Exploring the Physical World

Ancient Hominins Used Fire to Make Stone Tools

A study of these tools suggests skilled control over heating flint to produce different forms

• TAGS: Archaeology, Evolution, Chemistry

Potlid Flake Blade
(l-r) Pot-lid, flake, and blade (not shown to scale). Each was produced at a different temperature

REHOVOT, ISRAEL—October 5, 2020—Our ancestors not only knew how to use fire, they also developed sophisticated technologies for making tools. Researchers at the Weizmann Institute of Science recently employed cutting-edge technologies of their own to take a fresh look at a collection of stone tools. Their results suggest that the early humans who made the tools may have had a good understanding of the effects of heating the stone before flaking it into blades, and may even have used different temperatures to create different types of tools. The findings of this research were published in Nature Human Behaviour.

Qesem Cave, a site in Central Israel, was excavated by Prof. Avi Gopher and colleagues from Tel Aviv University. The findings in the cave were dated to 420,000 to 200,000 years ago, or the Lower Paleolithic, and the cave is assigned to the unique Acheulo-Yabrudian Cultural Complex. The ancient hominins (a group that includes us and the extinct members of our family tree) who lived in Qesem Cave left behind tens of thousands of stone tools. These tools are mainly made of flint, a material that is readily available throughout the country, and were produced using knapping – a process in which another rock or tool is used to chip off pieces of the target material, honing a sharp edge.

Somewhere between 300,000 and 400,000 years ago, the primary prey hunted by these hominins changed from elephants to fallow deer, necessitating a move toward finer tools. The question asked by the Weizmann research group was whether the ancient inhabitants of the area might have used fire to temper the flint before knapping it. Much more recent groups – less than 100,000 years ago – left evidence that they fired their flint, thus making the stone easier to shape. However, in sites as old as Qesem, there is usually almost no remaining organic matter that can give scientists conclusive evidence of fire use.

Dr. Filipe Natalio of the Institute’s Scientific Archaeology Unit explains that the first challenge in trying to understand whether flint has undergone a structural change, such as that produced by fire, is that the structure of raw flint can vary from site to site and even piece to piece, depending on the geological conditions in which it formed. Furthermore, traces of past heating in solid rock are typically microscopic or smaller – basically invisible. To approach these obstacles, he and postdoctoral fellow Dr. Aviad Agam, who specializes in prehistoric archaeology, turned to Dr. Iddo Pinkas of the Institute’s Chemical Research Support Department – an expert in a technique known as Raman spectroscopy.

The group first collected flint from areas near Qesem Cave and from around the country. After heating the flint pieces to different temperatures and cooling them again, the researchers examined them with the tools in Dr. Pinkas’s spectroscopy lab, thus revealing the makeup of the rocks down to their chemical and molecular structures.

The experiment yielded vast amounts of data – too much to analyze with standard methods. So the scientists turned to Dr. Ido Azuri of the Institute’s Bioinformatics Unit, part of the Life Sciences Core Facilities Department. Dr. Azuri is an expert in machine learning and artificial intelligence (AI), and so while the project was a departure from his usual biological research, finding patterns in large amounts of data was right up his alley. Indeed, he was delighted to find that not only could the spectroscopy data be analyzed through machine learning methods so as to sort out the changes caused by baking the rocks, but that the methods could also identify the temperature range in which each had been heated.

Next, the group applied spectroscopy and AI analysis to samples chosen randomly from the thousands of pieces of ancient knapped flint that Prof. Gopher had excavated from the cave. Dr. Azuri then took this new data and, using the model he’d created, evaluated the temperatures to which the early humans heated the ancient knapped flints.

“At first,” says Dr. Natalio, “the data seemed to be all over the place, and we did not know if we could say anything about these tools. But then Dr. Azuri created his model, and things just fell into place.”

Box Graph Filipe
A comparison of temperatures used to create the three types of tools reveals a small, relatively cool range for blades, in contrast with the other two

A distilled version of the findings compared three different types of flint artifacts and revealed three unique temperature ranges: one for each type. The first type, which the scientists call pot-lids, was comprised of small nicked and chipped shards. The analysis showed they had been exposed to fire hot enough to cause pieces of the flint to fly off of their own accord. That told the team their analysis was on the right track, as extremely high heat – up to 600⁰ Celsius (1,112⁰ Fahrenheit) – had been suggested, in other studies, to create the nicks and chips.

The second type is known as flakes, with blades – larger, knife-like tools with one long, sharp edge and a facing, thicker edge where they can be held – making up the third type of flint artifacts. The flakes, essentially smaller cutting tools than the blades, had been treated at a relatively broad range of temperatures, while the blades had been heated to lower temperatures (some 200-300⁰ degrees – low-to-medium oven settings), and the temperature range was much narrower. In other words, it appeared as though the cave’s inhabitants had intentionally used different heat-treatments to create different tools.

“We can’t know how they taught others the skill of toolmaking, what experience led them to heat the raw flint to different temperatures, or how they managed to control the process, but the fact that the longer blades are consistently heated in a different way than the other pieces does point to an intent,” says Dr. Natalio. “And that,” adds Dr. Pinkas, “is technology, as surely as our cell phones and computers are technology. It enabled our ancestors to survive and thrive.”

Dr. Filipe Natalio’s research is supported by the Braginsky Center for the Interface between Science and the Humanities; the Yeda-Sela Center for Basic Research; the Abramson Family Center for Young Scientists; the George Schwartzman Fund; and the estate of Olga Klein Astrachan.

Dr. Iddo Pinkas is the incumbent of the Sharon Zuckerman Research Fellow Chair.

