Or it might have just been overlooked—because most developmental research only aims to reveal how whole organisms or parts of them grow under normal or mildly manipulated conditions, Jablonka said. But Levin’s work has a new goal, she says: “Constructing an autonomous creature that has nothing to do with the specific form of the [original] organism.”
Xenobots normally live for about a week, subsisting on the nutrients passed down from the fertilized egg they came from. But in rare cases, by “feeding” them with the right nutrients, Levin’s team has been able to keep xenobots active for more than 90 days. The longer-lived ones don’t stay the same but begin to change, as though they are on a new developmental path—destination unknown. None of their incarnations look anything like a frog as it grows from an embryo to a tadpole.
Channels of Communication
Media reports of the earlier handmade xenobots both reveled in and worried about the idea of miniature robots made from living matter. Might they breed and develop minds of their own? In truth, neither possibility was remotely likely: The cells could survive in a nutrient medium, but they couldn’t replicate into new xenobots. And they didn’t have any nerve cells that might act like a mind.
But even though xenobots have no nervous system, that doesn’t mean the cells can’t communicate with one another. One cell might release a chemical that sticks to surface proteins on another cell, triggering a biochemical process within the recipient. This type of cell signaling happens constantly during embryonic development, and it’s one way that neighboring cells control one another’s fate—the type of tissue each cell ultimately becomes. Adhesive proteins enable cells to attach to one another and to sense mechanical forces and deformations. In developing embryos, mechanical cues like this may also guide to become the right tissue type.
Levin thinks that cells also commonly communicate electrically—that this isn’t just a property of nerve cells, although they may have specialized to make good use of it. In a xenobot, “there’s a network of calcium signaling,” Levin said—an exchange of calcium ions like that seen between neurons. “These skin cells are using the same electrical properties that you would find in the neural network of a brain.”
For example, if three xenobots are set spaced apart in a row, and one of them is activated by being pinched, it will emit a pulse of calcium that, within seconds, shows up in the other two—“a chemical signal that goes through the water saying that someone just got attacked,” Levin said.
He thinks that intercellular communications create a sort of code that imprints a form, and that cells can sometimes decide how to arrange themselves more or less independently of their genes. In other words, the genes provide the hardware, in the form of enzymes and regulatory circuits for controlling their production. But the genetic input doesn’t in itself specify the collective behavior of cell communities.
Instead, Levin thinks that it programs cells with an ensemble of tendencies that produce a repertoire of behaviors. Under the normal conditions of embryogenesis, those behaviors follow a certain path toward forming the organisms we know. But give the cells a very different set of circumstances, and other behaviors and new emergent shapes will appear.
“What the genome provides for the cells is some mechanism that allows them to undertake goal-directed activities,” Levin said—in effect, a drive to adapt and survive.
Innate Drives to Survive
One such goal that Levin and his colleagues think they have seen is known as infotaxis, a push for cells to maximize the amount of information they get from their neighbors. Cells may also seek to minimize “surprise,” the chance of encountering something unexpected. The best way to do that, Levin says, is to surround yourself with copies of yourself. Some other goals are based on pure mechanics and geometry, such as minimizing the surface area of a cluster.