Despite ageing being one the hottest topic
in the media recently, scientists have no coherent explanation for it. New
demographic data on humans, animals and plants for the first time unveil such
an extraordinary diversity of ageing processes that no existing evolutionary
theory can account for.
Both life spans and mortalities vary from
species to species. The fact that the probability of dying rises with age
applies to humans, but is not principally true.This
is shown by a catalogue of 46 species with their respective mortality and
fertility rates, which has now been published in the science journal Nature.
It is the result of a long-term data
collection project led by scientists at the Max Planck Institute for
Demographic Research (MPIDR) in Rostock, Germany, and at the Max-Planck Odense Centre
on the Bio-demography of Ageing (MaxO) in Odense, Denmark.
Not only are previous explanations unable
to deal with life spans ranging from a few days (fruit fly), to decades
(humans), to centuries (hydra), but they are also unable to account for
variations in the death rate. Common theories assert that the probability of
dying rises with age, as for humans.
However, the researchers catalogued species
such as the white mangrove and the desert tortoise whose probability of dying
actually decreases with age. In addition, fertility periods of some species
also challenge common theories.
Previous attempts to explain ageing claim
that creatures only invest in self-preservation until they have reproduced
successfully and raised their offspring. Following this line of reasoning, when
the end of the fertility period approaches, the body should start to decay –
which is known as senescence, or ageing.
Humans a real oddity
For humans this is only partly true.
According to the Nature study, mortality of modern Japanese women rises
constantly after childhood.
contradictorily, humans still live for a long time after fertility has ceased.
Today, many people stay healthy until they are grandparents and their
probability of dying is correspondingly small. Only at advanced ages is
mortality growing rapidly. For example, in Japanese women 100 years old,
mortality reaches more than 20 times their lifetime average.
This makes humans a real oddity. No other
species in the researcher’s catalogue has a mortality curve which rises that
sharply. Even among other mammals, death rates reach no more than five times
the lifetime average. Why evolution developed such big differences is a mystery
Ageing turned upside down
Current theories are especially at odds
with two groups of species for which the concept of ageing appears to be turned
upside down. On the one hand there are creatures whose mortality stays constant
throughout their whole life, like hydra or the hermit crab.
Their bodies do not seem to degenerate
during their lifetime which can be understood as the absence of ageing. And
there are even species whose probability of dying decreases as they grow older,
like the red gorgonian (a coral), the net leaf oak and the desert tortoise.
Their risk of dying obviously never becomes zero, but when they are old they
are more likely to survive until their next birthday than when they were in
There is another belief that the new data
catalogue disproves: the idea that species with a short life span die so soon
because they age so quickly. This would mean that their mortality rises
strongly throughout life.
However, sometimes the contrary is the
case, such as in the tundra vole. Its mortality increases only moderately until
it reaches two times its lifetime average at old age. Nevertheless, this vole
rarely survives beyond one year. Humans, however, are living for an entire
century more and more often, despite the fact that their risk of dying
skyrockets at old age (up to more than 20 times the lifetime average).
Unified theory of ageing
“Surprisingly, one can hardly imagine a
type of life course that is not found in nature,” says researcher Owen
Jones. This applies not only for mortality but also for fertility. While women
become infertile after a limited childbearing period in the first half of their
lives, fertility rises until almost the end of the lifespan for the alpine
swift. And the yellow baboon has offspring throughout its life without any
influence of age.
“One reason why we still lack a unified
theory of ageing is that our view on ageing was always biased because we had data
only for a very restricted selection of species,” says bio-demographer
Alexander Scheuerlein from MPIDR.
There have long been high quality
demographic records for hundreds of mammals and birds but very few for other
vertebrates or invertebrates. Extremely little is known about algae, fungi or
bacteria. In order to understand why evolution created ageing, much more
comprehensive data on all species have to be collected, says Alexander