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Especially relevant to University of Oxford and University of Cambridge.
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Evolutionary reasoning, physiological reserve and the ability to challenge the premise of a question.
A structure to build your own answer on — not a script.
Challenge the premise
'Perfectly well' — true for most people at rest, but reserve matters under stress, injury or disease.
Reserve and redundancy
Spare capacity protects against injury, infection and age-related decline; the remaining kidney increases its function (compensatory hypertrophy).
Developmental explanation
Bilateral symmetry: many paired organs arise from symmetrical development.
Evolutionary explanation
Organs we can survive losing one of may have been selected for redundancy; cost of a second kidney is relatively low compared with the benefit.
Contrast
Heart — a single pump works better; redundancy is only useful where a duplicate adds benefit.
A strong but not perfect answer. Read the notes to see why it works — then make your own version with your experiences.
I'd first question the word 'perfectly'1. Most people who donate a kidney live healthy lives, but the second kidney gives us reserve: if one is injured, infected or blocked, or when kidney function declines with age or disease, having two provides a safety margin. After someone donates a kidney, the remaining one enlarges and increases its filtration, which suggests the body normally has spare capacity rather than needing both at full stretch.
There are a couple of different types of explanation2. One is developmental: our bodies are built on a largely symmetrical plan, so many organs come in pairs — kidneys, lungs, eyes, ovaries — because they develop on both sides of the embryo. So the question could be 'why hasn't evolution got rid of one?' rather than 'why did we get two?'
The evolutionary answer might be that the extra kidney is relatively cheap to maintain, and the benefit — surviving an injury to one side, or an infection — could have been significant, especially before modern medicine.
The contrast with the heart3 is interesting. A second heart wouldn't simply be spare capacity; two pumps would have to coordinate, and the heart's structure already has four chambers doing different jobs. So redundancy seems to be kept where a duplicate genuinely adds safety at low cost.
I'd want to know more about4 whether people with one kidney from birth have any different outcomes over a lifetime — that would tell us how much the reserve matters.
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