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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? (continued)

Tobias Bast
School of Psychology, University of Nottingham, UK

I wanted to pick up on a previous response to this question, which suggested that neurons in our brains are continuously replaced and an 80-year-old would retain virtually no neurons from their 20-year-old brain.

The formation of new neurons in adult brains, known as adult neurogenesis, is well established in many animal species, although evidence for it in humans is somewhat controversial. Regardless, this process is limited to a few brain regions, including a small subregion of the hippocampus. Also, adult neurogenesis adds neurons to the existing network, rather than replacing them. Therefore, it is very likely that the majority of the neurons in your 65-year-old (or even in your 80-year-old) brain will be the same as those you had at 25 years old, or even at 20.

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