24 Jan, 2013
The evolution of human aging
Weismann: aging evolved as a benefit to the species (replaces the old with the young - group
selection) – Finch 2007
The concept was changed later to suggest that aging evolved because organisms that segregate
germ cells (re: reproduction) and soma cells (re: all other body functions) must invest additional
resources to reproduce instead of maintaining the soma, and this renunciation of the soma
results of aging.
We invest additional resources in the germ cells as we grow up and when we reproduce.
Williams, following Medawar proposed two mechanisms by which processes of senescence occur (and
might genetically determined rather than due to natural selection):
1) Antagonistic pleiotrophy is when an allele has beneficial effects in early life but acts to the
detriment of long life
2) Mutation accumulation/ age-specific gene action: alleles with detrimental effects are expected
to express these late in life after reproduction effort is complete
Both types of genetic propensities have been observed among laboratory animal models (insects,
rodents and worms) and proposed for aspects of human variation in senescence and life span
Weismann’s concept of germ/ soma later explained by Kirkwood: “disposable soma and
immortal germline theory of senescence” that organisms must reach a balance/ trade-off
between the resources they invest in soma maintenance vs. reproduction. This theory basically
says that aging occurs during adulthood after reproductive age where this is the feedback
effect of the body prioritizing germ cells for reproduction earlier on, as such the body now
must counter balance the effect by investing the resources into the soma cells.
the disposable soma theory predicts that aging occurs due to the accumulation of damage
during life (which, during reproductive years is at the expense of the soma/ body) and that
multiple defensive or repair mechanisms (particularly the investment in reproduction over soma
in earlier adulthood) contribute to aging
Aging is the result of a trade-off or adverse side effect between the beneficial quality of
reproduction vs. body maintenance in early adulthood.
Humans are distinctive among primates for the greatest longevity and most prolonged maturation. We
are highly pedomorphic as such chimps appear older than we are. Due to the different life history into adulthood between humans and chimpanzees, we notice that the
chimpanzees are aging a lot faster than we are (although we maintain the pedomorphic features)
Increase in size and complexity of the brain is an important anatomical change underlying both human
biological and behavioural evolution. Thus it is not unreasonable to conclude that this is also the most
significant factor leading to the increase observed in the human life span.
Note: our brains are about 3 times bigger than that of the chimpanzees. When a chimpanzee is born,
their expected life expectancy at birth was estimated to be around 15-20 years but their maximum life
span can be estimated to about 50-60 years.
Humans had a similar life expectancy at birth (30-40 years), it is almost doubled. And to top it all
off, during the recent human evolution, life spans have more than doubled.
We also evolved a unique social system of multi-generational caregiving and resource transfer
to the young which is important in brain evolution and reduced mortality/ increased life span.
This is a behaviour trait that played a vital role in the survival of our species and the prolonging
of our life span.
Evolution of human longevity is hypothesized to be an outcome of selection for low mortality
during prolonged maturation that is also selected for lower mortality in older individuals who
comprise the critical multi-generational human social matrix.
WHEN DID LIFE EXPECTANCY OF HUMAN ANCESTORS BEGIN TO INCREASE ABOVE THE GREAT APE
BASELINE OF 15-20Y
Body size references: larger animals have slower development and live longer than smaller ones;
Australopithecenes and other early hominids before 2 MYA were in the upper size range
chimpanzees, 20-40 kg, implying modestly longer life spans than 15-20 years.
Dental evidence: younger and older adults may be distinguished by tooth wear and presence of
adult molars/ M3 at the end of the juvenile period.
Upper paleolithic sample of modern H. Sapians in Europe had a ratio of old: young adults of 2.1
which was significantly higher than the <0.4 of old :young ratio for middle Paleolithic ‘early
modern’ H. Sapians, Neandertal, other earlier Homo and Australopithecus.
Caspari-Lee analysis indicates that the major increases in survival to adult ages occurred
relatively recently in the upper paleolithic i.e, approx. 30 Kya (probably to current forager life
expectancy at birth: 30-40 years)
There are graphs that show a clear trend in the increase of life expectancy with the increase of brain
growth and body size over time.
Life history factors and lifespan: the 2.5 y average age at weaning is several years earlier than in chimps and other great apes
who delay weaning until age 5-10. Long term weaning of a child, opts to decreases the chance of
pregnancy (early weaning allows a successive pregnancy before the child is fully able to forage
independently and sooner in chimps) .
Access to energy and nutrient dense meat is hypothesized to have facilitated early weaning. It is
hypothesized to have play a vital to our increased life expectancy due to additional nutrients
being possessed and obtained by eating meat.
Survival before age 5 depends on multigenerational provisioning and care by other adults/ kin
(alloparenting- an important human trait) which enables mothers to become pregnant again
sooner often within a year of weaning.
The 50% shorter interbrith interval in humans gives a huge reproductive advantage (population
growth) that offsets the 5 year longer maturation of humans (re: 1 births), relative to shared
ancestors of great apes with shorter life expectancy; maturation of specialized skills is even
more delayed into 20’s and 30’s for hunting with weapons.
Assisted birthing (due to larger head) would also be developed in this scenario (our pelvic area
has changed and as such we require assisted birthing to increases chances of survival)
Greater parental care is linked with longer male survival which may also be linked to increased
human life span
Humans are weaned till 2.5 y whilst its 5yrs in chimps, and chimps reach adulthood faster than humans (
14y as compared to 18y in humans)
The evolution of meat eating:
The human diet differs from other primates. The great apes are primarily herbivores though in
the case of chimps, they can occasionally eat meat by hunting monkeys and other small
mammals (omnivorous) – some males averaging 70g meat per day
In most chimps, typical caloric intake from animal tissues is about 5% of total diet (much lower
than human norms).
Regular consumption of meat not essential to chimp survival (some chimp communities have
not been observed to hunt or to eat meat) and in those that do ea