What was found
In 1950, the Belgian geologist Jean de Heinzelin de Braucourt was working at Ishango, a site on the shore of Lake Edward in what is now the Democratic Republic of Congo. There he found a small bone, usually identified as the fibula of a baboon, with a piece of quartz set into one end. The surface is covered with rows of short cuts, arranged in three long columns that run along the length of the bone. It is kept at the Royal Belgian Institute of Natural Sciences in Brussels.
The bone was first thought to be fairly young, from several thousand years ago. Later work on the site, which had been buried by a volcanic eruption, led to a much older estimate, and it is now commonly placed around 20,000 years ago. As with all such dates, it carries a margin of uncertainty.
The notches
The cuts are arranged in groups, and the totals in each group have attracted attention. The numbers below are those usually reported, listed in the order the groups are read along each column.
| Column | Groups of notches | Total |
|---|---|---|
| First | 11, 13, 17, 19 | 60 |
| Second | 3, 6, 4, 8, 10, 5, 5, 7 | 48 |
| Third | 11, 21, 19, 9 | 60 |
Each total can be checked: 11 + 13 + 17 + 19 = 60, 3 + 6 + 4 + 8 + 10 + 5 + 5 + 7 = 48, and 11 + 21 + 19 + 9 = 60. Counts of individual groups depend on how worn or ambiguous marks are read, and some published tables differ in small details.
What archaeologists have proposed
Several readings have been put forward, and none is settled.
Heinzelin himself suggested that the bone might show some sort of arithmetic. In the second column, 3 is followed by 6 and 4 by 8, which looks like doubling, and 10 is followed by 5, which looks like halving. In the other columns, the groups 11, 13, 17 and 19 are the prime numbers between 10 and 20. The third column’s 11, 21, 19 and 9 sit close to 10 and 20, being one more or one less. He and others also noted that 48 and 60 are both multiples of 12, a point sometimes tied to duodecimal counting.
A second family of explanations treats the bone as a calendar. In the 1970s Alexander Marshack proposed that the notches record lunar phases across about six months, with the columns tracking the moon over successive cycles. Other researchers have offered related ideas, including that the bone could have been a record kept by a woman to track the menstrual cycle. This idea is often credited to Claudia Zaslavsky, though it is a suggestion and not an established fact.
Why many specialists are cautious
The main objection is that patterns can be found in nearly any set of groupings. The primes 11, 13, 17 and 19 are four numbers from a short range, and nothing else on the bone requires an understanding of primality. A person making marks for another reason could easily have produced them without any such concept. Doubling and halving are likewise easy to find if one is free to choose which neighbours to compare.
The notches may also be decorative, or may have helped to grip the tool, or may record something unrelated to number. The quartz fragment could have been a tool for engraving, which suggests the bone was a working object, but it does not show what was recorded. The reading order of the groups is itself a choice made by modern interpreters, and reading in a different direction would change the apparent sequences.
For these reasons, careful accounts describe the Ishango bone as a deliberate and probably meaningful set of marks, rather than as proof of advanced mathematics.
How it fits the wider record
The bone belongs to a small group of older objects that carry regular marks, including the Lebombo bone from southern Africa and a wolf bone from the Czech Republic. Its position is unusual in that the notches are arranged in several organised groups instead of one long row. That makes it a stronger candidate for planned recording than a simple tally of items, but not enough to say what was recorded.
If the lunar reading is right, it would link the bone to the calendar systems that later cultures built on the moon’s cycle. If the arithmetic reading is right, it would be among the earliest known number sequences. The simplest conclusion supported by the evidence is that someone made these marks with care, for a reason we cannot confirm. Future finds of better-documented tallies may help, but for now the bone is best treated as a question, not an answer.