On February 21, 1986, Shigechiyo Izumi of Japan died at the remarkable age of 120 years and 237 days. Born in 1865, the year in which Abraham Lincoln died, he was the oldest human being on record up to that point.

 

Humans are the longest-lived of all mammals. Although an Indian elephant in captivity reached the age of 78, this is rare. A lion of 30, for example, is considered very old. A blue whale, the largest animal on Earth, is unlikely to live much beyond 40. Small, fast-moving animals are especially short-lived. Mice and shrews, for instance, rarely live more than two years.

 

Among reptile, the giant Galapagos tortoise is the old-age record-holder (see Figure 1). It has a life span as long as 150 years. But the oldest animals yet found live not on land, but under the sea. They are a type of ocean clam, called quahogs, which may live to be 200. Compare this with the mayfly, which usually dies on the same day that it becomes an adult.

 

No animal, however, can compare in age to the longest lived plants. Scientists estimate the age of some giant sequoia trees in California to be between 3,000 and 4,000 years old. One bristlecone pine tree, growing high on a mountain in Nevada, may have been alive for 4,600 years. This ancient tree may have been a tiny seedling at the time the great pyramids were being built in Egypt.

 

Limits to Life

As with every creature, human beings have a maximum life span. This is about 100 years, though a small proportion of people do live to be more than a century old. It seems likely that the maximum human life span has not changed much in thousands of years. What has changed greatly is the average life span (see Figure 2).

 

In stone age times, people had little protection from wild animals. They faced the constant threat of starving when they could not find food or freezing to death from bitter cold. If disease struck them, they could do nothing about it. As a result, more than 10,000 years ago, human beings probably had an average life span of 20 years or less. Few people lived to be 30.

 

Gradually, this situation improved. Humans built towns and cities in which they could live in great safety. They grew crops and reared farm animals so that they had a more reliable source of food. Advances in medicine and cleanliness, or hygiene, were slower to come. But, in time, doctors learned how to treat successfully more and more kinds of illnesses. They realized, too, that diseases which could kill millions were often spread by poor hygiene.

 

Sadly, even today in many parts of the world, these life-threatening problems remain. Thousands of children die every day in countries where there is a shortage of food, doctors, and vital medicines. Along with other problems, such reduce shortages greatly reduce the average life span in many nations of Africa, Asia, and South America. In Bangladesh, for instance, the average life expectancy is only 45 years.

 

Even with plenty of food and proper hygiene, there is still a limit to how long people can live. We cannot live longer because as we grow older, the powers of our bodies and minds gradually decline. Every human being, as he or she ages, undergoes the same basic changes.

 

Signs of Aging

Some of the most obvious signs of aging are not really important to a person's well-being (see Figure 3). Hair color, for instance, changes. The hair of a young person contains a colored substance called melanin. There is a dark brown melanin and a reddish melanin. Depending on how much of these two is in a person's hair, its color may be any shade from blond to black.

 

Later in life, no new melanin is added to the growing hairs. When this happens, they turn white or gray. Some people – mean in particular – gradually lose some or all of their hair.

 

Our teeth, too, develop problems as we grow older. If they are nor properly cleaned, or we eat too much sugar, large numbers of germs – tiny living particles – breed in the mouth. These then produce chemicals that attack the teeth and cause gum disease. Also, since an adult's teeth do not continue to grow, they slowly wear down. To humans this is not a serious problem. But when it happens to wild animals, they often starve because they can no longer hunt or chew effectively.

 

Another obvious change in old age is that our skin becomes less elastic. When we are young, fibers of a substance called collagen help to keep our skin tightly drawn and smooth. But as time goes on, these collagen fibers become tangled and frayed. As a result, our skin wrinkles.

 

From the middle twenties to the fifties, there is a slight drop in the sharpness of human vision. People often become far-sighted during this time. That means they can see distant objects more clearly than near ones. In old age, diseases such as cataracts and glaucoma are more common. A cataract affects the lens of the eye – the part that brings objects into focus – causing it to become cloudy. Glaucoma happens when pressure builds up in the jelly-like liquid inside the eyeball. This damages the main nerve connecting the eye to the brain and may cause blindness.

 

A slight loss of hearing also tends to accompany old age. This mainly affects the ability of people more than 50 to hear high-pitched sounds. After age 70, the tongue loses some of its taste buds so that our sense of taste declines a little. Our reflexes, or ability to react quickly, also slow down as we grow older.

 

Yet none of these outward signs of aging threaten a person's life. It is the gradual changes that take place deep inside our bodies that are more serious.

 

Vital Changes

Many people die in old age because of problems with their heart or blood circulation. The heart is a pump consisting mostly of muscles that work automatically. As we grow older, these muscles weaken. Between the ages of 25 and 65, the amount of blood pumped by the heart decreases by about 13 percent. By itself, this drop in heart muscle-power is not very important. The problem is that the condition of our blood vessels gets worse at the same time.

 

The large blood vessels that carry blood away from the heart are called arteries. In most young people the arteries are clean and wide open. This makes it easy for the heart to pump blood through them. But as we grow older, a fatty, porridge-like substance may collect on the inside walls of the arteries. As the opening of the artery narrows, the heart has to work much harder to push through the same amount of blood. Yet, the heart becomes weaker with age. At some point this may mean that too little blood is able to reach some vital part of the person's body, such as the brain. Then unless help comes quickly, the person is likely to die.

 

Bones also suffer from aging. The most important building-block of bones is calcium. But as people get older, bones lose some of their calcium, making them easier to break. In women, especially, osteoporosis can be a major problem. This is a condition in which hollow spaces develop inside the bones. It makes the bones weak and brittle. For a person with osteoporosis, even a minor fall can result in broken bones.

 

Finally, old age weakens the most important part of the human body, the brain. Every 10 years after the age of 20, about 10 percent of the brain cells controlling higher thought shrivel up and die. This is not as serious as it sounds because the brain has many spare cells that can take over from those that are lost. Still, the loss of brain cells does cause gradually weakening of our memory and power of concentration as we grow older.

 

Someday, it may be possible to slow or halt all of the changes that affect us as we age. However, this will not help us live much longer as long as there are many diseases from which we can die early. It is these more immediate threats to a long and healthy life that need to be dealt with first.

 

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