Darwin's Day Special: Are Some Non-Human Monkeys Evolving Into Stone Age?

There is a rhetorical question every anti-evolutionist asks.

If we evolved from the monkeys then why are not we seeing any monkeys evolve into humans now?

The question is fallacious because evolution through natural selection that Darwin put forth, does not say that monkeys evolved into us. It shows in more than one convincing way that we share common ancestry with the monkeys we see around. Or in the words of evolutionist Richard Dawkins, we share with the monkeys and non-human apes a ‘concestor’.

We shared a common ancestor with chimpanzees which lived eight to six million years ago. We shared common ancestors with other apes and monkeys some twenty five million years ago. These numbers change with a few million years either to the recent or the distant past with new discoveries of fossils. But the general picture is clear. We had common ancestors and evolved in branches.

So the question is wrong. But there is another component to the question. What about the other branches of monkeys and apes? Have they shown any evolution? Here we are talking not about evolving into humans but evolving culture similar to the ones we see in humans.

If that is the question then the answer is a very emphatic ‘yes’.

In 2005, primatologist Suchinda Malaivijitnond and her team from Chulalongkorn University of Thailand were making surveys of long-tailed macaques in an island national park situated in southern Thailand.

They reported that two of the three troops of long-tailed macaques in the island were using axe-shaped stones to crack rock oysters, detached gastropods, bivalves and swimming crabs. The primatologists reported that the monkeys ‘smashed the shells with stones that were held in either the left or right hand, while using the opposite hand to gather the oyster meat. Some monkeys used both hands to handle the stones.’ The observations were published in American Journal of Primatology in 2007.

In 2013 a team consisting of an archaeologist, a psychologist and primatologists including Suchinda Malaivijitnond published another study – analysing the use-wear patterns on wild Macaque stone tools. In this they hypothesized that there could be ‘a number of as-yet unidentified macaque tool use sites along the Thailand and Burmese coasts, with site locations and the spread of tool use by past macaque populations influenced by the dramatic sea level fluctuations of recent glacial period.’

By 2017 the team now joined also by Max Planck Institute for Evolutionary Anthropology, published a very astonishing paper: ‘Resource depletion through primate stone technology.’ The paper showed that the overharvest by the use of stone tools by macaque monkeys has reduced the population density of coastal shellfish. Technology based overharvest reducing the population of a species has been so far considered a uniquely human effected phenomenon.

In 2019 Suchinda and team published another paper. They had studied the characteristic features of 115 stone tools used by the monkeys from two islands.

These tools belonged to different groups but they came from almost the same kind of environment and were made for similar prey. But the tools used by different groups of the monkeys showed remarkable diversity.

This tool-diversity could not be explained by ecological diversity. With very similar environmental circumstances and other factors like availability of raw material and prey not differing between islands, the possibility that tool use behaviour in long-tailed macaques is socially learned cannot be dismissed. The paper said among other things:

"Our results highlight the possibility that tool selection in old world monkeys might also be affected through social learning and therefore might classify as a cultural behaviour".

Today a new discipline seems to be evolving – primate archaeology where the stone tool usage in non-human primates and even non-ape monkeys seem to provide clear insights into early evolution of hominids.

Meanwhile halfway around the planet in South America anthropologists and primatologists have been arriving at very similar discoveries.

If it is oysters in Thailand, it is nuts that are opened using stone tools in the semi-arid Northern Brazil’s Serra da Capivara National Park (SCNP).

In South America, wild Brazilian bearded capuchin monkeys use stones to break open the indigenous nuts including cashew nuts. They pound the nuts on a sandstone anvil with a quartzite hammer.

Scientists discovered that the capuchin monkeys had actually selected smooth quartzite hammers from available stones, and they strongly preferred using tabular sandstone for anvils.

Further anvil stones were ‘just over four times heavier than hammer stones, while hammers were in turn over four times heavier than natural stones in the same environment.’

The scientists who made the discovery pointed out that in discovering sites of non-human simian tool usage ‘anthropocentrism limits our comparative insight into the emergence and development of technology, weakening our evolutionary models.’

In their paper published in 2016 in Current Biology they estimated these stone tools to be as old as 700 years.

In November 2022, two Argentinian anthropologists Agustin Agnolin and Federico Agnolin studied some of the most ancient stone tools discovered at well known sites like Pedra Furdain, dated 50,000 years ago, that belonged to the Pleistocene epoch.

There have been debates about the origin of these tools which in turn are associated with questions about some of the earliest peopling of the continent.

Now anthropologists and primatologists have some features characteristically associated with monkey tool-making. Apart from the negative evidence of the absence of any specific human traits with respect to stone tools from these sites, the two anthropologists also discovered some of the features that are associated with non-hominin monkey tool making – for example, stone tools made of non-transported immediately available raw material sourced from within 20 m away. In the quartz and quartzite cobbles and flakes there was unidirectional flaking and absences of bipolar percussion.

This, if true, not only has real significance for the deep history of South America but also for Simian evolution. From 50,000 years BP to 700 years ago there have been use of stone tools by monkeys. This this perhaps the oldest and more importantly continuous stone tool usage and tool assemblage creation outside Africa. In Africa the oldest stone tool usage site is in a Western African nation, Côte d'Ivoire, and is more than 4,000 years old.

So now we can say we have at least three non-human species of monkey branch of evolution that have entered stone tool age.

As the authors of 'Pre-Columbian monkey tools' paper pointed out we are looking into the processes - 'social, ecological and cognitive — that support primate technological evolution.'

'They are also improvising through generations. And we the human primates have also introduced into Chimpanzees sign language, playing a catalytic role. Perhaps we will see a parallel evolution slowly, very slowly but steadily happening.