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.
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
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?