Next, Dr. Blackiston swept the skin cells into a little well, forming a milky ball. Soon the cells glued themselves back together. Then he cauterized away some parts of the ball, carving a tiny figurine — a living skin-sculpture, about the size of a fine grain of salt, that looked like Mr. Kriegman’s quadruped. Two weeks later, he showed a picture of the entity to the Vermont computer scientists.

“We were just dumbfounded,” Dr. Bongard said. “The moment we saw that, both labs really just kind of dove into this full time.”

Function follows form

In Vermont, he and Mr. Kriegman began crafting virtual worlds that would reward particular behaviors by the clumps of repurposed frog. Take walking: First an algorithm produced many random body designs; some just sat there, others rocked or waddled forward. Then the algorithm let the best of the walkers procreate into the next generation; from these, another generation was produced, and so on, each one improving on the best designs. Another simulation, aimed at finding designs that could carry an object, became crowded with bagel-like bodies that had evolved a central cavity to hold things.

After the process ran for about a day, it produced body shapes preprogrammed to execute the initial tasks. Then the Vermont team relayed the resulting best body shapes to Dr. Levin and Dr. Blackiston, who began trying to sculpt cellular figurines that resembled those designs: first with just skin cells, then also with cardiac tissue — an assemblage of muscle cells that contracts and expands. The Tufts team offered feedback, to make the next round of simulations better at predicting what would happen in a real petri dish. And so on, in a loop.

In their paper and in press coverage, the team hinted at what these xenobots might someday do. Sweep up ocean microplastics into a larger, collectible ball? Deliver drugs to a specific tumor? Scrape plaque from the walls of our arteries? The xenobots would biodegrade after using up the yolk inside their cells. And whatever their intended purpose, their bodies would be designed not by an engineer but by a simulacrum of real evolution built to encourage the right behavior in the target environment.

“What should a robot look like that’s crawling through your arteries?” Mr. Kriegman said. “Should it have four legs? I don’t know!”