Lifespan

And it is so simple and so robust that not only did it ensure life’s continued existence on the planet, it ensured that Earth’s chemical survival circuit was passed on from parent to offspring, mutating and steadily improving, helping life continue for billions of years, no matter what the cosmos brought, and in many cases allowing individuals’ lives to continue for far longer than they actually needed to.

The human body, though far from perfect and still evolving, carries an advanced version of the survival circuit that allows it to last for decades past the age of reproduction. While it is interesting to speculate why our long lifespans first evolved—the need for grandparents to educate the tribe is one appealing theory—given the chaos that exists at the molecular scale, it’s a wonder we survive thirty seconds, let alone make it to our reproductive years, let alone reach 80 more often than not.

But we do. Marvelously we do. Miraculously we do. For we are the progeny of a very long lineage of great survivors. Ergo, we are great survivors.

But there is a trade-off. For this circuit within us, the descendant of a series of mutations in our most distant ancestors, is also the reason we age.

And yes, that definite singular article is correct: it is the reason.

TO EVERYTHING THERE IS A REASON

If you are taken aback by the notion that there is a singular cause of aging, you are not alone. If you haven’t given any thought at all as to why we age, that’s perfectly normal, too. A lot of biologists haven’t given it much thought, either. Even gerontologists, doctors who specialize in aging, often don’t ask why we age—they simply seek to treat the consequences.

This isn’t a myopia specific to aging. As recently as the late 1960s, for example, the fight against cancer was a fight against its symptoms. There was no unified explanation for why cancer happens, so doctors removed tumors as best they could and spent a lot of time telling patients to get their affairs in order. Cancer was “just the way it goes,” because that’s what we say when we can’t explain something.

Then, in the 1970s, genes that cause cancer when mutated were discovered by the molecular biologists Peter Vogt and Peter Duesberg. These so-called oncogenes shifted the entire paradigm of cancer research. Pharmaceutical developers now had targets to go after: the tumor-inducing proteins encoded by genes, such as BRAF, HER2, and BCR-ABL. By inventing chemicals that specifically block the tumor-promoting proteins, we could finally begin to move away from using radiation and toxic chemotherapeutic agents to attack cancers at their genetic source, while leaving normal cells untouched. We certainly haven’t cured all types of cancer in the decades since then, but we no longer believe it’s impossible to do so.

Indeed, among an increasing number of cancer researchers, optimism abounds. And that hopefulness was at the heart of what was arguably the most memorable part of President Barack Obama’s final State of the Union address in 2016.

“For the loved ones we’ve all lost, for the family we can still save, let’s make America the country that cures cancer once and for all,” Obama said as he stood in the House of Representatives chamber and called for a “cancer moon shot.” When he placed then Vice President Joe Biden—whose son Beau had died of brain cancer a year earlier—in charge of the effort, even some of the Democrats’ staunch political enemies had trouble holding back the tears.

In the days and weeks that followed, many cancer experts noted that it would take far more than the year remaining to the Obama-Biden administration to end cancer. Very few of those experts, however, said it absolutely couldn’t be done. And that’s because, in the span of just a few decades, we had completely changed the way we think about cancer. We no longer submit ourselves to its inevitability as part of the human condition.

One of the most promising breakthroughs in the past decade has been immune checkpoint therapy, or simply “immunotherapy.” Immune T-cells continually patrol our body, looking for rogue cells to identify and kill before they can multiply into a tumor. If it weren’t for T-cells, we’d all develop cancer in our twenties. But rogue cancer cells evolve ways to fool cancer-detecting T-cells so they can go on happily multiplying. The latest and most effective immunotherapies bind to proteins on the cancer cells’ surface. It is the equivalent of taking the invisible cloak off cancer cells so T-cells can recognize and kill them. Although fewer than 10 percent of all cancer patients currently benefit from immunotherapy, that number should increase thanks to the hundreds of trials currently in progress.

