Figuring out if something is alive or not feels like it should be pretty cut and dry—does it consume energy? Does it reproduce? Does it respond to light, sound, or a poke with a sharp stick? If so, name it after David Attenborough and move on. But as the realm of human knowledge expands, the traditional scientific definition of life is starting to fray around the edges. On the microscale, you have the century-old debate among doctors and scientists over whether things like viruses or prions are living creatures. On the macro, you have questions about synthetic life—is it within our scientific and moral capacity to bring life from nothing? And if we try, how exactly do we know when we’ve been successful?
Science currently defines a live entity as something that can reproduce, metabolize, maintain homeostasis, adapt, grow, react to outside stimuli, and be organized in a meaningful way. But even this definition mischaracterizes some of the life we’re pretty sure is alive (say, viruses, which can only reproduce when attached to living cells and don’t really have much stasis to homeo) and includes some of the things we’re pretty sure aren’t (like mitochondria, which were probably once separate living entities but now can’t survive outside the cell to which they belong).
At some point the question becomes as much philosophy as science, which is probably why science fiction deals with it a lot more readily than science fact does. (The very first modern science-fiction novel was Frankenstein.) More recently, we ask ourselves if Johnny Five was really alive, or if Mr. Data deserves to be treated like something other than a really advanced sex toy (spoiler alert: yes).
But science fact is catching up. Earlier this month, Harvard researchers published a description of a stingray-shaped robot that they had built from genetically engineered rat heart muscles that were grown onto a skeleton made of breast-implant silicone and gold (yes, really).* The fingernail-sized robot—or “biorobot” since it’s made mostly of meat—has no will of its own, but it is able to swim towards light sources. It’s incapable of surviving outside of the lab, where it’s kept in a sterile nutrient broth, which it consumes. The robot can’t think, reproduce, feel, or fend for itself, but it can swim and tell light from dark, and it’s made of mammal, with the same oxygen-based metabolism that we have. So it swims and eats and breathes, but is it alive?
Currently, it’s missing too much of the main criteria to qualify. On the other hand, there’s a lot of wiggle room in the traditional definition of life. Consider the mule, which results from crossing a male donkey with a female horse, creating a creature that has a disjointed number of chromosomes and in almost all circumstances can’t reproduce. And yet no one would say a mule was less alive than, say, a forest fire, even though that almost always reproduces. A special working group from NASA was tasked with defining life so that they could know if they discovered it elsewhere in the universe. They concluded that it was a philosophical question and that scientists “shouldn’t be in the business of [defining life], but also it isn’t really possible.”
Kit Parker, the Harvard bio-engineer who spearheaded the development of the stingray biorobot, has aspirations beyond setting up an epic bad joke (“a bar of gold, a rat heart, and some fake boobs walk into a bar …”). While Parker’s specific interest in biorobotics is creating artificial human organs, he has said he believes that his creation is alive—not an organism in its own right but a life form. Parker refers to it as an “adaptive swimming animal” in the Science paper and says he sees it as the first step in developing autonomous, adaptive creatures “able to process multiple sensory inputs and produce complex behaviors in distributed systems.”
Regardless of how alive or not alive the stingray is, its existence reminds us that we might all benefit from having a clearer understanding of what defines life at all—otherwise we might not even know when we ultimately do create it in a lab.
*Correction, July 19, 2016: This article originally misstated that the robot used silicon. It was silicone. (Return.)