Every cell in the body carries an instruction manual for building proteins. It is written in RNA molecules, which are constantly copied from DNA. But copying brings typos: missing letters, extra insertions, premature stops. These typos pile up like damage in an old book, and the cell starts to work worse.
Scientists have figured out how to count such typos. They take a tissue sample, read all the RNA molecules, and look for four kinds of errors: premature stop signals, faulty assembly, activation of "jumping" regions, and fusion of different genes. The more of these errors, the more worn out the cell.
Tests in mice and humans showed that errors increase with age in almost all tissues. In mice given rapamycin or kept on a restricted diet, errors became fewer, and the clock showed a younger age. In people with Alzheimer's disease, the clock ran faster than in healthy people.
This way of measuring aging can be put to use right now: to check whether anti-aging drugs work, and to spot brain diseases earlier from a blood test.
The cerebellum turned out to be the most "damaged" region of the brain, even though the samples did not differ in age from other regions. Researchers link this to the near-total absence of neuron turnover in the cerebellum: the cells live a long time and accumulate errors.
The idea of measurable molecular damage as a clock of aging echoes Greg Egan's novel Permutation City, in which characters try to control biological age by manipulating cellular processes. In reality, though, we are still far from that kind of precision.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.