You may not have the universe-warping abilities of a Time Lord, but you certainly can regenerate like one.
Doctor Who’s Time Lords can die like anyone else. They can succumb to old age, disease, poison, injury, or severe fatigue. Like no human, however, Time Lords have a way out—twelve ways out to be exact. When death looms large (or sometimes by choice) Time Lords can undergo a total regeneration that essentially gives them a new life. Triple helix DNA twists and contorts cell by cell to produce a Peter Capaldi from a Matt Smith. It takes mere moments. But after every ten years or so, your whole body does the same thing.
Let’s start small.
Rebuilding the Basics
Scratch your arm and you’ll see regeneration on a small scale. The tiny wound gets inflamed, blood rushes to it (as does pain), and eventually a scab forms. Layers of cells then begin to knit the microscopic chasms closed and in a few days the skin looks like new. Cuts heal, broken bones reset, and hair grows back. You regenerate all the time.
But humans have got nothing on some of our animal kin.
Regenerating isn’t always about wounds. Think of a male deer’s antlers. Over the course of a year a male deer goes from unadorned to wearing a crown of horns. Regrowing all that material is a huge regenerative feat. Some deer can regrow up to 60 pounds (27 kilograms) of antlers in as little as three months.
Sea stars up the ante. Famously, a sea star can regrow a limb if lost to a predator, but how many limbs? Well, as long as the central ring of nerves at the center of the star is left intact, even if the poor creature is down to just one arm, the sea star can make itself whole again.
However, the smiling axolotl (pictured above) is hard to beat. This little salamander, which is perpetually in a larval state (hence the adorable gills), can come back from almost any beating it takes. Over the course of laboratory testing, we’ve learned that axolotls can recover muscle function after being paralyzed. They can regrow their limbs, eyes, tails, and parts of the heart. You can induce a stroke in an axolotl and part of its brain will grow back.
For full body regeneration, though, planarian worms are the masters. Planarians are flatworms—simple animals that if cut in half have the odd ability to regenerate into two new flatworms. Some planarians can regenerate a new body from a single cell. If we could only unlock such potential in people it would change human life. The genes are there, but we don’t know how to make a human like a flatworm.
Your Inner Salamander
Regeneration on the scale of new limbs or body parts was first documented in 1740 when Abraham Trembley discovered that a microscopic pond monster could regrow a severed head. He dubbed it a “hydra” (pictured above) after the Greek myth. Trembley’s discovery changed the way a lot of scientists at the time thought about the regenerative capacity of life. But in the intervening centuries, we aren’t that much closer to understanding how to unlock such power in people.
We aren’t even close to uncovering the secrets to human regeneration for two big reasons. First, in a fascinating column for the BBC, science journalist Ed Yong notes that while there are a number of teams all over the world researching regeneration, they are disparate groups researching a gigantic question without much communication between them. Second, animals that are easy to work with in the lab typically don’t have the regenerative abilities that the Doctors of the animal world do. “Salamanders are the ideal choice, since they regenerate very well and have limbs with the same basic structure as ours. But they make for poor laboratory subjects,” Yong writes. It can take months to years for an axolotl to regenerate a lost limb.
Of course, just because we don’t understand how to unlock our inner salamander doesn’t mean that the human body isn’t already adept at regenerating. In fact, you are having a Doctor Who-like regeneration as you read this.
Small regenerations build up the whole; they are what keep you alive. And the body’s normal cellular turnover—replacing dead, dying, or malfunctioning cells—is our main form of regeneration. Because of the harsh acids in your stomach, for example, the outermost cells lining your gut are recycled every five days. The surface layer of your skin, called the epidermis, is replaced every two weeks. Journalist Nicholas Wade describes the lifespan of our blood cells in The New York Times this way: “The red blood cells, bruised and battered after traveling nearly 1,000 miles through the maze of the body’s circulatory system, last only 120 days or so on average before being dispatched to their graveyard in the spleen.”
Different cells have different lifespans. While the lining of the stomach may last less than a week, the cells in your rib bones may linger for 15 years. But when using ageing techniques to look at the body’s different tissues, one thing is clear: Whoever “you” are, whatever your consciousness is, you are much older than your body. If you average out all the lifespans of the trillions of cells that make you up, you get about 7-10 years.
Think about that astounding fact for a moment. Before you are a teenager, you have a new body with cells you weren’t born with. By the time you become an octogenarian, you may have had a dozen new bodies. Going through life as one stream of consciousness with maybe a dozen new but recognizable bodies sounds vaguely familiar, doesn’t it?
You are regenerating right now, just like any Time Lord in Doctor Who, only slower.
If our bodies are eventually replaced, piece-by-piece, why can’t we live a Time Lord’s lifespan? Well, if you knew the answer to that, you’d win every prize the scientific establishment has to offer. It’s not clear what puts a limit on our regenerations, but we do have some clues. We suspect that over time our DNA accumulates dangerous mutations that prevent unhindered regrowth. Or it could be that the powerhouses which live inside each one of our cells, our mitochondria, don’t have the adequate repair mechanisms to keep us going indefinitely. Or maybe the stem cells that act as our bodies’ handymen get too “feeble” as we age. Maybe you just can’t beat entropy.
But the most important part of our regenerations is that not everything is replaced. Even though the Doctor changes appearance and personality whenever he regenerates, there is still a through-line that makes each iteration a true Doctor Who. The one thing you don’t want to fully regenerate is the brain.
As far as we know, certain parts of the body are not replaced over your many cycles. Muscle cells of the heart are one part; the cells of the cerebral cortex are another. If every bit of your brain was replaced, memories would fade and shatter, learned skills would become insurmountable efforts, and language might even leave you. No, you remain who you are precisely because of what doesn’t regenerate. Your personality may change, as may your appearance, but even with as many cycles as a Time Lord you are still, through and through, a Doctor You.
Kyle Hill is the Chief Science Officer of the Nerdist enterprise. Follow the continued nerdery on Twitter @Sci_Phile.