The Fibonacci Sequence
The sequence 1, 1, 2, 3, 5, 8, 13, 21, 34… where each number is the sum of the previous two — and which appears all over biology.
Origin
Introduced to Western Europe by Leonardo of Pisa (Fibonacci) in his 1202 book Liber Abaci, as a thought experiment about breeding rabbits. The sequence had been known earlier in Indian mathematics (Pingala, around 200 BCE). Its biological appearances — sunflower seed spirals, pine cone scales, the arrangement of leaves on a stem — are real and follow from the math of efficient packing. The ratio of consecutive Fibonacci numbers approaches the golden ratio.
Modern usage
The default math reference for 'nature follows numbers' — used in everything from popular-science writing to Dan Brown novels. Stock-market 'Fibonacci retracement' is a trading-chart technique with extremely thin evidence. The sequence is also a common toy example in introductory coding interviews.
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