It is time for an answer to this very old question.

      Now, finding a universal explanation for anything—let alone something as complicated as aging—doesn’t happen overnight. Any theory that seeks to explain aging must not just stand up to scientific scrutiny but provide a rational explanation for every one of the pillars of aging. A universal hypothesis that seems to provide a reason for cellular senescence but not stem cell exhaustion would, for example, explain neither.

      Yet I believe that such an answer exists—a cause of aging that exists upstream of all the hallmarks. Yes, a singular reason why we age.

      Aging, quite simply, is a loss of information.

      You might recognize that loss of information was a big part of the ideas that Szilard and Medawar independently espoused, but it was wrong because it focused on a loss of genetic information.

      But there are two types of information in biology, and they are encoded entirely differently. The first type of information—the type my esteemed predecessors understood—is digital. Digital information, as you likely know, is based on a finite set of possible values—in this instance, not in base 2 or binary, coded as 0s and 1s, but the sort that is quaternary or base 4, coded as adenine, thymine, cytosine, and guanine, the nucleotides A, T, C, G of DNA.

      Because DNA is digital, it is a reliable way to store and copy information. Indeed, it can be copied again and again with tremendous accuracy, no different in principle from digital information stored in computer memory or on a DVD.

      DNA is also robust. When I first worked in a lab, I was shocked by how this “molecule of life” could survive for hours in boiling water and thrilled that it was recoverable from Neanderthal remains at least 40,000 years old.39 The advantages of digital storage explain why chains of nucleic acids have remained the go-to biological storage molecule for the past 4 billion years.

      The other type of information in the body is analog.

      We don’t hear as much about analog information in the body. That’s in part because it’s newer to science, and in part because it’s rarely described in terms of information, even though that’s how it was first described when geneticists noticed strange nongenetic effects in plants they were breeding.

      Today, analog information is more commonly referred to as the epigenome, meaning traits that are heritable that aren’t transmitted by genetic means.

      The term epigenetics was first coined in 1942 by Conrad H. Waddington, a British developmental biologist, while working at Cambridge University. In the past decade, the meaning of the word epigenetics has expanded into other areas of biology that have less to do with heredity—including embryonic development, gene switch networks, and chemical modifications of DNA-packaging proteins—much to the chagrin of orthodox geneticists in my department at Harvard Medical School.

      In the same way that genetic information is stored as DNA, epigenetic information is stored in a structure called chromatin. DNA in the cell isn’t flailing around disorganized, it is wrapped around tiny balls of protein called histones. These beads on a string self-assemble to form loops, as when you tidy your garden hose on your driveway by looping it into a pile. If you were to play tug-of-war using both ends of a chromosome, you’d end up with a six foot-long string of DNA punctuated by thousands of histone proteins. If you could somehow plug one end of the DNA into a power socket and make the histones flash on and off, a few cells could do you for holiday lights.

      In simple species, like ancient M. superstes and fungi today, epigenetic information storage and transfer is important for survival. For complex life, it is essential. By complex life, I mean anything made up of more than a couple of cells: slime molds, jellyfish, worms, fruit flies, and of course mammals like us. Epigenetic information is what orchestrates the assembly of a human newborn made up of 26 billion cells from a single fertilized egg and what allows the genetically identical cells in our bodies to assume thousands of different modalities.40

      If the genome were a computer, the epigenome would be the software. It instructs the newly divided cells on what type of cells they should be and what they should remain, sometimes for decades, as in the case of individual brain neurons and certain immune cells.

      That’s why a neuron doesn’t one day behave like a skin cell and a dividing kidney cell doesn’t give rise to two liver cells. Without epigenetic information, cells would quickly lose their identity and new cells would lose their identity, too. If they did, tissues and organs would eventually become less and less functional until they failed.

      In the warm ponds of the primordial Earth, a digital chemical system was the best way to store long-term genetic data. But information storage was also needed to record and respond to environmental conditions, and this was best stored in analog format. Analog data are superior for this job because they can be changed back and forth with relative ease whenever the environment within or outside the cell demands it, and they can store an almost unlimited number of possible values, even in response to conditions that have never been encountered before.41

      The unlimited number of possible values is why many audiophiles still prefer the rich sounds of analog storage systems. But even though analog devices have their advantages, they have a major disadvantage. In fact, it’s the reason we’ve moved from analog to digital. Unlike digital, analog information degrades over time—falling victim to the conspiring forces of magnetic fields, gravity, cosmic rays, and oxygen. Worse still, information is lost as it’s copied.

      No one was more acutely disturbed by the problem of information loss than Claude Shannon, an electrical engineer from the Massachusetts Institute of Technology (MIT) in Boston. Having lived through World War II, Shannon knew firsthand how the introduction of “noise” into analog radio transmissions could cost lives. After the war, he wrote a short but profound scientific paper called “The Mathematical Theory of Communication” on how to preserve information, which many consider the foundation of Information Theory. If there is one paper that propelled us into the digital, wireless world in which we now live, that would be it.42

      Shannon’s primary intention, of course, was to improve the robustness of electronic and radio communications between two points. His work may ultimately prove to be even more important than that, for what he discovered about preserving and restoring information, I believe, can be applied to aging.

      Don’t be disheartened by my claim that we are the biological equivalent of an old DVD player. This