Lifeboat Foundation

Scientists have captured the fleeting moment when a heart starts beating, a feat made all the more remarkable when you consider the relative brevity of a zebrafish’s life.

In a tight window about 20 hours into zebrafish development, the embryos’ developing hearts jumped into action, emerging as one from an ensemble of single cells.

“It was like somebody had flipped on a switch,” says Harvard University biophysicist Adam Cohen, senior author of the new study, which imaged zebrafish embryos snug in custom-made agarose molds to capture this once-in-a-lifetime event.