Why Do We Age and Die? The Biology of Aging

Everyone ages, and nearly every animal does too. But why? If evolution favors survival, why hasn’t it eliminated aging? Biologists have been working on this puzzle for more than 70 years. This article explains the main answers and what is still unknown.

What is aging, scientifically?

A decline in performance and fitness with advancing age. This is how the Annual Review of Entomology defines senescence, the scientific word for aging. The review calls it a nearly universal feature of multicellular organisms, and says understanding why it happens is a long-standing problem in biology.

Why doesn’t natural selection remove it?

Because selection loses its force with age. A review in PubMed Central explains that the key idea, from the biologist Peter Medawar, is that natural selection becomes less efficient after an animal reproduces. Genes that act late in life face little pressure from selection, because by then the animal has usually already passed on its genes. This idea is the basis for all the evolutionary theories of aging. A more recent review calls the weak force of selection at later ages the “selection shadow.”

What are the three classic theories?

They are:

• Mutation accumulation (Medawar, 1952). Harmful mutations that only show up late in life are not weeded out, so they build up in a population over time.
• Antagonistic pleiotropy (Williams, 1957). Some genes help early in life but cause harm later. Selection favors them because the early benefit counts more.
• Disposable soma (Kirkwood, 1977). An organism has limited energy, and must divide it between growth, reproduction, and repairing its body. It puts reproduction first, so the body is, in a sense, disposable.

The PubMed Central review notes that the first two were formulated over 50 years ago, before molecular biology, and the third more than 30 years ago.

Do these theories explain everything?

No. An arXiv review concludes that none of the traditional theories gives a full account of the mechanism of aging. A review in the journal Aging adds that the evolutionary theory explains why we age, but not how. It also notes that some researchers doubt that the energy trade-off in the disposable soma theory is the main cause of aging. A review by University College London’s David Gems calls antagonistic pleiotropy a well-supported principle, but says that its biological mechanisms need more study.

How do scientists study the “how”?

By looking at the body’s own processes. The Annual Review of Entomology describes mechanistic theories that focus on stress from reactive molecules (oxidative stress), cell signaling, and diet as ways to change lifespan. The hardest question, it says, is which of these mechanisms actually cause the differences in aging that we see in nature.

What does this mean for living longer?

That the biology is complicated. Researchers are testing drugs and diets, as described in Can Humans Live Forever?. Some animals seem to avoid aging, as explained in Is Any Animal Immortal?. For how long people live now, see How the Life Expectancy of Americans Changed Over the Last Century.