A London furniture conservator has been credited with a crucial discovery that has helped understand why Ice Age hunter-gatherers drew cave paintings.
Ben Bacon analysed 20,000-year-old markings on the drawings, concluding they could refer to a lunar calendar.
It led to a specialist team proving early Europeans made notes about the timing of animals' reproductive cycles.
Mr Bacon said it was "surreal" to work out for the first time what hunter-gatherers were saying.
Cave paintings of animals such as reindeer, fish and cattle have been found in caves across Europe.
But archaeologists had been stumped by the meaning of dots and other marks on the paintings. So Mr Bacon decided he would try to decode them.
He spent numerous hours on the internet and in the British Library consulting pictures of cave paintings and "amassed as much data as possible and began looking for repeating patterns".
In particular, he examined a 'Y' sign on some paintings, which he felt might be a symbol for "giving birth" because it showed one line growing out from another.
With his research advancing, he brought in friends and senior academics. They encouraged him to continue with his investigations despite Mr Bacon being "effectively a person off the street".
He collaborated with a team including two professors from Durham University and one from University College London and, by working out the birth cycles of similar present-day animals, they deduced that the number of marks on the cave paintings was a record, by lunar month, of the animals' mating seasons.
The team's findings were published in the Cambridge Archeological Journal.
Prof Paul Pettitt, of Durham University, said he was "glad he took it seriously" when Mr Bacon contacted him.
"The results show that Ice Age hunter-gatherers were the first to use a systemic calendar and marks to record information about major ecological events within that calendar."
He added: "In turn, we're able to show that these people, who left a legacy of spectacular art in the caves of Lascaux [in France] and Altamira [in Spain], also left a record of early timekeeping that would eventually become commonplace among our species."
Mr Bacon said our ancestors were "a lot more like us than we had previously thought. These people, separated from us by many millennia, are suddenly a lot closer".
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