Ageing. Ageing or aging 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 broader senses, 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 may cause biological systems to fail, or to the programmed ageing concept, whereby the internal processes inherently may cause ageing. Programmed ageing should not be confused with programmed cell death. 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, 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. Such organisms multiply by fission into daughter cells; thus do not age and are potentially immortal under favorable conditions. Ageing and mortality of the individual organism became possible with the evolution of sexual reproduction, which occurred with the emergence of the fungal/animal kingdoms approximately a billion years ago, and the evolution of seed-producing plants 320 million years ago. The sexual organism could henceforth pass on some of its genetic material to produce new individuals and could itself become disposable with respect to the survival of its species. This classic biological idea has however been perturbed recently by the discovery that the bacterium E. coli may split into distinguishable daughter cells, which opens the theoretical possibility of age classes among bacteria. Even within humans and other mortal species, there are cells with the potential for immortality: cancer cells which have lost the ability to die when maintained in a cell culture such as the HeLa cell line, and specific stem cells such as germ cells. In artificial cloning, adult cells can be rejuvenated to embryonic status and then used to grow a new tissue or animal without ageing. Normal human cells however die after about 50 cell divisions in laboratory culture. A number of characteristic ageing symptoms are experienced by a majority or by a significant proportion of humans during their lifetimes. Teenagers lose the young child's ability to hear high-frequency sounds above 20 kHz. Wrinkles develop mainly due to photoageing, particularly affecting sun-exposed areas. After peaking in the mid-20s, female fertility declines. After age 30 the mass of human body is decreased until 70 years and then shows damping oscillations. People over 35 years of age are at increasing risk for losing strength in the ciliary muscle of the eyes which leads to difficulty focusing on close objects, or presbyopia. Most people experience presbyopia by age 45-50. The cause is lens hardening by decreasing levels of alpha-crystallin, a process which may be sped up by higher temperatures. Around age 50, hair turns grey. Pattern hair loss by the age of 50 affects about 30-50% of males and a quarter of females. Menopause typically occurs between 44 and 58 years of age. In the 60-64 age cohort, the incidence of osteoarthritis rises to 53%. Only 20% however report disabling osteoarthritis at this age. Almost half of people older than 75 have hearing loss inhibiting spoken communication. Many vertebrates such as fish, birds and amphibians do not develop presbycusis in old age as they are able to regenerate their cochlear sensory cells, whereas mammals including humans have genetically lost this ability.