(ABC NEWS) -- Scientists may be able to make substantial gains in extending not only the length of human life, but the quality of life as we age, according to many researchers. That won't be limited to breakthroughs in the laboratory. To a significant extent, it will depend on how we live our lives.
As for the scientists, first they have to answer a very basic question. Why do humans live longer than any other mammals?
For starters, we are big. Long ago scientists recognized a relationship between body size and longevity. Humans just narrowly edge out the elephant (so size isn't the whole story) to win the Olympic gold for living longer, but recent research reveals that's just part of the story.
We also have huge brains compared to the size of our bodies. We are mobile, have few predators except for other humans, and there's a drugstore on every corner.
It wasn't always that way. During most of recorded history any human who reached the mid thirties had beaten the system. Over the past century we gained a global average of 30 years, about 25 of which are attributed to improvements in public health, according to federal statistics.
Today, the global life expectancy is 67.2 years. It's around 78 years in the United States, and a few years more in Japan, the world leader for sticking around.
Genetics, of course, play a key role in longevity. In recent years, when we entered the golden age of genetics, many hoped to discover the "longevity gene" that allowed an increasing number of humans to live more than a century. For awhile, they thought they had found it.
One gene produces sirtuins, a protein thought to increase lifespan in several organisms, and that protein quickly became the darling of producers of anti-aging creams. But last year an international team of researchers found that sirtuins have no effect on animal longevity.
That came as no surprise to scientists at the University of California, San Francisco, who had determined that there is no longevity gene. As has often been the case in genetics in recent years, it's much more complicated than that.
It turns out that there are many genes that affect lifespan, but each of those genes has a very limited role. The San Francisco researchers found that some genes make proteins that fight bacterial infections, while others ward off oxidative stress and protein damage, commonly associated with aging. But all these genes don't just do their own thing. They are apparently controlled by at least two other genes that act as drill sergeants. Research by these scientists found that when all these genes work right, the lifespan of the roundworm, C. elegans, doubled. That worm is used in much research because it is a simple organism that shares many genes with humans.
But will the same thing work for humans? Maybe.
In a related study, scientists at the University of Liverpool reported earlier this year that some proteins change over time in long living species, including humans. Joao Pedro Magalhaes and his colleagues studied 30 mammals and found that these proteins evolve during the course of the lifetime "to cope with biological processes impacted by aging, such as DNA damage." In other words, animals that live longer are better equipped to make repairs in tissues and organs that help them fight the aging process.
There is a huge body of evidence showing that size really does matter, both in terms of body mass and cerebral tissue. Researchers in Barcelona studied 493 mammal species and found that a larger brain leads to a longer life.
A smarter animal is better equipped to deal with environmental challenges and less likely to take silly chances, like picking a fight with a much bigger animal. That may seem obvious, but it's less clear why body size should contribute to longer lifespan. Among mammals, the top four are humans, followed by elephants, horses and hippopotamuses, but most likely the hippo wouldn't score all that high on an IQ test.
The turkey buzzard tops the list for birds at 118 years, maybe because it's smart enough to wait for road kill instead of attacking a live animal.
But the giant tortoise is the real champ. The world mourned the passing of Lonesome George in the Galapagos Islands earlier this year. The actual age of old George is unknown, although it's clear he made it well past the century mark. Among the superachievers was Tu'I Malila, who was presented to the royal family of Tonga by Capt. James Cook in 1777. He was thought to be 188 when he died in 1965. That still leaves the question of why size matters. Adrian Bejan, a mechanical engineering professor at Duke University, has spent years studying the relationship between size and lifespan, and he is out with a new idea.
Bejan argues in a paper published this week in Nature Scientific Reports that big animals live longer because they travel farther, thus giving them access to more resources. Mobility is the key. Get off the couch.
If he's right, then that leaves longevity largely in our own hands. Do the right thing and you'll live longer. Physicians tell us that all the time. Don't smoke. Get plenty of exercise. Eat right. Researchers at Newcastle University in England think they have figured out why something like eating a low calorie diet can increase lifespan. Aging is strongly influenced by senescence, the end of a cell's ability to replicate itself. They fed mice a low calorie diet and the accumulation of senescent cells plummeted, thus defeating much of the aging process.
It worked even for older mice, suggesting that eating less – or at least fewer calories – may be our best defense against aging and an early death.
No more ice cream? I'm waiting for a magic pill.