Ageing

Ageing (or aging in American English) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In a broader sense, ageing can refer to single cells within an organism which have ceased dividing, or to the population of a species. In humans, ageing represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes. Reaction time, for example, may slow with age, while memories and general knowledge typically increase. Ageing increases the risk of human diseases such as cancer, Alzheimer's disease, diabetes, cardiovascular disease, stroke and many more. Of the roughly 150,000 people who die each day across the globe, about two-thirds die from age-related causes. Current ageing theories are assigned to the damage concept, whereby the accumulation of damage (such as DNA oxidation) may cause biological systems to fail, or to the programmed ageing concept, whereby the internal processes (epigenetic maintenance such as DNA methylation) inherently may cause ageing. Programmed ageing should not be confused with programmed cell death (apoptosis). Obesity has been proposed to accelerate ageing, whereas dietary calorie restriction in non-primate animals slows ageing while maintaining good health and body functions. In primates (including humans), such life-extending effects remain uncertain. Human beings and members of other species, especially animals, age and die. Fungi, too, can age. In contrast, many species can be considered potentially immortal: for example, bacteria fission to produce daughter cells, strawberry plants grow runners to produce clones of themselves, and animals in the genus Hydra have a regenerative ability by which they avoid dying of old age. Early life forms on Earth, starting at least 3.7 billion years ago, were single-celled organisms.