Research

Using cutting-edge technology to trace ancient footsteps

In Tajikistan, the sands of time have literally buried the traces of the area’s first inhabitants. A DTU-led project is uncovering these ancient clues to determine when the region was first inhabited and what kind of climate these people lived in.

Tuesday 06 February 2024

Miriam Meister

“It never ceases to amaze me when I stand there with a newly uncovered stone tool thinking that the last time someone touched this was perhaps hundreds of thousands of years ago,” says DTU Senior Researcher Jan-Pieter Buylaert as his face lights up in a broad smile.

In his hand is a crude tool, a so-called lithic. The untrained eye could easily mistake it for just a small rock. But to people with knowledge of early stone tool making, the way bits of the stone have been deliberately removed by hitting it with another stone reveals that it is in fact a man-made instrument.

Jan-Pieter Buylaert is standing in the lush green hills of the Khovaling region of Tajikistan in Central Asia where—for the past four years—a group of international scientists have battled on despite the disruption of a global pandemic and a war in the region to establish when humans first arrived in this area.

“We are using different methods to help understand where we have come from as a species and what the climate conditions were like for the first people to populate this part of the planet,” Jan-Pieter Buylaert says of the rationale behind the interdisciplinary, NordForsk-funded THOCA-project that he heads.

Here, a three-hour drive from the capital Dushanbe in the mountainous country, the researchers have carved out three excavation trenches in outcrops that have evolved over hundreds of thousands of years to become more than 100 metres tall.

The outcrops are made up of layers of loess—a light-coloured, fine-grained dust that blows in constantly, but especially during colder, drier periods. Between them are layers of darker soils that have formed in the loess under warmer, moister conditions.

Bjergside i Khovaling regionen i Tajikistan. Foto: Miriam Meister — Kuldara is one of three excavation sites in the Khovaling region that are being studied by the THOCA project partners. Photo: Ramona Schneider, Uppsala University

Trapper gravet ind i siden af en bakkeskråning i Khovaling regionen i Tajikistan. Miriam Meister — The trenches at Kuldara that allow access to the excavation sites are all dug by hand. Photo: Ramona Schneider, Uppsala University

Jordlag ses i mørke og lyse bånd i en bakkeskråning i Khovaling regionen i Tajikistan. Foto: Miriam Meister — The outcrops expose how the hills are made up of layers: Lighter layers consist mainly of fine-grained windblown dust (loess), while darker layers have a higher concentration of organic matter (such as plant materials and animal residues). Photo: Ramona Schneider, Uppsala University

Providing the x-axis

Central Asia is widely accepted as a key route for human expansion into Asia and plenty of lithics can be found in the hills of the Khovaling region, showing that the migratory paths of the first humans to leave Africa went through here.

There has long been an interest in establishing when humans arrived: Fifty odd years ago Soviet archaeologists excavated huge amounts of lithics. However, the science wasn’t available to put an exact age on the artefacts and thus establish when people occupied those landscapes. “They could only guess by counting layers of soil,” Jan-Pieter Buylaert explains.

While this method, called pedostratigraphy, can reveal relative ages of the soil layers, only absolute dating techniques can establish exact dates, he says: “Broadly speaking, pedostratigraphy works, but you can make huge errors of up to hundreds of thousands of years. That’s where we come in as luminescence specialists to date the top 250,000 years of the accumulated dust layers.”

Luminescence is a method that can determine when material like sand was last exposed to sunlight (see box below for explanation). So, by analyzing loess samples collected in lightproof tubes at regular intervals down through the excavation trenches, the DTU-specialists can help accurately date the loess layers that make up the hills they are studying. Knowing the age of the soil where lithics are found is important in establishing when they were left there—presumably as inhabitants moved on.

What is luminescence dating?

A method to determine the age of certain materials such as sand, ceramics or stone since they were last exposed to daylight or were heated to temperatures above 300 ºC. It is based on the amount of energy it has absorbed over time through exposure to natural sources of radiation (decay of natural radionuclides such as uranium, thorium, and potassium). This energy begins to accumulate when the sand grains are shielded from light or heating has stopped.

