Humans have wanted to live forever for as long as we've lived at all. It's an obsession that stretches back so far that it feels like it's somehow hard-coded into our DNA. Over the years, immortality (to a greater or lesser extent) has been promised by everyone from religions and cults to the cosmetics industry, big tech companies and questionable food blogs.

It's also a staple of fiction, all the way back to the earliest surviving great work of literature. The Epic of Gilgamesh, carved onto stone tablets in 2100 BC, depicts its titular king hunting for the secret of eternal life, which he finds in a plant that lives at at the bottom of the sea. He collects the plant by roping stones to his feet, but then a snake steals it while he's having a pre-immortality bath. Gilgamesh has a little cry, then gives up.

A cuneiform tablet containing part of The Epic of Gilgamesh.

The reason why we age is still the subject of major scientific debate, but it basically boils down to damage accumulating in our cells throughout our lives, which eventually kills us. By slowing that damage - first by making tools, then controlling fire, inventing writing, trade, agriculture, logic, the scientific method, the industrial revolution, democracy and so on, we've managed to massively increase human life expectancy.

There's a common misconception that to live forever we need to somehow pause the ageing process. We don't. We just need to increase the rate at which our lifespans are lengthening. Human lifespan has been lengthening at a constant rate of about two years per decade for the last 200 years. If we can speed that up past the rate at which we age then we hit what futurist Aubrey de Grey calls "longevity escape velocity" - the point we become immortal.

There's a common misconception that to live forever we need to somehow pause the ageing process. We don't. We just need to increase the rate at which our lifespans are lengthening.

That all sounds rather easy, and of course it's not quite that simple. It's all we can do at the moment to keep up with the Moore's Law of increasing lifespans. But with a major research effort, coordinated around the world, who knows? Scientific history is filled with fields that ticked along slowly and then suddenly, massively, accelerated. Computer science is one. Genetics is another recent example.

To understand what we need to do to hit longevity escape velocity, it's worth looking at how life expectancy has increased in recent history. The late statistician Hans Rosling made a powerful case that average lifespans rise alongside per capita income. Take a couple of minutes to watch this video and you'll be convinced:

Reducing the gap between the global rich and poor, therefore, is probably the fastest way to boost the world average life expectancy figure, but it's limited. And it won't do much for people in rich countries.

To boost the lifespans of the people living in countries that are already pretty wealthy, we need to look closer at the countries that are forecast to have the highest life expectancies in the coming years. A study published earlier this year in the Lancet shows what life expectancy might look like in 2030 in 35 industrialised countries, using an amalgamation of 21 different forecasting models.

South Korea tops the chart with women living on average beyond 90, while France, Japan, Switzerland and Australia are not far behind. Most of the countries at the top of the chart have high-quality healthcare provision, low infant deaths, and low smoking and road traffic injury rates. Fewer people are overweight or obese. The US, meanwhile, is projected to see only a modest rise - due to a lack of healthcare access, and high rates of obesity, child mortality and homicides.

The study results are interesting, not only because they're the best possible guess at our future but because they clearly show how social policies make a massive difference to how long people live. There are unknowns, of course - no-one could have predicted the 80s AIDS epidemic, for example, and no doubt further pandemics lurk in humanity's future. But ban smoking, fight obesity, and introduce autonomous cars and personalised medicine, and you'll see lifespans rise.

The US is projected to see only a modest rise in lifespan - due to a lack of healthcare access, and high rates of obesity, child mortality and homicides.

The other interesting thing is that the study's results are a shot across the bows of scientists who claim that there are hard limits to human lifespan.

"As recently as the turn of the century, many researchers believed that life expectancy would never surpass 90 years, lead author Majid Ezzati of Imperial College London told the Guardian back in February.

That prediction mirrors another, published in Nature in October 2016, that concluded that the upper limit of human age is stuck at about 115 years.

"By analysing global demographic data, we show that improvements in survival with age tend to decline after age 100, and that the age at death of the worlds oldest person has not increased since the 1990s," wrote the authors - Xiao Dong, Brandon Milholland & Jan Vijg.