We continue to rail against a disease we once accepted as fate, pouring billions of dollars into research each year, and the effort is paying off. Survival rates for once lethal cancers are increasing dramatically. Thanks to a combination of a BRAF inhibitor and immunotherapy, survival of melanoma brain metastases, one of the deadliest types of cancer, has increased by 91 percent since 2011. Between 1991 and 2016, overall deaths from cancer in the United States declined by 27 percent and continue to fall.[19 - J. B. Iorgulescu, M. Harary, C. K. Zogg, et al., “Improved Risk-Adjusted Survival for Melanoma Brain Metastases in the Era of Checkpoint Blockade Immunotherapies: Results from a National Cohort,” Cancer Immunology Research, 6, no. 9 (September 2018): 1039–45, http://cancerimmunolres.aacrjournals.org/content/6/9/1039.long; R. L. Siegel, K. D. Miller, and A. Jemal, “Cancer Statistics, 2019,” CA: A Cancer Journal for Clinicians 69, no. 1 (January–February 2019): 7–34, https://onlinelibrary.wiley.com/doi/full/10.3322/caac.21551.] That’s a victory measured in millions of lives.

Aging research today is at a similar stage as cancer research was in the 1960s. We have a robust understanding of what aging looks like and what it does to us and an emerging agreement about what causes it and what keeps it at bay. From the looks of it, aging is not going to be that hard to treat, far easier than curing cancer.

Up until the second half of the twentieth century, it was generally accepted that organisms grow old and die “for the good of the species”—an idea that dates back to Aristotle, if not further. This idea feels quite intuitive. It is the explanation proffered by most people at parties.[20 - As far back as Aristotle, scientists and philosophers have struggled to resolve the enigma of aging, the authors wrote. D. Fabian and T. Flatt, “The Evolution of Aging,” Nature Education Knowledge 3, no. 10 (2011): 9, https://www.nature.com/scitable/knowledge/library/the-evolution-of-aging-23651151.] But it is dead wrong. We do not die to make way for the next generation.

In the 1950s, the concept of “group selection” in evolution was going out of style, prompting three evolutionary biologists, J. B. S. Haldane, Peter B. Medawar, and George C. Williams, to propose some important ideas about why we age. When it comes to longevity, they agreed, individuals look out for themselves. Driven by their selfish genes, they press on and try to breed for as long and as fast as they can, so long as it doesn’t kill them. (In some cases, however, they press on too much, as my great-grandfather Miklós Vitéz, a Hungarian screenwriter, proved to his bride forty-five years his junior on their wedding night.)

If our genes don’t ever want to die, why don’t we live forever? The trio of biologists argued that we experience aging because the forces of natural selection required to build a robust body may be strong when we are 18 but decline rapidly once we hit 40 because by then we’ve likely replicated our selfish genes in sufficient measure to ensure their survival. Eventually, the forces of natural selection hit zero. The genes get to move on. We don’t.

Medawar, who had a penchant for verbiage, expounded on a nuanced theory called “antagonistic pleiotropy.” Put simply, it says genes that help us reproduce when we are young don’t just become less helpful as we age, they can actually come back to bite us when we are old.

Twenty years later, Thomas Kirkwood at Newcastle University framed the question of why we age in terms of an organism’s available resources. Known as the “Disposable Soma Hypothesis,” it is based on the fact that there are always limited resources available to species—energy, nutrients, water. They therefore evolve to a point that lies somewhere between two very different lifestyles: breed fast and die young, or breed slowly and maintain your soma, or body. Kirkwood reasoned that organisms can’t breed fast and maintain a robust, healthy body—there simply isn’t enough energy to do both. Stated another way, in the history of life, any line of creature with a mutation that caused it to live fast and attempt to die old soon ran out of resources and was thus deleted from the gene pool.

