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Evolution, the theory goes, guarantees survival to the fittest. But we can blame evolution for some of today's most pressing health problems, such as cancer, obesity, diabetes and cardiovascular disease.
A 2009 Gallup poll found that 44% of Americans believe that God created human beings in their present form within the past 10,000 years. Many of them also think the human body is perfectly designed.
But most scientists -- including biologists, anthropologists, paleontologists and geneticists -- see the 21st century human body as a collection of compromises, jury-rigged by evolution as our ancestors adapted to changing conditions.
'In many ways, we are maladapted for modernity,' says Stephen Stearns, a Yale evolutionary biologist. He and others in the field are urging medical schools to include more evolutionary thinking when teaching doctors about modern diseases.
For example, the immune system was honed to fight off epidemics like malaria and cholera, which proliferated along with urbanization. According to the 'hygiene hypothesis,' asthma and auto-immune diseases are increasing because the human immune system doesn't face enough challenges in today's cleaner environments and is picking fights with the body's own systems instead.
The current epidemic of obesity also has prehistoric roots. Our hunter-gatherer forebears were tall, lean long-distance runners who subsisted on plants and protein. When populations shifted to agriculture about 10,000 years ago, a carbohydrate-rich diet became the norm. Early farmers had more calories but less nutrition, and average heights dropped from 5-foot-9 to 5-foot-3 for men, and from 5-foot-5 to 5-feet for women. Metabolisms adjusted over the millennia -- but populations that shifted to agriculture more recently, like Polynesians and American Indians, have the highest rates of obesity and Type 2 diabetes today.
Evolution even plays a role in yo-yo dieting, according to Rudolph Leibel, an obesity expert at Columbia University Medical Center and one of the discoverers of leptin, the hormone that usually signals the body to stop eating. Leptin levels fall rapidly when people lose weight, setting off a cascade of physiological changes that burn fewer calories and act to pile weight back on. 'That was great during times of famine, but these days, it's counterproductive,' says Dr. Leibel.
With fewer predators and more resistance to disease, humans are now living long enough that cells have more chance to go haywire. Reproductive cancers may be a function of longer lives and changing cultures. For long stretches of history, the average woman had only about 100 menstrual cycles in her lifetime, because frequent pregnancies and breastfeeding kept her from menstruating. A typical woman today has 400 cycles, creating more stress on her ovaries and subjecting her breasts to more hormonal swings.
One of the best known holdovers is the 'fight-or-flight' mechanism that pumps out adrenaline, cortisol and other hormones when the brain senses danger, making muscles tense up, blood vessels constrict, the digestive tract slow and the heart beat faster. That was useful for outrunning mastodons -- but counterproductive for sitting at the computer.
Even anxiety dreams -- where you're unprepared for a test or falling off a cliff -- may be a leftover from ancient times when people had to be constantly on guard for predators in the night, says Deirdre Barrett, a Harvard psychologist and editor of the journal Dreams.
Scientists studying genetic variations around the world say that about 1,800 genes -- some 7% of the human genome -- have been evolving rapidly in the past 10,000 to 40,000 years. The precise function of many isn't known, but researchers have identified several that provided a key advantage for survival and got passed along.
Many recent mutations developed in response to infectious diseases, particularly as people started living in large communities. In Africa, some 25 new gene variations and an entire blood type have emerged to help people resist malaria in the past 10,000 years. About 10% of people in Europe today have a gene variation that makes them resistant to HIV/AIDS.
But many evolutionary advantages came with trade-offs. It's long been known, for example, that gene variations that protect some Africans from malaria make them vulnerable to sickle-cell anemia. Genes that helped early Africans retain salt guarded against dehydration in tropical climates now put some African-Americans at risk for high blood pressure today.
And some body parts that provided a benefit at some time in human history pose challenges today -- a phenomena Texas Tech University geneticist Lewis I. Held Jr. calls 'bislagiatt,' an acronym for 'but it seemed like a good idea at the time.'
Among the body's bislagiatt parts Dr. Held catalogs in his book, 'Quirks of Human Anatomy,' are men's testicles that hang outside the body because sperm develop best at slightly cooler temperatures -- but that makes them vulnerable to injury.
In women, the mismatch between mother's narrow pelvis (which facilitates walking upright) and a newborn's large head (which facilitates cognitive development) makes childbirth a painful and sometimes dangerous process.
The appendix, which scientists think served as a fermentation chamber for helpful intestinal bacteria in primates, is less needed now that people have varied diets and cook food.
The human mouth has also evolved unevenly. Teeth shrank considerably as agriculture changed our ancestors' diets from mostly meat and plants to mostly carbohydrates. The human jaw shrank even faster, making wisdom teeth largely useless and creating the overcrowding that people face today.
Why haven't years of evolution corrected these quirks? 'Many features of our anatomy operate 'under the radar' of natural selection,' says Dr. Held. That is, they generally aren't problematic enough to affect people's survival before they reach reproductive age, so they keep getting passed on. Some experts think that wisdom teeth and the appendix may be slowly on their way out -- some people are already born without them -- since they do sometimes cause life-threatening infections. 2010-04-20
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