While natural selection is best known for weeding out the weak, it may also be partly responsible for the apparent rise of some disorders, such as autism, autoimmune diseases and reproductive cancers, according to researchers.
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Since evolutionary factors play a role in disease, the two fields should have some crossover, say a group of scientists who have studied various aspects of the link between evolution and medicine.
"This work points out linkages within the plethora of new information in human genetics and the implications for human biology and public health, and also illustrates how one could teach these perspectives in medical and premedical curricula," said researcher Peter Ellison, an anthropologist at Harvard University.
The results, they say, could save lives.
"Evolutionary medicine got going in the '80s and early '90s, but it has been energized in the last decade by the discovery that it really makes a difference," researcher Stephen Stearns of Yale University told LiveScience. "In the last 10 years we have found out that taking an evolutionary perspective really helps to reduce suffering and to reduce the risk of death."
Evolution and disease
Stearns and a long list of scientists presented their findings on this evolution-medicine link at the Arthur M. Sackler Colloquium in the spring of 2009. The results, announced publicly today, are now published in the journal Proceedings of the National Academy of Sciences.
For example, previous work in evolutionary medicine has helped to explain some reasons why disease is so prevalent and difficult to prevent: Natural selection favors reproduction over health; biology evolves more slowly than culture; and pathogens evolve more quickly than humans.
They describe these and other connections between evolution and sickness along with possible explanations. Here are the highlights:
- Humans evolved alongside beneficial bacteria and parasitic worms, and so our ancestors built up immunity to such bugs. But nowadays with increased hygiene, we've eliminated the bacteria and worms. The result: Since our immune systems aren’t used to these good bugs, our bodies fight them as foreigners. That can result in allergies, asthma and autoimmune diseases, such as Graves' disease in which a person has an overactive thyroid.
- Humans have higher rates of cancer than other species. One reason: We aren't adapted to the new risk factors of modern society, including tobacco, alcohol, a high-fat diet and contraceptives, researchers have found.
- Certain adaptations that once benefited us might be helping several ailments to persist in spite of, or perhaps because of, advancements in modern culture and medicine, according to researchers.
With respect to evolution and culture, here's a case in point: Harmful mutations are often recessive, and so both parents must pass on the gene in order for the disease to show up in offspring. And while natural selection has supported outbreeding (breeding with people other than close relatives), culture hasn't always followed suit. Across the globe, about 10 percent of spouses are second cousins or closer, the researchers say, with the prevalence ranging from 1 percent to 50 percent in different cultures.
The inbreeding can cause recessive genes that should only have a small effect on mortality to have a much larger impact.
Autism and evolution
Autism and schizophrenia also have ties with evolutionary science. Essentially, they boil down to a battle of the sexes.
Past studies beginning in the 1960s have built on one another to suggest mom and dad are in evolutionary conflict over investment of resources to their offspring. A mother knows all of her babies are hers and so should give evenly to all. But fathers only want to invest in their biological kids (not offspring from another male) and so a father’s genes will pressure mom to skew investment toward those offspring.
Studies in genetically engineered mice have shown that when certain paternal genes get expressed, the baby mice are 10 percent heavier than normal.
The results should translate to humans and carry into early childhood, affecting children’s behaviors, the researchers suggest.
For instance, when the paternal form of a gene on chromosome 15 gets expressed, and not the mother's, the resulting offspring will be more demanding, sleep poorly, want to suckle frequently and have a 40 percent to 80 percent chance of having autism as an adult. (Humans normally have 46 chromosomes in each cell.) While scientists think genes play a role in developing autism, the complex causes of this disease are still unknown.
Similar findings have shown psychoses such as schizophrenia can develop when the maternal form of certain genes gets expressed.
Educating physicians on evolution
Stearns suggests evolutionary perspectives should be integrated into curricula as early as undergraduate school for students planning to attend medical school. The knowledge, Stearns said, would complement traditional studies undertaken in medical school.
We're trying to design ways to educate physicians who will have a broader perspective and not think of the human body as a perfectly designed machine," Ellison said. "Our biology is the result of many evolutionary trade-offs, and understanding these histories and conflicts can really help the physician understand why we get sick and what we might do to stay healthy."
The take-home message: "Evolution and medicine really do have things to say to each other, and some of these insights actually reduce suffering and save lives," Stearns said.
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