Kirkwood’s theory is best illustrated by fictitious but potentially real-life examples. Imagine you are a small rodent that is likely to be picked off by a bird of prey. Because of this, you’ll need to pass down your genetic material quickly, as did your parents and their parents before them. Gene combinations that would have provided a longer-lasting body were not enriched in your species because your ancestors likely didn’t escape predation for long (and you won’t, either).

Now consider instead that you are a bird of prey at the top of the food chain. Because of this, your genes—well, actually, your ancestors’ genes—benefited from building a robust, longer-lasting body that could breed for decades. But in return, they could afford to raise only a couple of fledglings a year.

Kirkwood’s hypothesis explains why a mouse lives 3 years while some birds can live to 100.[21 - A bat from Siberia set a world record when it reached 41 years of age. R. Locke, “The Oldest Bat: Longest-Lived Mammals Offer Clues to Better Aging in Humans,” BATS Magazine 24, no. 2 (Summer 2006): 13–14, http://www.batcon.org/resources/media-education/bats-magazine/bat_article/152.] It also quite elegantly explains why the American chameleon lizard, Anolis carolinensis, is evolving a longer lifespan as we speak, having found itself a few decades ago on remote Japanese islands without predators.[22 - Small colonies of lizards on a series of Caribbean islands were likely to explore islands where there weren’t predators, while less adventurous animals survived better when predators were present. O. Lapiedra, T. W. Schoener, M. Leal, et al., “Predator-Driven Natural Selection on Risk-Taking Behavior in Anole Lizards,” Science 360, no. 3692 (June 1, 2018): 1017–20, http://science.sciencemag.org/content/360/6392/1017.]

These theories fit with observations and are generally accepted. Individuals don’t live forever because natural selection doesn’t select for immortality in a world where an existing body plan works perfectly well to pass along a body’s selfish genes. And because all species are resource limited, they have evolved to allocate the available energy either to reproduction or to longevity, but not to both. That was as true for M. superstes as it was and still is for all species that have ever lived on this planet.

All, that is, except one: Homo sapiens.

Having capitalized on its relatively large brain and a thriving civilization to overcome the unfortunate hand that evolution dealt it—weak limbs, sensitivity to cold, poor sense of smell, and eyes that see well only in daylight and in the visible spectrum—this highly unusual species continues to innovate. It has already provided itself with an abundance of food, nutrients, and water while reducing deaths from predation, exposure, infectious diseases, and warfare. These were all once limits to its evolving a longer lifespan. With them removed, a few million years of evolution might double its lifespan, bringing it closer to the lifespans of some other species at the top of their game. But it won’t have to wait that long, nowhere near that. Because this species is diligently working to invent medicines and technologies to give it the robustness of a much longer lived one, literally overcoming what evolution failed to provide.

CRISIS MODE

Wilbur and Orville Wright could never have built a flying machine without a knowledge of airflow and negative pressure and a wind tunnel. Nor could the United States have put men on the moon without an understanding of metallurgy, liquid combustion, computers, and some measure of confidence that the moon is not made of green cheese.[23 - Richard Dawkins eloquently made this point in River Out of Eden, arguing that primitive societies don’t have a place in science, using as an example their belief the moon is an old calabash tossed into the sky. R. Dawkins, River Out of Eden (New York: Basic Books, 1995).]

In the same way, if we are to make real progress in the effort to alleviate the suffering that comes with aging, what is needed is a unified explanation for why we age, not just at the evolutionary level but at the fundamental level.

But explaining aging at a fundamental level is no easy task. It will have to satisfy all known laws of physics and all rules of chemistry and be consistent with centuries of biological observations. It will need to span the least understood world between the size of a molecule and the size of a grain of sand,[24 - See “The Scale of Things” at the end of this book.] and it should explain simultaneously the simplest and the most complex living machines that have ever existed.

It should, therefore, come as no surprise that there has never been a unified theory of aging, at least not one that has held up—though not for lack of trying.

