By comparing the marks to the animals’ life cycles, researchers showed that the number of dots or lines in a given image strongly correlates to the month of mating across all the analyzed examples, which included aurochs (an extinct species of wild cattle), bison, horses, mammoth and fish. What’s more, the position of the symbol “Y” in a sequence was predictive of birth month, suggesting that “Y” signifies “to give birth.”

The finding is one of the earliest records of a coherent notational system, the researchers say. It indicates that people at the time were able to interpret the meaning of an item’s position in a sequence and plan ahead for the distant future using a calendar of sorts — reinforcing the suggestion that they were capable of complex cognition.

“This is a really big deal cognitively,” says Ben Bacon, an independent researcher based in London. “We’re dealing with a system that has intense organization, intense logic to it.”

A furniture conservator by day, Bacon spent years poring through scientific articles to compile over 800 instances of these cave markings. From his research and reading the literature, he reasoned that the dots corresponded to the 13 lunar cycles in a year. But he thought that the hunter-gatherers would’ve been more concerned with seasonal changes than the moon.

In the new paper, he and colleagues argue that rather than pinning a calendar to astronomical events like the equinox, the hunter-gatherers started their calendar year with the snowmelt in the spring. Not only would the snowmelt be a clear point of origin, but the meteorological calendar would also account for differences in timing across locations.