© Anthony Gerace

When Walker began his scientific career, he focused on the female reproductive system as a model of “pure ageing”: a woman’s ovaries, even in the absence of any disease, slowly but inevitably slide into the throes of menopause. His studies investigated how food, light, hormones and brain chemicals influence fertility in rats. But academic science is slow. He hadn’t cured ageing by his 40th birthday, nor by his 50th or 60th. His life’s work was tangential, at best, to answering the question of why we’re mortal, and he wasn’t happy about it. He was running out of time.

So he went back to the drawing board. As he describes in his book, Why We Age, Walker began a series of thought experiments to reflect on what was known and not known about ageing.

Ageing is usually defined as the slow accumulation of damage in our cells, organs and tissues, ultimately causing the physical transformations that we all recognise in elderly people. Jaws shrink and gums recede. Skin slacks. Bones brittle, cartilage thins and joints swell. Arteries stiffen and clog. Hair greys. Vision dims. Memory fades. The notion that ageing is a natural, inevitable part of life is so fixed in our culture that we rarely question it. But biologists have been questioning it for a long time.

It’s a harsh world out there, and even young cells are vulnerable. It’s like buying a new car: the engine runs perfectly but is still at risk of getting smashed on the highway. Our young cells survive only because they have a slew of trusty mechanics on call. Take DNA, which provides the all-important instructions for making proteins. Every time a cell divides, it makes a near-perfect copy of its three-billion-letter code. Copying mistakes happen frequently along the way, but we have specialised repair enzymes to fix them, like an automatic spellcheck. Proteins, too, are ever vulnerable. If it gets too hot, they twist into deviant shapes that keep them from working. But here again, we have a fixer: so-called ‘heat shock proteins’ that rush to the aid of their misfolded brethren. Our bodies are also regularly exposed to environmental poisons, such as the reactive and unstable ‘free radical’ molecules that come from the oxidisation of the air we breathe. Happily, our tissues are stocked with antioxidants and vitamins that neutralise this chemical damage. Time and time again, our cellular mechanics come to the rescue.

Which leads to the biologists’ longstanding conundrum: if our bodies are so well tuned, why, then, does everything eventually go to hell?

One theory is that it all boils down to the pressures of evolution. Humans reproduce early in life, well before ageing rears its ugly head. All of the repair mechanisms that are important in youth – the DNA editors, the heat shock proteins, the antioxidants – help the young survive until reproduction, and are therefore passed down to future generations. But problems that show up after we’re done reproducing cannot be weeded out by evolution. Hence, ageing.

Most scientists say that ageing is not caused by any one culprit but by the breakdown of many systems at once. Our sturdy DNA mechanics become less effective with age, meaning that our genetic code sees a gradual increase in mutations. Telomeres, the sequences of DNA that act as protective caps on the ends of our chromosomes, get shorter every year. Epigenetic messages, which help turn genes on and off, get corrupted with time. Heat shock proteins run down, leading to tangled protein clumps that muck up the smooth workings of a cell. Faced with all of this damage, our cells try to adjust by changing the way they metabolise nutrients and store energy. To ward off cancer, they even know how to shut themselves down. But eventually cells stop dividing and stop communicating with each other, triggering the decline we see from the outside.

Scientists trying to slow the ageing process tend to focus on one of these interconnected pathways at a time. Some researchers have shown, for example, that mice on restricted-calorie diets live longer than normal. Other labs have reported that giving mice rapamycin, a drug that targets an important cell-growth pathway, boosts their lifespan. Still other groups are investigating substances that restore telomeres, DNA repair enzymes and heat shock proteins.

During his thought experiments, Walker wondered whether all of these scientists were fixating on the wrong thing. What if all of these various types of cellular damage were the consequences of ageing, but not the root cause of it? He came up with an alternative theory: that ageing is the unavoidable fallout of our development.

The idea sat on the back burner of Walker’s mind until the evening of 23 October 2005. He was working in his home office when his wife called out to him to join her in the family room. She knew he would want to see what was on TV: an episode of Dateline about a young girl who seemed to be “frozen in time”. Walker watched the show and couldn’t believe what he was seeing. Brooke Greenberg was 12 years old, but just 13 pounds, and 27 inches long. Her doctors had never seen anything like her condition, and suspected the cause was a random genetic mutation. “She literally is the Fountain of Youth,” her father, Howard Greenberg, said.

Walker was immediately intrigued. He had heard of other genetic diseases, such as progeria and Werner syndrome, which cause premature ageing in children and adults respectively. But this girl seemed to be different. She had a genetic disease that stopped her development and with it, Walker suspected, the ageing process. Brooke Greenberg, in other words, could help him test his theory.