Ageing

Ageing is the biological process of becoming older over time. It refers to the accumulation of changes in humans, many animals, and fungi. In a broader sense, ageing can describe single cells that stop dividing or entire populations of a species. While many living things age and eventually die, some species are considered potentially immortal. For example, the genus Hydra has a regenerative ability that helps it avoid dying of old age.

Bacteria can also be potentially immortal through fission, which is when they split to create new cells. However, evidence shows that some single-celled organisms, such as the bacterium Escherichia coli and yeast, do experience ageing and death.

Scientists use two main theories to explain why ageing happens. The first is the damage concept. This theory suggests that the accumulation of damage, such as DNA oxidation, causes biological systems to fail. The second is the programmed ageing concept. This theory proposes that internal processes, like DNA methylation, inherently cause ageing through a biological timetable. It is important not to confuse programmed ageing with apoptosis, which is programmed cell death.

Researchers study these mechanisms by looking at how genes regulate maintenance, repair, and defense responses in an organism.

Evolution has played a major role in how ageing works. About one billion years ago, the emergence of the fungal and animal kingdoms introduced eukaryotic sexual reproduction. This allowed organisms to pass genetic material to new individuals. This meant the original organism could become disposable to ensure the survival of its species. Later, about 320 million years ago, seed-producing plants evolved. Even in modern medicine, we see the limits of cell life. In 1961, Leonard Hayflick discovered the Hayflick limit. He found that normal human cells in a laboratory culture only divide about 50 times before they stop.

Human ageing involves many distinct physical changes. As people age, they may experience presbyopia, which is difficulty focusing on close objects. This usually happens between ages 45 and 50 because the eye lens hardens. By age 55, hair often turns grey, and pattern hair loss affects 30% to 50% of males. Many people also experience presbycusis, or hearing loss, as they get older. While many vertebrates like birds can regenerate their hearing cells, mammals like humans have lost this ability.

Additionally, the incidence of osteoarthritis rises to 53% in the 60–64 age group, though only 20% report it as disabling.

The human body also undergoes significant structural and internal changes. After age 30, body mass typically decreases until age 70. Muscles may experience sarcopenia, which is the loss of muscle mass and strength. The cardiovascular system also changes through atherosclerosis, a condition that can lead to strokes or heart attacks. The heart may increase in left ventricular mass, which can eventually lead to heart failure. Vision can also be affected by macular degeneration, which impacts nearly 12% of people over age 80. This is caused by changes in the circulation of waste products and abnormal vessel growth near the retina.

Cognitive changes are another major part of the ageing process. While general knowledge and vocabulary often remain steady or increase, other types of intelligence may decline. After age 20, there is a 10% reduction each decade in the total length of the brain's myelinated axons. Some people may develop dementia, which becomes more common in older age groups. For example, nearly half of people over age 85 may live with dementia. This includes neurodegenerative diseases such as Alzheimer's, Parkinson's, and Lou Gehrig's disease.

However, some studies show that older adults may paradoxically report improved moods despite declining physical health.

Ageing is a massive factor in global health and mortality. Of the roughly 150,000 people who die every day worldwide, about two-thirds die from age-related causes. In industrialized nations, this number reaches 90%. The maximum human lifespan is suggested to be around 115 years. The oldest reliably recorded person was Jeanne Calment, who lived to be 122. Scientists continue to use model organisms like the nematode C. elegans to study these patterns. Because C. elegans only lives for two to three weeks, it allows for easier genetic manipulation to understand how to extend lifespans.