When can we call something alive? This question is more difficult than you may think and has several important implications, such as attempting to detect extraterrestrial life on other planets and even during medical procedures when sterile conditions have to be maintained.
But let’s start with when life originated on our planet. A variety of organic molecules got together to form a unit, which eventually became a cell. Nowadays, the reproduction of a parent cell to a daughter cell is accomplished by just one very special molecule: DNA. However, the first cells on Earth did not likely use DNA, but RNA, or perhaps even some other simpler molecule.
But what about viruses? Are they alive? It has been suggested that viruses are derived from ancient cells that lost their ability to reproduce on their own without a host. Part of the reasoning is that many viruses reproduce using RNA. However, if viruses are derived from these ancient cells, they would be considered “alive” before becoming viruses and transitioning into a non-life stage. Thus, there may not be some great transition or evolutionary step from a chemical system to a living system, but a kind of continuum in which life from non-life would be difficult to distinguish and where “systems” switch between life and non-life with relative ease— similar to how some organisms can switch between a single-cellular and a multicellular life style.
Intuitively, though, we experience a big difference between something that is alive and something that is not. The more complex the organism, the more obvious it is. If you feel your pulse and breathe, you are alive. If your pulse is not detectable after a relatively short time period of a few minutes, then you are (in most cases) dead— especially if there are no brain waves detectable anymore.
The observation of telltale signs of life in animals likely contributed to the idea of vitalism becoming widespread as a biological hypothesis in the 18th and 19th century. Scientists supporting vitalism at that time thought that the difference between life and non-life could not be reduced to physical and chemical processes alone. French philosopher Henri Bergson invoked a vital principle called the élan vital which included spiritual aspects. But the idea of vitalism, at least the more far-reaching parts of it, were widely discredited. For example, it could be shown that organic (and biologically important) compounds like urea could be synthesized by inorganic components and processes.
To verify any hypothesis in the sciences, it has to pass various tests, but no experiment could provide proof that such an élan vital, or vital force, exists. Even the famous French chemist and microbiologist Louis Pasteur couldn´t prove it even though he tried! No credible evidence was found that there is something in addition to mechanical physical and chemical processes and building blocks. Nevertheless, far-reaching ideas of vitalism are still around (just think about the Force in Star Wars!) and it is still discussed in science and philosophy, and also how it relates to medicine.
But let´s get back to the origin of life issue. Would there be something intrinsic that distinguishes a biological system such as a cell from a chemical system? Would or could cells have something equivalent to a rhythmic pattern or motion similar to the breathing and circulation processes within an animal? Is that rhythmic pattern perhaps an emergent property that arises once the transition to life has been achieved? In a new paper, Marina Walther-Antonio from the Mayo Clinic and I provided suggestions on how we might find out. There are various types of motions within any type of cell from the myriad of internal processes, and also at the cell boundaries when a microorganism interacts with its natural environment. These myriads of ion exchanges and other processes going on at the cellular membranes should be detectable with state-of-the-art microscopy technologies. In our paper, we hypothesize that all living microbes will exhibit a rhythmic pattern analogous to a living pulse in more evolved life forms. We expect the frequency and the magnitude of that signal to be different depending on the species—just like it is the case for animals.
Hints of the existence of such a “living pulse” in microorganisms comes from nanomechanical oscillators, which detect forces in the order of a piconewton (which is 1 trillionth of a newton!) and which were used to characterize living specimens and their metabolic cycles. Cantilevers were used to investigate the activity of a cell’s molecular motors and the particular vibrations of living yeast.
Whether a “living pulse” exists in cells is not only a theoretical question, but would have far-reaching practical implications. If the hypothesized “living pulse” can be detected, it could mean the discovery of a universal biosignature independent of an organism´s specific biochemistry. It would be a tool to detect life in extreme environments on Earth and in extraterrestrial locations, where we don´t know whether it exists. It would also help ensure that life is not present where sterilizing conditions are critical, such as for planetary protection purposes (not to bring Earth life to Mars, and perhaps more importantly not to possibly bring indigenous Martian life to Earth), in the food-processing industry, and during medical procedures (for example, on the operating table). It could be a first step in the direction of building a kind of tricorder as envisioned in Star Trek.