"Our results strongly suggest that the maximum lifespan of humans is fixed and subject to natural constraints."

The maximum length of a human lifespan remains up for debate.

Other researchers, however, disagree. Bryan G. Hughes & Siegfried Hekimi wrote in the same journal a few months later that their analysis showed that there are many possible maximum lifespan trajectories.

We just dont know what the age limit might be. In fact, by extending trend lines, we can show that maximum and average lifespans, could continue to increase far into the foreseeable future, Hekimi said.

Three hundred years ago, many people lived only short lives. If we would have told them that one day most humans might live up to 100, they would have said we were crazy.

That's all big-picture stuff, so let's dive down to a more personal level. Assuming that you can't change your genetics or your life up until the point that you're currently at, what can you personally do to live longer?

Here's the list: Don't smoke. Exercise your body and mind on a daily basis. Eat foods rich in whole grains, vegetables, fruits, and unsaturated fat. Don't drink too much alcohol. Get your blood pressure checked. Chop out sources of stress and anxiety in your life. Travel by train. Stay in school. Think positive. Cultivate a strong social group. Don't sit for long periods of time. Make sure you get enough calcium and vitamin D. Keep your weight at a healthy level. And don't go to hospital if you can help it - hospitals are dangerous places.

All of those things have been correlated with increased lifespan in scientific studies. And they're all pretty easy and cheap to do. If you want to maximise your longevity, then that's your to-do list. But there are also strategies that have a little less scientific merit. The ones that people with too much money pursue when they realise they haven't been following any of the above for most of their life.

Inside the Cryonics Institute.

Cryonics is probably the most popular. First proposed in the 1960s by US academic Robert Ettinger in his book "The Prospect of Immortality", it involves freezing the body as soon as possible after death in a tube kept at -196C, along with detailed notes of what they died of. The idea is that when medicine has invented a cure for that ailment, the corpse can be thawed and reanimated.

Calling someone dead is merely medicines way of excusing itself from resuscitation problems it cannot fix today, reads the website of top cryogenics firm Alcor.

The problem is the brain. First, it's so dense and well-protected that it's extremely difficult for the cryonics chemicals to penetrate it. It's almost impossible that it doesn't get damaged in the freezing process.

The 21,000,000,000 neurons and ~1,000,000,000,000,000 synapses in the human brain means that it'll be a while until we have the computational resources to map it.

Secondly, your neurons die quickly - even if you're immersed within minutes of death, you're still likely to suffer substantial brain damage. To which cryonics proponents argue: "What do I have to lose?" If the choice is between probably never waking up again and never waking up again, and it's your money to spend, then why not give it a shot?

An alternative to deep freeze is storing your brain in a computer. Not literally a lump of grey matter, but a database detailing in full all of the connections between the neurons in your brain that make you you (known as your connectome). Future doctors could then either rewire a real or artificial brain to match that data, resurrecting you in a new body (or perhaps even as an artificial intelligence).

A close look at a slice of mouse brain. Credit: Robert Cudmore

So far, we've only managed to map the full connectome of one animal - the roundworm C. elegans. Despite the worm's mere 302 neurons and 7,500 or so synapses, the resulting data is about 12GB in size - you can download it in full at the Open Connectome Project, and even install it in a robot, which will then act like a worm.

Unfortunately the human brain is a somewhat larger undertaking. The Human Connectome Project is making a start, and AI is helping, but the 21,000,000,000 neurons and ~1,000,000,000,000,000 synapses in the human brain means that it'll be a while until we have the computational resources to get it done. It's worth noting that this isn't an unassailable goal, especially if we can somehow figure out which bits are actually important to our personality and who we are as individuals and which bits are just used to remember the lyrics of Spice Girls songs.

For now, though, my recommendation would be to stick to the list of simple life extension strategies above. It's probable that in time we'll have new ways of augmenting our bodies that will extend our lifespans (we've already started with cyborg technology - just look at pacemakers and artificial hips).

But if you want to be at the front of the waiting list then you'll need to arrive at that point with as youthful a body as possible.

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How to live forever - TechRadar