Leonardo of Pisa (Fibonacci)

Leonardo of Pisa (around 1170–1250), nicknamed Fibonacci, was an Italian merchant's son who did something quietly world-changing: he taught Europe to count. Growing up around the bustling ports of the Mediterranean, he learned the Hindu–Arabic numerals — the 0 through 9 you use every day — and realised they were vastly better than the clunky Roman numerals Europe was still struggling with.

The concepts left behind

His famous Fibonacci sequence — 1, 1, 2, 3, 5, 8, 13, where each number is the sum of the two before it — first appeared as a playful puzzle about breeding rabbits. Yet it turns up astonishingly often in nature: in the spirals of sunflower seeds, pinecones, and pineapples, and in the way petals cluster. As the numbers grow, their ratios creep toward the golden ratio, that endlessly pleasing proportion \varphi \approx 1.618 beloved by artists.

Fibonacci's real masterpiece wasn't the rabbits at all — it was his 1202 book Liber Abaci ("The Book of Calculation"), which patiently showed merchants how to do arithmetic, convert currencies, and keep accounts using the new numerals. It was so useful that it helped drag European commerce out of the dark ages of tally sticks and Roman numerals. The rabbit sequence was just one fun example buried inside — but that little side-note became the thing everyone remembers, which would probably have amused him no end.