Biff Eddington
Anthropology 4467
24 April 2011
What is life history theory? What is a primate life history pattern? What is the relationship between brain size and life history?
Life history theory is a branch of biology that analyzes the selective forces that have managed the evolution of the schedule and duration of key events in an organism’s lifetime related to investments in growth, reproduction, and survivorship (Bogin, O’Rourke and Stinson 547). The reason that there are differences between major events in species related to growth, maintenance, and reproduction is because each species employs different evolutionary strategies to increase reproductive fitness. Each species has a unique life history; this is a pattern of how it distributes its finite energy between five different phases: Growth, development, raising its young, staying alive, and reproduction. The assumptions that underlie life history theory are not complicated, but are simple and impactful: There is limited time and there is limited energy, so this energy must be distributed among growth, reproduction, and survivorship recognizing the trade-offs that cannot be avoided.
Primates have a unique life history pattern that is different from other orders of mammals. Relative to other mammals primates have: a longer gestation period, fewer offspring overall, more care given to each individual offspring, infants that are born developmentally advanced, a longer period of parental dependency, a strong bond between mother and child. After the infancy stage relative to other mammals primates have: a longer juvenile period growth, a lengthened adult period, and a longer life span.
The relationship between brain size and life history is important because the brain has such a huge impact on development that a change in the brain size will inevitably affect life history. There is a directly proportional relationship between brain size and the pace of a life history. In humans who are highly encephalized this has led to the slow pace of their life history. Because large brain size slows life history it is thought that subadulthood is extended because of the length of time it takes for a brain to grow to a large size. Evidence that supports this theory is that human subadulthood is 6 years longer than chimpanzees, yet only one more year is used to grow a larger brain. Chimpanzees in fact have a similar percentage brain size at birth compared to humans. Chimpanzees and humans share a similar pattern of brain growth trajectories from birth. So, in fact primates grow a larger brain from an increased rate of growth, not by their brains growing longer. A slower life history allows there to be an opportunity to change the rate and timing of brain growth (Bogin, O’Rourke, and Stinson 401-403).

What is a life table? What values comprise it and how is it used?
A life table is an arrangement of mortality rates by age for a particular population. Life tables also have some derived measures of mortality, such as how much longer you are expected to live at any given age. Life tables can be applied to any kind of “duration,” but are most often used for studying mortality (Bogin, O’Rourke, and Stinson 698-700). The earliest life tables have been found to be from Roman times. Prior to the 19th century, all demography was relegated to the life table. The first formal life history was done by John Gaunt starting in 1662 on the “Bill of Mortality.”
There are two types of life tables: Kaplan-Meier and actuarial. Actuarial life tables usually categorize individuals into categories that are fix-aged, most often by using 5-year age categories. Kaplan-Meier life tables use each person’s exact age at death. Actuarial life tables are the type of life table that is most often used in the field of human biology. There are five standard columns in a life table. The value ex is how long the individual is expected to live at age x. The value Tx is the total future person-years lived after age x (Biology-online.org defines person-years as “The total sum of the number of years that each member of a study population has been under observation; e.g., years treatment with a certain drug). The value Lx is the person-years that are lived between an age x and a chosen interval. The value lx is the number of people who are born who are able to continue living to age x. The value qx is the likelihood that a person who is able to continue living until age x dies within the interval used (Bogin, O’Rourke, and Stinson 700).
A life table is used by assuming that the life table is representative of an imagined cohort of people who are exposed to a mortality schedule. Bogin, O’Rourke, and Stinson define a cohort as, “Any designated group of people (or other organisms) who share characteristics of interest to a researcher” (809). The imaginary cohort is called the radix (l0) of the life table; the radix is chosen arbitrarily. Then the qx column is estimated, which allows all the other columns to be computed from it. Finally, by looking at the life table you can see how the risk of dying changes with the age of a person in a population (Bogin, O’Rourke, and Stinson 701). How do the Hayflick limit, telomere length, and DNA repair mechanisms relate to senescence and aging?
What causes degradation of the physiological fitness of humans? This was answered by Professor of Anatomy Leonard Hayflick. He observed that cells can only divide so many times before they stop dividing and for a long time he could not figure out why this would happen. He eventually noticed that the rate at which cells stop dividing declined linearly with age until the cells were no longer viable and had reached their limit of divisions possible. This limit he figured was set by natural selection.
The way the Hayflick limit is set is by Telomere length. As cells divide a little of the telomere is lost. Each chromosome is a telomere. At the end of the telomere there is a long repeat. This repeat acts as a buffer for the coding information on the inside of the chromosome. Over time as cell division occurs the telomere gets shorter and shorter until it eventually eats into coding and creates errors. The exception to this rule is cancer cells, which the telomere lengthens and endless replication occurs. As the telomere length of the cells in a human’s body become shorter and shorter over time it leads to overall decline of physiological function. A human’s maximum functional capacity is on average at the age of 30. Ubiquitin is a regulatory protein that is found throughout the tissues of the body. It is involved in the regulation of protein degradation via proteasome activity. It is also involved in the modification of cellular proteins. It aids in regulation of transcription, DNA repair, and stabilization. As organisms age there ubiquitin becomes less and less effective repairing DNA.