This is because energy becomes trapped within the atomic structure of the material—kind of like a battery that is being charged. When the material is exposed to light, part of the trapped energy is released in the form of luminescence which can be measured in the laboratory.

Using specialized equipment, scientists can expose samples of sand to light and measure the amount of stored energy (luminescence) that has accumulated since burial. By dividing the amount of energy in a sample that was trapped there since burial by the known rate at which the energy accumulates, the scientists can determine when the sample was last exposed to sunlight or heated.

Once covered by even the thinnest layer, the sand will start to absorb energy anew.

Experts from Aarhus University use a method called cosmogenic radionuclide dating to date the lithics that Tajik and Uzbek project partners are excavating to help establish when the tools were buried by dust. Using luminescence and cosmogenic dating concurrently allows the researchers to set up a robust timeline for when the different layers of the loess hills were deposited.

“We can provide a highly resolved chronology that will provide the x-axis for the other work that goes on in this project. Because if we haven’t established the age of the different layers, we are just guessing when it comes to the rest of it,” Jan-Pieter Buylaert points out.

“Our colleagues in Uppsala for example measure carbon and oxygen isotopes in the soil so they can establish what the temperature and precipitation was like at different times—in other words the data for the y-axis when it comes to understanding the climate over the years.”

He adds that having detailed knowledge of past climates globally is crucial to developing the most accurate models to predict our future climate. Knowing what the climate was like can also help understand why different groups of humans may have moved into or abandoned a habitat in the past.

One man standing in a whole in the ground passing soil samples to another man. Photo: Miriam Meister — Senior Researcher Jan-Pieter Buylaert (right) retrieves soil from the top of Khonako and passes it to Professor Andrew Murray, who bags it up. Analyzing the samples to find their water content is an important reference point for samples taken further down the site. Photo: Miriam Meister, DTU

Two women and two men studying soil samples that one man is holding in his hand. Photo: Miriam Meister — Professor Farhad Khormali (right) is inspecting soil samples with PhD students Olga Tokareva (left) and Ramona Schneider, and Senior Scientist Jan-Pieter Buylaert. Photo: Miriam Meister, DTU

Senior Scientist Jan-Pieter Buylaert retrieving samples from one of the excavation sites. Photo: Redzhep Kurbanov

Ancient stone tools put in plastic bags and laid out on the ground. Photo: Miriam Meister — Just a few of the hundreds of lithics that the THOCA partners have excavated. Photo: Miriam Meister, DTU

Ancient stone tools laid out on a table. Photo: Miriam Meister — The archeologists working on the THOCA project look at e.g., the shape and size of the unearthed lithics to try and establish what they were used for. Photo: Miriam Meister, DTU

Three images of: handcarved steps at an excavation site in the Khovaling region of Tajikistan, a marker to help mark out the site, and holes in the site where sediment samples have been removed respectively. Photos: Miriam Meister — The steps of the trenches (left) are dug by hand and need to be cleaned up before each excavation season as rain, snow and wind erode them over time. Identifying markers are applied up and down the trenches (middle). The DTU-experts initially tried using plastic tubes to collect sediment samples for luminescence dating, but they proved too fragile for the densely compacted sediment and were swapped out for metal tubes (right). Photos: Miriam Meister, DTU

Light tight metal tubes containing sediment samples. Photo: Miriam Meister — Samples are collected in 20-40 cm long metal tubes to ensure the samples are not exposed to light as they are extracted from the outcrops. Before a sample is examined in the lab, a small portion at each end of the tube is discarded as it will have been exposed to some sunlight on the way out, so measuring that part would give an incorrect result. Photo: Miriam Meister, DTU.

World leading instrument

Jan-Pieter Buylaert normally spends his working day at DTU’s Risø Campus—home to some of the world’s preeminent luminescence dating experts and where the world-leading luminescence instrument, the Risø TL/OSL-reader, first saw the light of day in 1982.

Since its initial development, more than 550 of these machines have been sold and installed in laboratories in more than 38 countries around the world, with new orders continually ticking in. The senior scientist and his mentor and THOCA-collaborator Professor Andrew Murray from Aarhus University estimate that some 90 percent of luminescence dates published globally have been derived using this instrument.

