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The Evolution of the Human Life Cycle

 

bogin_528

Barry Bogin is an American physical anthropologist trained at Temple University that researches physical growth in Guatemalan Maya children, and is a theorist upon the evolutionary origins of human childhood. He is currently at Loughborough University in the UK. He is noted for the idea that evolution added two new stages into human development; childhood and adolescence.

 

Smith,B Holly

B. Holly Smith is a Associate Research Scientist in the Department of Anthropology at the University of Michigan at Ann Arbor where she got both her Master and Ph.D. She is interested in  how humans differ from other mammals in life cycle and life span, why we differ and whether we can reconstruct the evolutionary history of our life cycle from the fossil record.

This chapter was about the evolution and alterations of the human life cycle. The main questions that guided this research were:

  • How can human biologists identify the shared and novel features of the human life cycle?
  • Can the time of origin of the novel features be determined?
  • Can the reasons for the evolution of new growth development and maturation patters be determined?

 

Stages in the Life cycle

There are four main stages in the human life cycle Birth, Postnatal Development, Adulthood, and Death. Of these, both postnatal development and adulthood are divided up into sections. Pregnancy (the period before birth) is divided into trimesters and during this gestational period, the fetus grows and changes and experiences critical periods. These are times when a fetus is particularly susceptible to outside factors such as diseases or lack of nutrients. During this time the fetus can undergo epigenetic modification.

What other outside factors can affect a fetus in vivo?

 

After the pregnancy comes birth, a rapid transition from a fairly stable liquid environment to a volatile gaseous one. And after this period come the postnatal development. This is the most complex of the stages and is divided up into these sections

  • Neonatal period
  • Infancy
  • Childhood
  • Juvenile
  • Puberty
  • Adolescence

Which of these sections is the longest and why do you think that is?

In which of these stages is proper nutrition the most critical for brain development, and why is this so?

 

Why did new life stages evolve?

If we look at the life cycles of other large primates we see that although humans experience delays in Molar 1 eruptions, menarche and 1st births, humans have less spacing between births (3 years for humans, 6 for chimpanzees). This gives humans the advantage to give birth to more offspring. So we find that our evolution of childhood gives us the reproductive advantage although it does come with some drawbacks. Children need specialized diets and extended periods of care, as they do not become self sufficient until post-adolescence.

Although we cannot study the life cycles of an extinct organism, we can postulate it by looking at currently living species. In looking at archaeological evidence, we can see that there is an increase in brain size in cubic centimeters and that because of this, there had to have been an increase in postnatal stages. When we get to homo sapiens we see the appearance of adolescence.

Which organism(s) would be useful in looking at early human life cycles?

What physiological changes needed to occur in early human ancestors to accommodate larger brains?

 

Food for thought

  • How would we be different if we had a shorter postnatal period?
  • Would anything be different if humans waited (on average) twice as long between children?

6 thoughts on “The Evolution of the Human Life Cycle

  1. Meghan Steel

    The most interesting part of this chapter was reading about the evolutionary implications for changes in life history. It makes sense that any alteration in life history theory would culminate in speciation as the patterns of the growth of youngsters has a huge impact on reproductive strategies. I was particularly interested in the section about menopause since my personal research focus is on gerontological anthropology. I would hypothesize, and hopefully eventually explore, the relationship between a culture's reliance on alloparenting by grandparents and the corresponding length of post-reproductive "healthy years" by each parent. I would also hypothesize that grandmothers with grandchildren from a daughter will be healthier than grandmothers who only bore male children, due to the absence of any parental uncertainty. Would anyone agree (or disagree)?

  2. rebeccaleon

    What other outside factors can affect a fetus in vivo?

