by the development of cooking. "Larger brains require a lot of energy to grow and maintain, and cooking increases the energy available from food relative to a raw diet," says Rosati.
It's not clear when humans first invented cooking. It may have been long after our brains began swelling, which would mean Wrangham's idea is probably wrong.
But if he is correct, it suggests that a 2015 study by Rosati and her Harvard colleague Felix Warneken is very significant. Chimps might not have learned to control fire, but Rosati and Warneken found that they have enough smarts to appreciate the benefits of cooking.
In a series of experiments, Rosati and Warneken introduced chimps to an "oven": a container into which the apes could place food, which would later be returned to them in a cooked state.
It's not clear whether they will have the opportunity to advance their Stone Age technology
The chimps were far more likely to put raw potato chips into this "oven" than into a second container that returned the food still raw.
What's more, when the chimps were given wood chips as well as raw potato chips, they generally didn't bother placing the wood into the "oven". That suggests they didn't see it simply as a cooked food dispenser, but understood that it would only cook edible things.
The chimps were even prepared to carry raw food from a remote location to the "oven" to have it cooked. This reflects the way our ancestors must have begun transporting food to the fireside millions of years ago.
Of course, until chimps do learn to control fire – if they ever make that leap – they won't be able to put their appreciation of cooked food to use. But Rosati and Warneken's work suggests that the relevant brain pathways, which perhaps allowed our ancestors to develop bigger brains and more advanced stone tools, are present in chimpanzees too.
It's possible that chimpanzees – and macaques and capuchins – haven't yet reached the limits of their technological capabilities, says Haslam.
But it's not clear whether they will have the opportunity to advance their Stone Age technology.
"We are shrinking their populations dramatically through habitat destruction and hunting," says Haslam. "Smaller populations cannot spread and sustain complex technologies as well as larger groups."
In other words, chimps and monkeys might have the capacity to make much more sophisticated stone tools, but they may never get the chance to achieve that potential: all because of another group of primates that became master stone tool manufacturers.