Xenobots were first created last year, using cells taken from the embryo of the frog species Xenopus laevis. Under the right lab conditions, the cells formed small structures that could self-assemble, move in groups and sense their environment.

Now, the researchers behind the work have found that xenobots can also self-replicate. Josh Bongard at the University of Vermont and Michael Levin at Tufts University in Massachusetts and their colleagues began by extracting rapidly dividing stem cells that are destined to become skin cells from frog embryos.

When the cells are brought together in clumps, they form spheres of around 3000 cells within five days. Each clump is around half a millimetre wide and covered in minuscule hair-like structures. These act like flexible oars, propelling the xenobots forward in corkscrew paths, says Bongard.

The team noticed that individual clumps of cells appeared to work together in a swarm, pushing other loose cells in the dish together. The resulting piles of cells gradually formed new xenobots.

Further experiments revealed that groups of 12 xenobots placed in a dish of around 60,000 single cells appear to work together to form either one or two new generations.

“One [xenobot] parent can begin a pile and then, by chance, a second parent can push more cells into that pile, and so on, generating the child,” says Bongard.

Each round of replication creates slightly smaller xenobot offspring, on average. Eventually, offspring that comprise fewer than 50 cells lose their ability to swim and reproduce.

In an attempt to create additional generations of xenobots, the team turned to artificial intelligence. Using an algorithm modelled on evolution, the team predicted which starting shapes of xenobots might generate the most offspring.

The simulation predicted that C-shaped clusters would give rise to the most generations. When the team cut spherical xenobots into C-shapes, the altered xenobots produced up to four generations, double that generated by spherical xenobot parents.

“By manipulating the shape of the parents, you can make a better shovel to move more cells,” says Bongard.

It is the first time multicellular organisms have been found to self-replicate in a way that doesn’t involve growth on the organism’s own body. “This work shows there was a previously unknown way that life could self-replicate,” says Bongard.

Some of the team members hope to use the xenobots to investigate how the first organisms on Earth may have reproduced.