7.2: Age

Humans have the longest life span of any primate. Orangutans live to be about 60 but we average another third of a life on top of that. We are the longest living out of any mammal and most life on the planet. It makes us ask why natural selection selected so many old people.

Epigenetics

Epigenetics refers to changes beyond the four forces of evolution. Your chromosomes can change during your lifetime. The term genetic mosaic refers to a group of cells in the body with variation in their DNA. Don't confuse mosaic evolution with genetic mosaicism.

Notes

DNA responds to signals from outside the cell.

Article on Genetic Mosaic in the Brain

Article on Mother's diet can lead to alterations in her child's DNA

Evo Devo

Evo Devo went beyond the Modern Synthesis and focused evolutionary theory on the development of the embryo (Stanford 2012).

Embryology

People who don't understand evolution often struggle with trying to explain why human fetuses have things that look like tails and gills.

Figure \(\PageIndex{2}\) - Embryonic stages of a bat, a gibbon and a human. Ernst Haeckel. 1905.

But if you understand our ancestry it makes perfect sense that our DNA is just a bunch of variations away from fish DNA. There is a grandiloquent phrase to describe this phenomenon, which you can use to impress people at cocktail parties: "Ontogeny recapitulates phylogeny." Ontogeny is the development of the individual through various life stages. To recapitulate is to briefly summarize. Phylogeny is the evolution of a species, that we studied in the paleontology section. So, this idea is that the development of the individual is a brief summary of the development of the species. The idea goes way back to Lamarck's time, and has been abused and misused for a long time, and you should not think of it as a scientific law that applies to all cases, and be very vary of people who talk about our "reptilian brain", for example, because our brain is not just a fish brain, enclosed by a reptile brain, enclosed by a human brain, and that kind of simplification can be very misleading. But, I think the idea is useful as a metaphor, something not true, but just a good reminder that we share relatives with fish, and that evolution tends to build on structures that are already there.
(see: Gould, Stephen Jay. 1977. Ontogeny and Phylogeny. Cambridge Mass.: Belknap Press of Harvard University Press. for more info.)

Human Life Cycles

We can use apply evolutionary theory, cellular biology, primatology, genetics, and most of the other subfields we've studied so far to better understand how humans change over our lifetimes.

Childhood

During childhood body growth slows slightly until adolescence, but brain growth peaks around age 5.

* Article on peak glucose use in childhood brains

Adolescence

Hypotheses for risk taking; insurance companies

Research on Bonobo thyroid hormones and life stages vocabulary: ontogenetic

Secular Trend

The average age of menarche has continued to fall since the 1800s, and as good anthropologists we should beware of unicausal arguments and consider all the possible contributing factors, like improved diet and health, and environmental contaminations such as hormones (from discarded birth control pills and animal products) and similar chemicals found in plastics.

Senescence

One factor in both aging and epigenetic change are the ends of your chromosomes, your telomeres which tend to wear out when they get bumped around.

Grandmother Hypothesis

Most animals reproduce until they die. Natural selection selects against a"wasted" life. So why do humans live so long? Why do women tend to live so long after they stop reproducing?

Notes

Testing the grandmother hypothesis in Utah

* with killer whales

Vocabulary

• birth
• conception
• grandmother hypothesis
• k selection
• life cycle
• menarche
• menopause
• puberty
• R selection
• secular trend
• weaning