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Dr Frank Tulenko, Australian Regenerative Medicine Institute, Monash University

Q

Why do we have five fingers and five toes, and why do so many creatures also have five fingers? William Forrest, Tamworth, NSW

A

Modern tetrapods (limbed vertebrates) generally have hands and feet with five digits. This was not the case for the first tetrapods that moved from water to land as early as almost 400 million years ago – they had up to eight digits. It wasn’t until about 350 million years ago that a five-digit hand stabilised and was inherited. We don’t really understand why the five-digit model evolved, though biomechanical advantages for walking have been suggested. Developmental biologists have begun to tell a fascinating story of the genetic changes underpinning how limbs evolved from fins, and how these changes may have affected digit count. Digits have often been lost within tetrapods, but they almost never increase. This observation suggests a strong constraint – perhaps related to the use of the same genes for making different body parts – on what is possible through evolutionary tinkering.