Writing is only half the problem, Kim noted. You also have to read it back out of each cell, so the team redesigned its recording "tape" to make recovery easier.

What did 13.5 days of recording reveal?

Embryo No. 3's recording captured a permanent mark distinguishing the first two cells. Researchers traced nearly every profiled cell back to one founder or the other.

"One of these two founding cells did go on to produce more descendants than the other," Kim said, "but they produced diverse cell types at almost equal ratios."

That finding complicates the older idea that the first two cells of a mammalian embryo contribute asymmetrically to the body's tissues.

When do cells pick their final identities?

The recordings showed specialization happens on staggered timelines, not all at once:

• Blood and retinal cells committed relatively early.
• Outer skin cells waited noticeably longer.
• Many tissues stayed flexible well into organ formation.

Different cell types settled into identity on their own separate schedules — some early, some late.

Why does this matter for cancer and birth defects?

Shendure led the study with Chengxiang Qiu of Dartmouth College. Both serve as senior authors.

"We are interested in tracking how cells' early ancestry in an embryo influences their later fates," Shendure said. "Such research could eventually advance knowledge about congenital malformations, neurodevelopmental conditions, genetic disorders or cancer."

Co-first authors Qi Yu and Sophie Seidel, both at UW's Department of Genome Sciences, round out the team.

What's the catch?

Researchers worked entirely in mice. Mouse gestation runs 19 to 21 days, so the 13.5-day window covers roughly two-thirds of development and stops just short of birth.

Only one embryo yielded a near-complete record — a sign of how technically demanding the recordings still are.

The team says the approach offers something earlier tools couldn't: a single animal's complete developmental biography, written in its own DNA.