One hypothesis, proposed independently by Peter Medawar and Leo Szilard, was that aging is caused by DNA damage and a resulting loss of genetic information. Unlike Medawar, who was always a biologist, who built a Nobel Prize–winning career in immunology, Szilard had come to study biology in a roundabout way. The Budapest-born polymath and inventor lived a nomadic life with no permanent job or address, preferring to spend his time staying with colleagues who satisfied his mental curiosities about the big questions facing humanity. Early in his career, he was a pioneering nuclear physicist and a founding collaborator on the Manhattan Project, which ushered in the age of atomic warfare. Horrified by the countless lives his work had helped end, he turned his tortured mind toward making life maximally long.[25 - Szilard spent his last years as a fellow of the Salk Institute for Biological Studies in La Jolla, California, as a resident fellow. He lived in a bungalow on the property of the Hotel del Charro and died on May 30, 1964.]

The idea that mutation accumulation causes aging was embraced by scientists and the public alike in the 1950s and 1960s, at a time when the effects of radiation on human DNA were on a lot of people’s minds. But although we know with great certainty that radiation can cause all sorts of problems in our cells, it causes only a subset of the signs and symptoms we observe during aging,[26 - R. Anderson, “Ionizing Radiation and Aging: Rejuvenating an Old Idea,” Aging 1, no. 11 (November 17, 2009): 887–902, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2815743/.] so it cannot serve as a universal theory.

In 1963, the British biologist Leslie Orgel threw his hat into the ring with his “Error Catastrophe Hypothesis,” which postulated that mistakes made during the DNA-copying process lead to mutations in genes, including those needed to make the protein machinery that copies DNA. The process increasingly disrupts those same processes, multiplying upon themselves until a person’s genome has been incorrectly copied into oblivion.[27 - L. E. Orgel, “The Maintenance of the Accuracy of Protein Synthesis and Its Relevance to Ageing,” Proceedings of the National Academy of Sciences of the United States of America 49, no. 4 (April 1963): 517–21, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC299893/.]

Around the same time that Szilard was focusing on radiation, Denham Harman, a chemist at Shell Oil, was also thinking atomically, albeit in a different way. After taking time off to finish medical school at Stanford University, he came up with the “Free Radical Theory of Aging,” which blames aging on unpaired electrons that whiz around within cells, damaging DNA through oxidation, especially in mitochondria, because that is where most free radicals are generated.[28 - Harman concluded that the diseases related to aging, as well as aging itself, stem fundamentally from “the deleterious side attacks of free-radicals on cell constituents and on the connective tissues.” The source of the free radicals, he continued, was “molecular oxygen catalyzed in the cell by the oxidative enzymes” and metal traces. D. Harman, “Aging: A Theory Based on Free Radical and Radiation Chemistry,” Journal of Gerontology 11, no. 3 (July 1, 1956): 298–300, https://academic.oup.com/geronj/article-abstract/11/3/298/616585?redirectedFrom=fulltext.] Harman spent the better part of his life testing the theory.

I had the pleasure of meeting the Harman family in 2013. His wife told me that Professor Harman had been taking high doses of alphalipoic acid for most of his life to quench free radicals. Considering that he worked tirelessly on his research well into his 90s, I suppose, at the very least, it didn’t hurt.

Through the 1970s and 1980s, Harman and hundreds of other researchers tested whether antioxidants would extend the lifespan of animals. The results overall were disappointing. Although Harman had some success increasing the average lifespan of rodents, such as with the food additive butylated hydroxytoluene, none showed an increase in maximum lifespan. In other words, a cohort of study animals might live a few weeks longer, on average, but none of the animals was setting records for individual longevity. Science has since demonstrated that the positive health effects attainable from an antioxidant-rich diet are more likely caused by stimulating the body’s natural defenses against aging, including boosting the production of the body’s enzymes that eliminate free radicals, not as a result of the antioxidant activity itself.

