> if an event or behaviour affects a person, it means that there must be a physical record of it

Yes, and it (or at least one of them) has now been found. Maybe you don't care about knowledge for its own sake, but as the "irritating" press release goes on to point out,

“There are currently no treatments for what early-life stress does to the brain, partially because we have not had a clear picture of what molecular mechanisms to target,” Peña said. “This work is exciting because it reveals a clear mechanism, and also helps explain why the impact of stress is both latent and broad.