I am 65. Assuming that I still weigh what I did at 25 (I wish!), what percentage of material from 40 years ago remains as part of me today?
@Nik_Henville
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While the 鈥渁verage鈥� cell in our bodies lasts between seven and 10 years, the range is from 4 to 8 hours for immune cells called granulocytes to a full human lifetime for some neurons.
But percentage-wise, not a lot of your 25-year-old self remains.
James Stone
Great Hucklow, Derbyshire, UK
Maybe there is no need to worry about how much of the biological material from your youth is retained in your 65-year-old body.
Neurons in the hippocampus (a brain region vital for memory) are replaced at the rate of 1.75 per cent each year in adults. A naive calculation, assuming each neuron gets replaced only once, predicts that the entire hippocampus would be replaced after about 57 years.
If this applies to other brain structures, then it implies that an 80-year-old retains no neurons from their 20-year-old brain. Despite this, an 80-year-old can recall events from their youth.
So it isn鈥檛 the preservation of material, but the preservation of information, that matters.
John Peterson,
via Facebook
What remains? Your teeth and your memories.
Guy Cox
Sydney, Australia
Your teeth remain, for a start. The enamel doesn鈥檛 grow once formed and has no repair mechanism. Plus, a few cells of the body, such as spinal neurons, aren鈥檛 replaced, so you have the original cells there.
However, some cells 鈥� such as those in skin and hair, as well as red blood cells 鈥� are continually replaced, so none of these will be the same ones 40 years later.
Even long-lived cells are metabolically active, with a turnover of material taking place in them, so the actual atoms in the cells are most unlikely to have been there for 40 years.
Ian Sanderson
via Facebook
It depends on how you define 鈥渕e鈥�. If 鈥渕e鈥� is your physical body, not much. If 鈥渕e鈥� is your personality, most of it.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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