Literature review (IT)

The Human Lifecycle Humans are unique in that they have more stages of life development than other primates.

This is largely due to the fact that humans require longer post-natal stages in order to

properly develop. We pass through periods of infancy, childhood, puberty and adolescence

before finally reaching adulthood. Human offspring take longer to mature, yet humans also

reproduce more quickly than chimpanzees (3 years between births, versus 6 for chimps).

When given the opportunity, we have quickly populated the entire planet at a rate much

faster than any other mammalian species.

Humans experience two kinds of changes as they develop into an adult: physical growth,

a series of changes in size and form; and maturation, a sequence of developmental

changes. Physical growth is a set of processes that are strongly regulated by genetic

factors. Generally children tend to reach a height close to the average height of their

parents, although, as in all complex genetic characters, variation exists as well. Processes

of growth are controlled by hormones, such as thyroxine, growth hormone, androgen and

estrogen. The growth processes require adequate nutrition and an absence of disease or

environmental stress that can disrupt them. Therefore, both genes and environment

contribute to growth, and there can be substantial deviations from the expected growth

curve of an individual. After the critical growth period (neonatal through one year old), it is

considerably easier to recover from malnutrition or stress through a “catch-up” growth

period.

Puberty represents a fundamentally important type of growth and development.

The adolescent growth spurt occurs between the ages of 12.5 years and 15.5 years in

boys and about 2 years earlier in girls on average. The age at which this occurs varies, but

the sequence of changes is consistent. In boys there is accelerated growth of the testes,

scrotum, and pubic hair, as well as facial and underarm hair.

In girls the first signs of puberty are the development of breast-buds and pubic hair. The

uterus and vagina develop as the breasts mature. Menarche (the first menstrual period)

typically occurs after the maximum growth spurt in height. The first year or two after

menarche represent a period of continued growth and relative infertility – three quarters of

cycles are not true ovulations. However, better-fed girls tend to have a higher proportion of

ovulatory cycles right after menarche. The complicated interplay between diet, health and

culture has resulted in an overall decrease in the age of first menarche and general

increase in childhood growth, termed the secular trend in puberty. (In this instance, the

word secular refers to a long, generational process as opposed to a cyclical or short-term

process.) Increased food consumption is only part of the picture. Many of the products

(http://www.ncbi.nlm.nih.gov/pubmed/19478717) in our daily environment, from pesticides to

plastics (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1280355/) , have been shown to

disrupt the endocrine system in ways that affects boys and girls alike (links optional).

Before discussing aging, menopause is a process that needs discussion from an

evolutionary perspective. In females, the cycles of uterine and ovarian functions that are

associated with fertility completely ceases at about the age of 50 years. The immediate

cause of menopause is associated with changes in the ovaries, notably the decrease of

necessary hormones for reproduction and loss of sex cells. The process of menopause

occurs in all human females. To date, the only other species recorded in the wild to

experience menopause are a few species of whale, including the short-finned pilot whale,

the narwhal, and the orca ("killer whale"). Cessation of reproduction has been observed in

several species of mammals in captivity, but not at the complete rate that it occurs in all

humans. The question we must ask is why human females outlive their reproductive

capacity for so long.

Short-finned pilot whale and her newborn calf.

Your text provides some information on the issue of menopause. To date, the argument

receiving the most attention is the grandmother hypothesis, which links the evolution of

female longevity past the age of menopause to the need for help from grandmothers, who

help care for children while allowing mothers to bear their next baby sooner. But how do

humans accomplish this, when menopause is accompanied by the end of ovarian estrogen

secretion? Estrogen is crucial to the maintenance of many physiological systems aside

from fertility. Human females remain strong and healthy beyond menopause, while other

primates display strong symptoms of aging while still menstruating and rarely even survive

their fertile years. In this video lecture, Dr. Kristin Hawkes points out the evidence for non-

ovarian sources of estrogen in the maintenance of post-menopausal human females that

are not found in other primates. Importantly, she also points out why the idea that women

are outliving their original lifespans (presented in your text as one possible hypothesis)

is not a good argument in support of menopause.

…

After puberty, especially after the adolescent growth spurt, mitosis has a progressively

more difficult time maintaining the numbers of cells that make up our body tissues. With

older ages, mitosis declines and body tissues do not maintain their cell

reproducibility. Senescence is in part a generalized atrophy resulting from progressive loss

and degradation of the ability of tissues to function. Many people are studying senescence,

the processes of degenerative changes that lead to death, in hopes of better understanding

it as well as perhaps delaying it. This brief video touches on some of the theories about

why we age and die:

Key theories to keep in mind include how over time, living organisms experience damage

to DNA, mitochondrial deterioration and shortening telomeres that inhibit regular cell

production. More broadly, we are subject to certain pleiotropic genes that initially provide

us with necessary, positive traits that eventually have negative effects that contribute to the

aging process and death. The idea of mutation accumulation states that as a species we

have acquired many deleterious traits that are not expressed until after reproductive age.

In this case, it is very difficult for evolution to select against these mutations, since

individuals do not experience the negative effects of the mutation until the traits have

already been passed on to offspring. Research is focusing on many of the more

straightforward ways of “solving the problem” of aging, such as manipulating specific genes

to alter a specific outcome. However, issues of pleiotropy in general will be considerably

more difficult to deal with, since essentially this indicates a kind of controlled obsolescence

—the body did not evolve to live forever.

One piece of evidence for this is that, while the average life span has greatly increased

recently, largely due to cultural advances in food production and medicine,

the maximum lifespan has not increased at all. There have always been a very few people

that have lived extraordinarily long lives. In a planet of 7 billion people, it is not surprising

that a tiny fraction live well past the age of 100. We are not seeing people now living to the

age of 130 or 140 just because the average lifespan has increased. Again, this is strong

evidence that senescence, while plastic to some degree, is fairly firmly programmed in our

genes.

Jeanne Louise Calment lived to be nearly 123 years old, the oldest confirmed person to

have ever lived. (Please remember to vet your sources if you find something on the

internet claiming someone is 150 years old!)