Lifetimes of organisms

A definition of life is problematic. As with time, everyone thinks they know what it means, but an exact definition is impossible. According to some perspectives, life is a process that involves organized units called cells that can absorb energy and nutrients from their environment and reproduce. Another perspective is that life is any relatively persistent pattern of particles that has a finite existence, with the ability to create replicas of itself before that pattern disappears.26 But where then is the dividing line between life and inanimate life? Could we say that stars were alive? They are complex patterns of atoms that, like our Sun, were created when other stars died in supernova explosions. Some scientists have considered viruses to be life in that they reproduce. But others disagree, noting that viruses need host organisms in order to do that. Viruses therefore may be classified as a form of endophysical life. Other forms of life do not have this limitation and can be classified as exophysical life: they can reproduce autonomously. Of course, resources are required in all cases, so this classification may be unhelpful.

An interesting question is: how long do lifeforms persist in time? Is there any limit? If an organism never died, the proverbial immortal being, then there would be no need for reproduction. Time itself would have little meaning to such a creature. There are various science fiction stories touching on this theme, such as the Star Trek Original Series ‘Requiem for Methuselah’. In that episode, Flint, the immortal man, laments the price paid for his immortality: ‘I have married a hundred times, Captain. Selected, loved, cherished. Caressed a smoothness, inhaled a brief fragrance. Then age, death, the taste of dust.

The great variation in species lifespan has preoccupied many scientists for decades. For instance, a mouse has a maximum lifespan of about four years whilst some species of whale may exceed 200 years. There are also some animals that appear to be immortal, such as flatworms and lobsters. Plants too show great variation in lifespan: annuals live one year whilst some perennials such as brittlecone pine may live as much as 5,000 years.

For over a century, a popular belief amongst bioscientists has been that there is statistical correlation between animal lifespan, metabolic rate, and mass. In 1908, Max Rubner formulated his rate of living theory, which posited that slow metabolism was correlated with longer lifespan and with larger mass. Later, work by Kleiber on possible correlation of surface area to metabolic rate led him to formulate Kleiber’s law. This asserts that for many animal species, their individual metabolic rate, i, scales as some power of the animal’s mass m: specifically, Kleiber found a relationship i ^ m3/4 [Kleiber, 1932]. However, recent studies have shown a more complex picture: ‘We confirm the idea that age at maturity is typically proportional to adult life span, and show that mammals that live longer for their body size, such as bats and primates, also tend to have a longer developmental time for their body size’ [De Magalhaes et al., 2007].

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