Exploring the Physical World

Ancient Hominins Used Fire to Make Stone Tools

A study of these tools suggests skilled control over heating flint to produce different forms

• TAGS: Archaeology , Evolution , Chemistry

Potlid Flake Blade
(l-r) Pot-lid, flake, and blade (not shown to scale). Each was produced at a different temperature

REHOVOT, ISRAEL—October 5, 2020—Our ancestors not only knew how to use fire, they also developed sophisticated technologies for making tools. Researchers at the Weizmann Institute of Science recently employed cutting-edge technologies of their own to take a fresh look at a collection of stone tools. Their results suggest that the early humans who made the tools may have had a good understanding of the effects of heating the stone before flaking it into blades, and may even have used different temperatures to create different types of tools. The findings of this research were published in Nature Human Behaviour.

Qesem Cave, a site in Central Israel, was excavated by Prof. Avi Gopher and colleagues from Tel Aviv University. The findings in the cave were dated to 420,000 to 200,000 years ago, or the Lower Paleolithic, and the cave is assigned to the unique Acheulo-Yabrudian Cultural Complex. The ancient hominins (a group that includes us and the extinct members of our family tree) who lived in Qesem Cave left behind tens of thousands of stone tools. These tools are mainly made of flint, a material that is readily available throughout the country, and were produced using knapping – a process in which another rock or tool is used to chip off pieces of the target material, honing a sharp edge.

Somewhere between 300,000 and 400,000 years ago, the primary prey hunted by these hominins changed from elephants to fallow deer, necessitating a move toward finer tools. The question asked by the Weizmann research group was whether the ancient inhabitants of the area might have used fire to temper the flint before knapping it. Much more recent groups – less than 100,000 years ago – left evidence that they fired their flint, thus making the stone easier to shape. However, in sites as old as Qesem, there is usually almost no remaining organic matter that can give scientists conclusive evidence of fire use.

Dr. Filipe Natalio of the Institute’s Scientific Archaeology Unit explains that the first challenge in trying to understand whether flint has undergone a structural change, such as that produced by fire, is that the structure of raw flint can vary from site to site and even piece to piece, depending on the geological conditions in which it formed. Furthermore, traces of past heating in solid rock are typically microscopic or smaller – basically invisible. To approach these obstacles, he and postdoctoral fellow Dr. Aviad Agam, who specializes in prehistoric archaeology, turned to Dr. Iddo Pinkas of the Institute’s Chemical Research Support Department – an expert in a technique known as Raman spectroscopy.

The group first collected flint from areas near Qesem Cave and from around the country. After heating the flint pieces to different temperatures and cooling them again, the researchers examined them with the tools in Dr. Pinkas’s spectroscopy lab, thus revealing the makeup of the rocks down to their chemical and molecular structures.

The experiment yielded vast amounts of data – too much to analyze with standard methods. So the scientists turned to Dr. Ido Azuri of the Institute’s Bioinformatics Unit, part of the Life Sciences Core Facilities Department. Dr. Azuri is an expert in machine learning and artificial intelligence (AI), and so while the project was a departure from his usual biological research, finding patterns in large amounts of data was right up his alley. Indeed, he was delighted to find that not only could the spectroscopy data be analyzed through machine learning methods so as to sort out the changes caused by baking the rocks, but that the methods could also identify the temperature range in which each had been heated.

Next, the group applied spectroscopy and AI analysis to samples chosen randomly from the thousands of pieces of ancient knapped flint that Prof. Gopher had excavated from the cave. Dr. Azuri then took this new data and, using the model he’d created, evaluated the temperatures to which the early humans heated the ancient knapped flints.

“At first,” says Dr. Natalio, “the data seemed to be all over the place, and we did not know if we could say anything about these tools. But then Dr. Azuri created his model, and things just fell into place.”

Box Graph Filipe
A comparison of temperatures used to create the three types of tools reveals a small, relatively cool range for blades, in contrast with the other two

A distilled version of the findings compared three different types of flint artifacts and revealed three unique temperature ranges: one for each type. The first type, which the scientists call pot-lids, was comprised of small nicked and chipped shards. The analysis showed they had been exposed to fire hot enough to cause pieces of the flint to fly off of their own accord. That told the team their analysis was on the right track, as extremely high heat – up to 600⁰ Celsius (1,112⁰ Fahrenheit) – had been suggested, in other studies, to create the nicks and chips.

The second type is known as flakes, with blades – larger, knife-like tools with one long, sharp edge and a facing, thicker edge where they can be held – making up the third type of flint artifacts. The flakes, essentially smaller cutting tools than the blades, had been treated at a relatively broad range of temperatures, while the blades had been heated to lower temperatures (some 200-300⁰ degrees – low-to-medium oven settings), and the temperature range was much narrower. In other words, it appeared as though the cave’s inhabitants had intentionally used different heat-treatments to create different tools.

“We can’t know how they taught others the skill of toolmaking, what experience led them to heat the raw flint to different temperatures, or how they managed to control the process, but the fact that the longer blades are consistently heated in a different way than the other pieces does point to an intent,” says Dr. Natalio. “And that,” adds Dr. Pinkas, “is technology, as surely as our cell phones and computers are technology. It enabled our ancestors to survive and thrive.”

Dr. Filipe Natalio’s research is supported by the Braginsky Center for the Interface between Science and the Humanities; the Yeda-Sela Center for Basic Research; the Abramson Family Center for Young Scientists; the George Schwartzman Fund; and the estate of Olga Klein Astrachan.

Dr. Iddo Pinkas is the incumbent of the Sharon Zuckerman Research Fellow Chair.