Another way DNA repair mechanisms relate to senescence and aging is related to Nuclear DNA. Nuclear DNA is constantly attacked by oxidation, alkylation, and hydrolysis. Bases become mutated, deleted, and strands of Nuclear DNA can be broken. DNA repair enzymes try to keep Nuclear DNA healthy by fixing damage or putting cells into senescence and apoptosis. As people get older DNA repair enzymes become less effective, thus leading to more and more damage to the Nuclear DNA which is a direct cause of aging (Best 1-2).
Describe what you consider the most unique aspect of human biology (the physiological system, not the study thereof). What makes humans stand out among all the great spectrum of life on earth? Use details and examples from this course to support your argument. I’ve often mentioned that humans are among the most widespread mammals on the planet, what about your chosen aspect of human biology aided in this success?
Scientists agree that cognitive ability and brain size are directly correlated. The brain controls the organs of the body and is the center of the nervous system in humans. As much as scientists know that the brain determines one’s intellectual abilities and emotions much of the processes of the brain are shrouded in mystery. The only problem with the theory that cognitive ability and brain size are directly correlated is that some animals have brains that are equal to or larger than the human brain and humans do not see these animals as equal to or more intelligent than them. For example: dolphins have the same average size of brain as humans do and some whales have an average brain size that is more than 5 times larger than humans. What scientists have to come to understand is that it is not the size of the brain of an animal that determines its intelligence, but rather how encephalized (the brain size to body size ratio) the animal is. That humans are the most encephalized species on earth and also the most intelligent gives credence to this theory. What does the fossil record show about the encephalization of human ancestors? Were they more, less, or the same regarding encephalization?
The fossil record shows that 30-40 million years ago the brain was expanding in simians. A major change occurred between 2-3 million years ago, which is the time when our ancestor australopithicenes evolved into the first species in the Homo genus. Over this time our direct ancestors became one-third more encephalized than any other primate that had lived up to that time. Another major change in the ancestors of humans’ brain size to body ratio happened around 100-200 thousand years ago. At this time our direct ancestors were 3 times more encephalized than any living or extinct primate that has ever lived. The brain not only grew in proportion to our ancestor’s body size, but it also changed. The temporal lobes became bigger and the Broca’s region formed, which is associated with speech (Bogin, O’Rourke, and Stinson 94). Did the energetics of humans change to accommodate this new, larger brain or did something else occur?
Basal metabolic rate (BMR) is how much energy is used daily by any given animal. If you plot brain weight versus BMR on a graph with other primates, the graph will show that humans are outliers because they have by far the highest brain to BMR ratio of any primate. In fact, of the total energy budget of humans the adult human brain uses 20-25% of this energy. Animals other than primates use 8-10% of their total energy budget on their brain and mammals use 3-5%. What makes this so interesting is that the overall metabolic maintenance costs for humans are about the same as other primates and mammals (Bogin, O’Rourke, and Stinson 270-271, 355). If humans use more energy for their brain than any other animal it seems logical to think that some other aspect of human physiology became more efficient energetically and/or a function diminished or even ceased to exist. Is this what happened?
Anthropologists Leslie Aiello and Peter Wheeler posited a theory to answer the question of what aspect of human physiology became more efficient energetically and/or what function diminished or even ceased to exist. They named their theory the expensive-tissue hypothesis which says that metabolic requirements of large brains are offset with a corresponding reduction of the gut (199). Aiello and Wheeler believe that because the liver and gastro-intestinal tract are the only organs that use as much energy as the brain and because the gut is the only organ that is smaller in relation to body size compared to other primates that there was a coevolution between the digestive system and the brain. They believe that for this to happen our ancestors would have had to transition to easily digestible, high-quality food (199,219). In fact there is evidence for this theory in the fossil record. Nariokotome boy, a Homo ergaster, is a skeleton that is 1.7 million years old that has a noticeably narrow rib cage and it just happens that at this very time brains were beginning to increase in size in the fossil record. It is thought that this high-quality food was meat and that the process of hunters following game animals is what led humans out of Africa. Three other changes are thought to have occurred to allow humans to have enough energy for a large brain: Reduced muscularity, babies getting fatter, and an increase in the size of females because it would allow them to give more energy to their offspring (Bogin, O’Rourke, and Stinson 355).
Because Homo sapiens are the most encephalized species on the Earth, they are the most intelligent species on Earth. Their brain has allowed them create, communicate, and solve problems in ways that other species can’t even contemplate. Because of this brain humans live on all five continents. Even beyond that humans can travel in vessels they have created deep into the ocean or even into space. Even though there have been other important human biological changes the most important is a process that occurred hundreds of thousands of years ago. This process is the coevolution of the human brain getting bigger and the gut shrinking. This is the process that is most responsible for human success.

Works Cited
Aiello, Leslie C., and Peter Wheeler. “The Expensive-Tissue Hypothesis.” Current Anthropology 36.2 (Apr. 1995): 199-221. JSTOR Web. April 2013
Best, Benjamin P. “Nuclear DNA Damage as a Direct Cause of Aging.” Rejuvenation Research 12.3 (2009): 1-22 Web. Apr. 2013
Biology-online.org, Biology-Online.org., 2013. Web Apr. 2013.
Bogin, Barry, O’Rourke, Dennis, and Sara Stinson. Human Biology: An Evolutionary and Biocultural Perspective. Hoboken: John Wiley and Sons, 2012 Print.