If old habits die hard, the free-radical idea is heroin. The theory was overturned by scientists within the cloisters of my field more than a decade ago, yet it is still widely perpetuated by purveyors of pills and drinks, who fuel a $3 billion global industry.[29 - Nutraceuticals World predicts that a rising appetite for synthetic antioxidants at the same time as a fall in costs, combined with increasing demand for them by food and beverage companies, will power market growth for the next few years. “Global Antioxidants Market Expected to Reach $4.5 Billion by 2022,” Nutraceuticals World, January 26, 2017, https://www.nutraceuticalsworld.com/contents/view_breaking-news/2017-01-26/global-antioxidants-market-expected-to-reach-45-billion-by-2022] With all that advertising, it is not surprising that more than 60 percent of US consumers still look for foods and beverages that are good sources of antioxidants.[30 - The sharp growth in demand for drinks with a health benefit, a beverage industry website finds, goes hand in hand with consumers wanting ingredients they value. A. Del Buono, “Consumers’ Understanding of Antioxidants Grows,” Beverage Industry, January 16, 2018, https://www.bevindustry.com/articles/90832-consumers-understanding-of-antioxidants-grows?v=preview.]

Free radicals do cause mutations. Of course they do. You can find mutations in abundance, particularly in cells that are exposed to the outside world[31 - I. Martincorena, J. C. Fowler, A. Wabik, et al., “Somatic Mutant Clones Colonize the Human Esophagus with Age,” Science 362, no. 6417 (November 23, 2018): 911–17, https://www.ncbi.nlm.nih.gov/pubmed/30337457.] and in the mitochondrial genomes of old individuals. Mitochondrial decline is certainly a hallmark of aging and can lead to organ dysfunction. But mutations alone, especially mutations in the nuclear genome, conflict with an ever-increasing amount of evidence to the contrary.

Arlan Richardson and Holly Van Remmen spent about a decade at the University of Texas at San Antonio testing if increasing free-radical damage or mutations in mice led to aging; it didn’t.[32 - The authors concluded that their data “calls into serious question the hypothesis that alterations in oxidative damage/stress play a role in the longevity of mice.” V. I. Pérez, A. Bokov, H. Van Remmen, et al., “Is the Oxidative Stress Theory of Aging Dead?,” Biochimica et Biophysica Acta 1790, no. 10 (October 2009): 1005–14, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2789432/.] In my lab and others, it has proven surprisingly simple to restore the function of mitochondria in old mice, indicating that a large part of aging is not due to mutations in mitochondrial DNA, either, at least not until late in life.[33 - A. P. Gomes, N. L. Price, A. J. Ling, et al., “Declining NAD(+) Induces a Pseudohypoxic State Disrupting Nuclear-Mitochondrial Communication During Aging,” Cell 155, no. 7 (December 19, 2013): 1624–38, https://www.ncbi.nlm.nih.gov/pubmed/24360282.]

Although the discussion about the role of nuclear DNA mutations in aging continues, there is one fact that contradicts all these theories, one that is difficult to refute.

Ironically, it was Szilard, in 1960, who initiated the demise of his own theory by figuring out how to clone a human cell.[34 - W. Lanouette and B. Silard, Genius in the Shadows: A Biography of Leo Szilard: The Man Behind the Bomb (New York: Skyhorse Publishing, 1992).] Cloning gives us the answer as to whether or not mutations cause aging. If old cells had indeed lost crucial genetic information and this was the cause of aging, we shouldn’t be able to clone new animals from older individuals. Clones would be born old.