Collaboration is key

In Tajikistan, it seems the team never really take a break from work when they come together. Even though days start early, and the work is hard and temperatures hot, breakfast quickly becomes a mix of tea and flatbread, dried fruits and fruitful discussions on methods and approaches to the work.

Throughout the day passionate debates break out when experts from different disciplines come together to crack the hardest nuts: discussions take place in the field when new lithics or tricky soils have been unearthed. And they continue when the team are back and showered at camp after a dusty day’s work, as they sit in the green plastic chairs of the common area with walls adorned with colourful woven blankets and whiteboards detailing the decisions of scientific negotiations.

Team members also pitch in where needed if they have a free hand: they gather in the evening to label some of the many plastic bags used to collect soil samples and, in the field, they help carry each other’s samples and equipment up and down the steep steps to the excavation sites.

“We are all a big team, and we help each other where we can. We are respectful and friendly, which I find important, because science these days is already really hard,” Jan-Pieter Buylaert says.

A woman and a man sitting at a table putting labels on small plastic bags. Photo: Miriam Meister — PhD student Ramona Schneider and Professor Andrew Murray are busy labelling some of the thousands of bags that are used to collect soil samples. Photo: Miriam Meister, DTU

Working on strong hypotheses

The THOCA-project has unearthed more than 5000 lithics from their sites. The oldest lithics found so far have been dated using cosmogenic dating at approximately 500,000 years old. Archaeologists have studied the skill level involved in making the various lithics—because more advanced tool making is linked to later stages of human evolution, and this helps to establish the kinds of people that made them.

So far, the group’s excavations in Khovaling have not provided human ancient DNA, and so the scientists will compare their stone tool technologies with those from sites in the Middle East to identify the human species most likely to have made them. “This will hopefully tell us the kinds of people that migrated through Central Asia on their way further east to China and Siberia, and ultimately into the Americas” says Jan-Pieter Buylaert.

Pushing the boundaries

The experts at DTU Risø are not unfamiliar with scientific breakthroughs. In 2008, a group of scientists from there along with Andrew Murray from Aarhus University improved on the luminescence dating techniques making it possible to date loess as old as 250,000 years—compared with 50,000 years previously.

Jan-Pieter Buylaert is hoping that the work in Tajikistan will pave the way for similar breakthroughs. A post doc in the project for example is working on finding new ways to further extend the age range of luminescence dating.

“We are really trying to push the next boundary to say half a million years in the Tajik samples. Now that would be something. Not just for THOCA, but for other sites around the world,” he adds enthusiastically.

And at Oslo University a PhD student is looking for traces of human excrement in the various loess sections. He hopes to use a characteristic stable biomarker (a cholesterol compound) to indicate how population density has changed through time. Others have shown this approach can work over a few tens of thousands of years. THOCA hopes to extend this to more than half a million.

Jan-Pieter Buylaert is aware that the ambitious goals, which the team have set for themselves, might not be met. “But that’s science. We should not fool ourselves into thinking that we will be able to do everything. But we will try.”

FACTS

About loess plateaus

Loess is found on nearly every continent, just not in the quantities that you find in East Asia and Central Asia. The loess plateaus in China and Tajikistan are exceptionally dense and expansive—and unlike anything else in the world.

The loess deposits alternate between layers of loess (windblown dust) and paleosols (ancient soils that formed in the loess under warmer, moister conditions). Paleosols get a darker colour from the higher concentration of organic matter derived from plant materials and animal residues.

The paleosol layers are called pedocomplexes, or PC. They are numbered from top to bottom (PC1, PC2, etc.) and the tallest outcrop in the Khovaling Loess Plateau consists of more than 30 PCs presumably stretching back more than 2 million years in time. THOCA focuses on three different sections in Khovaling: Obi Mazar/Lakhuti, Kuldara and Khonako.

About THOCA

The project partners of the NordForsk-funded project are:

DTU
Aarhus University
Uppsala University
University of Oslo
National Academy of Sciences of Tajikistan.

Find out more on the project's website: Timing and ecology of the Human Occupation of Central Asia.

Contact

Jan-Pieter Buylaert Senior Researcher Phone: +45 46774926 jabu@dtu.dk