Ishango Bone: Notched Tally Stick from Ice Age Congo

The Ishango bone is a dark, notched bone tool found in 1950 at Ishango, on the shore of Lake Edward in what is now the Democratic Republic of the Congo. It is usually dated to about 20,000 years ago. Its grouped notches have made it famous as possible evidence of early counting, although what the marks really meant remains uncertain.

Discovery at Ishango

Ishango lies where the Semliki River flows out of Lake Edward, inside today’s Virunga National Park. In 1950 the Belgian geologist Jean de Heinzelin de Braucourt led excavations there, in what was then the Belgian Congo.

The team uncovered the remains of a settlement of people who fished in the lake and hunted on its shores. Among thousands of finds, including bone harpoon points, was a small bone handle with rows of notches. De Heinzelin described it to a wide public in a 1962 article in Scientific American.

The bone was taken to Brussels and is now held by the Royal Belgian Institute of Natural Sciences.

The people of Ishango

The layers at Ishango record a community of fisher-hunter-gatherers who returned to the lakeshore over long periods. Fish bones show that they caught large numbers of lake fish, while mammal bones point to hunting of antelope and other animals in the surrounding grasslands. Fragments of human bones were also found.

The most characteristic tools are barbed harpoon points made of bone. These show that the people of Ishango had developed specialised equipment for fishing, part of a wider tradition of bone harpoons found around the lakes and rivers of central and eastern Africa.

Lake Edward lies in the Albertine Rift, a branch of the East African Rift system. During the last Ice Age, when much of Africa was drier than today, lakes like this one were valuable sources of water and food.

What the bone looks like

The object is about 10 centimetres long and is generally identified as the fibula (lower leg bone) of a baboon. A sharp fragment of quartz is fixed into one end, which suggests it was used as an engraving tool or handle.

Along its length are three columns of notches, divided into groups of different sizes. Most accounts give the groups as follows:

These counts come from readings of the original object. Some notches are worn, so different researchers have produced slightly different counts.

Dating the bone

De Heinzelin first suggested a much younger age, roughly 8,000 to 11,000 years. Later work at Ishango, including radiocarbon and other dating of the site’s layers from the 1980s onward, placed the occupation much earlier. The bone is now usually dated to about 20,000 years ago, with published estimates ranging from around 18,000 to 25,000 years.

Interpretations

The neat patterns invited bold readings. De Heinzelin suggested the notches showed some knowledge of arithmetic. He noted that the first column holds the prime numbers between 10 and 20, and that the second shows 10 plus and minus 1 and 20 plus and minus 1.

In the 1970s the American researcher Alexander Marshack argued that the marks recorded the phases of the Moon over about six months. Others have proposed that it was a counting tool in a number system based on 12, or a record kept by a woman tracking a monthly cycle.

Many archaeologists are more cautious. The grouping of notches might have helped grip the tool, been decorative, or recorded quantities of things now unknown. Because there is only one such object, there is no way to test which idea is right.

Research since de Heinzelin

In the 1980s the American archaeologists Alison Brooks and Catherine Smith returned to the Ishango finds and the wider Semliki valley. Their work, published from 1987, re-examined the dating of the deposits and placed the site much earlier than de Heinzelin had first proposed.

Mathematicians have also continued to study the bone. In 1999 Vladimir Pletser and Dirk Huylebrouck explored whether its groupings could fit a counting system based on 12, an idea that remains speculative. Belgian institutions have promoted the bone as a symbol of early science, and it has been shown in exhibitions in Brussels.

Myths and facts

The Ishango bone is often described online as proof that prehistoric Africans knew prime numbers or made a calendar. Neither claim is established. The bone does show that people at Ishango made deliberate, organised marks, which is evidence of careful record-keeping of some kind.

It is also sometimes called the oldest mathematical object. The Lebombo bone from southern Africa is considerably older, and even earlier engraved patterns come from sites such as Blombos Cave.

Timeline

• c. 20,000 years ago: Fisher-hunter-gatherers live at Ishango; the notched bone is made.
• 1950: Jean de Heinzelin’s excavations at Ishango recover the bone.
• 1962: De Heinzelin publishes “Ishango” in Scientific American, proposing arithmetic patterns.
• 1972: Alexander Marshack’s The Roots of Civilization presents a lunar-calendar reading.
• 1987: Alison Brooks and Catherine Smith publish a reassessment of the Ishango site.
• 1980s onward: New dating studies push the site’s age back to about 20,000 years.

Why it matters

The Ishango bone is one of the clearest early examples of humans grouping marks in structured sets. Whether it was a tally, a calendar or something else, it reminds us that the roots of counting and record-keeping lie deep in prehistory, and that Africa holds some of the most important evidence for them.