compromise between them to ensure an optimal cost-benefit tradeoff. Running efficiency in women, and birth canal size Encephalization, and gut size Skin pigmentation protection from UV, and the skin synthesis of vitamin D Speech and its use of a descended larynx, and increased risk of choking Competition effects Different forms of competition exist and these can shape the processes of genetic change. mate choice and disease susceptibility genomic conflict between mother and fetus that results in pre-eclampsia Lifestyle Humans evolved to live as simple hunter-gatherers in small tribal bands, while contemporary humans have a more complex life. This change may make present-day humans susceptible to lifestyle diseases. Diet In contrast to the diet of early hunter-gatherers, the modern Western diet often contains high quantities of fat, salt, and simple carbohydrates, such as refined sugars and flours. Trans fat health risks Dental caries High GI foods Modern diet based on "common wisdom" regarding diets in the paleolithic era Among different countries, the incidence of colon cancer varies widely, and the extent of exposure to a Western pattern diet may be a factor in cancer incidence. Life expectancy Examples of aging-associated diseases are atherosclerosis and cardiovascular disease, cancer, arthritis, cataracts, osteoporosis, type 2 diabetes, hypertension and Alzheimer's disease. The incidence of all of these diseases increases rapidly with aging (increases exponentially with age, in the case of cancer). Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes. In industrialized nations, the proportion is much higher, reaching 90%. Exercise Many contemporary humans engage in little physical exercise compared to the physically active lifestyles of ancestral hunter-gatherers. Prolonged periods of inactivity may have only occurred in early humans following illness or injury, so a modern sedentary lifestyle may continuously cue the body to trigger life preserving metabolic and stress-related responses such as inflammation, and some theorize that this causes chronic diseases. Cleanliness Contemporary humans in developed countries are mostly free of parasites, particularly intestinal ones. This is largely due to frequent washing of clothing and the body, and improved sanitation. Although such hygiene can be very important when it comes to maintaining good health, it can be problematic for the proper development of the immune system. The hygiene hypothesis is that humans evolved to be dependent on certain microorganisms that help establish the immune system, and modern hygiene practices can prevent necessary exposure