    There are numerous factors, probably more than most pregnant women would like to know, which can affect a fetus in utero. Some of these factors include diet (e.g., nutritious value), air quality (e.g., pollution), stress, how much weight the mother gains or doesn’t gain, and age of the mother. I find studies that examine the factors that can have negative and positive effects on a fetus during pregnancy very interesting. I think this specific avenue of research is very important for informing women of the factors that can play a role in either positively or negatively affecting their growing fetus. Seeing as how diet is the easiest factor for pregnant women to control, I think that is the topic public education should place the most emphasis on. There are so many popular misconceptions that both men and women continue to believe and adhere to, such as pregnant women should eat twice as much food because they are eating for two. Unfortunately for those who believe this misconception, they could end up gaining more weight than is necessary during pregnancy and potentially giving birth to a high weight baby. Although most research has focused on the implications of low birthweights, a recent study has found a positive association between high birthweight (>~8.8 Ibs) and an increased risk of both childhood and adult malignancies (i.e., malignant tumors) (Ross 2006). My husband I want to start having kids in the next couple of years, so having this type of information is personally very important to me. I have a pretty good general idea of the things I should and should not do or be aware of during pregnancy. However, there are some of factors that I know I won’t be able to control, such as pollution.

  3. Emily Barron

    We, unlike other primate species, have an extended period of adolescence. It is our longest life stage before adulthood. This could be because we learn how to maneuver through society during adolescence. Having large, complex social groups requires that humans have a longer time to develop their social skills. Most other mammals that have large social groups go through some sort of juvenile stage, although it does not match human adolescence. Human adolescence is also characterized by skeletal growth spurts, which is interesting because it doesn't happen in any other great ape.

    How would we be different if we had a postnatal period?

    I figure I just missed this part of the reading, but what is meant by postnatal period? I googled and got that it was the same as the postpartum period in women, which we do have.

  4. Madeleine Cheatham

    -outside factors that can affect a fetus-

    Diet seems like the most obvious factor that could and can affect an unborn baby, this is also the factor that would affect me the most. The risk of mercury or methylmercury poisoning (found in seafood and freshwater fish) could cause great harm to a fetus. Unfortunately I am an avid seafood eater and this will be hard to accommodate if I ever find myself to be with child. The United States food and drug administration advises pregnant women to not eat swordfish, shark, king mackerel or tilefish. Some other factors that can affect a fetus are chemical ones. Alcohol (even a little) can lead to problems with brain development. Mass amount of alcohol can lead to fetal alcohol syndrome. Smoking can also have negative affects on an unborn child and has been known to slow the growth of the fetus. Other factors include lead, dioxins, air pollution and pesticides. Many books and blogs recommend that preggo women stay away from paint supplies, check the quality of their tap water, make sure they aren't living in a home painted with lead paint, wash all produce thoroughly and that they avoid all cleaning products labeled toxic.

  5. Andrea Morris

    What other outside factors can affect a fetus in vivo?

    There are many factors that can affect a fetus in vivo. There are factors that vary from the environment to the diet of the mother. We have discussed in class before how air pollution in the air quality that the mother inhales during pregnancy can affect the fetus. The diet of the mother is also very important because this strongly influences the development of the baby. For example Fetal Alcohol Syndrome is the conscience of a mother consuming alcohol during pregnancy and the result is the child disfigured and can have mental disabilities as well. This syndrome intrigues me because there is not much known about what causes it. I learned about my freshman year in a biology class so there might be more research that has been conducted on it that I'm unaware of.

  6. Taylor Burbach

    What other outside factors can affect a fetus in vivo?
    Everyone else seems to be answering this question but it affects me personally so I will too! I've heard for ages that drinking alcohol is forbidden during pregnancy, but recently I've been hearing it is OK to have a little bit every now and then during pregnancy. I think because of all the unknown factors, it's still advised against by the CDC etc. but that is definitely something that intrigued me the first time I heard it.
    What physiological changes needed to occur in early human ancestors to accommodate larger brains?
    In addition to larger skulls (duh) there had to be changes to the birth canal and postnatal development. Females' skeletal structure needed to be able to accommodate their big-headed babies when giving birth. And even when these changes occurred, human brains were still too large, so a good bit of development happens in the first few months after birth.
    I'm not sure I understand what you mean by your questions under "Food for Thought."

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