It’s a misconception that cloned animals age prematurely. It has been widely perpetuated in the media and even the National Institutes of Health website says so.[35 - According to the NIH fact sheet, “clones created from a cell taken from an adult might have chromosomes that are already shorter than normal, which may condemn the clones’ cells to a shorter life span.” “Cloning,” National Human Genome Research Institute, March 21, 2017, https://www.genome.gov/25020028/cloning-fact-sheet/.] Yes, it’s true that Dolly, the first cloned sheep, created by Keith Campbell and Ian Wilmut at the Roslin Institute at the University of Edinburgh, lived only half a normal lifespan and died of a progressive lung disease. But extensive analysis of her remains showed no sign of premature aging.[36 - In the debates over Dolly the cloned sheep, the question that has proved to be challenging to answer is how old an animal is at birth when cloned from an adult’s cell. The answer an author on the site The Conversation found was that other clones born from the same cell as Dolly lived normal lifespans. “The new Dollies are now telling us that if we take a cell from an animal of any age, and we introduce its nucleus into a nonfertilized mature egg, we can have an individual born with its lifespan fully restored.” J. Cibell, “More Lessons from Dolly the Sheep: Is a Clone Really Born at Age Zero?,” The Conversation, February 17, 2017, https://theconversation.com/more-lessons-from-dolly-the-sheep-is-a-clone-really-born-at-age-zero-73031.] Meanwhile, the list of animal species that have been cloned and proven to live a normal, healthy lifespan now includes goats, sheep, mice, and cows.[37 - Though some cloned animals match their species’ rates of normal aging, it’s a field that still needs further analysis to get beyond the largely anecdotal evidence so far collected. J. P. Burgstaller and G. Brem, “Aging of Cloned Animals: A Mini-Review,” Gerontology 63, no. 5 (August 2017): 417–25, https://www.karger.com/Article/FullText/452444.]

Because of the fact that nuclear transfer works in cloning, we can say with a high degree of confidence that aging isn’t caused by mutations in nuclear DNA. Sure, it’s possible that some cells in the body don’t mutate and those are the ones that end up making successful clones, but that seems highly unlikely. The simplest explanation is that old animals retain all the requisite genetic information to generate an entirely new, healthy animal and that mutations are not the primary cause of aging.[38 - University of Bath researchers found in cloned mice that the telomeres protecting the ends of chromosomes were, surprisingly, slightly longer in successive generations and demonstrated no evidence of premature aging. T. Wakayama, Y. Shinkai, K. L. K. Tamashiro, et al., “Ageing: Cloning of Mice to Six Generations,” Nature 407 (September 21, 2000): 318–19. “Despite the length of telomeres reported in different studies, most clones appear to be aging normally. In fact, the first cattle clones ever produced are alive, healthy, and are 10 years old as of January 2008”; “Myths About Cloning,” U.S. Food & Drug Administration, August 29, 2018, https://www.fda.gov/animalveterinary/safetyhealth/animalcloning/ucm055512.htm.]

It’s certainly no dishonor to those brilliant researchers that their theories haven’t withstood the test of time. That’s what happens to most science, and perhaps all of it eventually. In The Structure of Scientific Revolutions, Thomas Kuhn noted that scientific discovery is never complete; it goes through predictable stages of evolution. When a theory succeeds at explaining previously unexplainable observations about the world, it becomes a tool that scientists can use to discover even more.

Inevitably, however, new discoveries lead to new questions that are not entirely answerable by the theory, and those questions beget more questions. Soon the model enters crisis mode and begins to drift as scientists seek to adjust it, as little as possible, to account for that which it cannot explain.

Crisis mode is always a fascinating time in science but one that is not for the faint of heart, as doubts about the views of previous generations continue to grow against the old guard’s protestations. But the chaos is ultimately replaced by a paradigm shift, one in which a new consensus model emerges that can explain more than the previous model.

That’s what happened about a decade ago, as the ideas of leading scientists in the aging field began to coalesce around a new model—one that suggested that the reason so many brilliant people had struggled to identify a single cause of aging was that there wasn’t one.

In this more nuanced view, aging and the diseases that come with it are the result of multiple “hallmarks” of aging:

● Genomic instability caused by DNA damage