Genes that may have been advantageous in the past may be critically unfit for individuals in today’s environment. Natural selection is not a perfect process; if an organism is “fit enough” to survive a particular environment and reproduce, its genes are passed on to the next generation. Some of these genes may increase an organism’s fitness while some may even be slightly disadvantageous. This seeming paradox is the origin of evolutionary baggage, which is the collectively inherited traits that evolved in a different environment from the present. The result is a “Paleolithic” genome, which is not adapted to today’s environment and is contributing to “diseases of civilization,” including atherosclerosis and obesity. Other hypotheses link evolutionary baggage to genetic diseases. Since all organisms descended from a common ancestor, evolutionary baggage can be seen universally among organisms.
Type I diabetes 
It has been hypothesized that genetically-linked type I diabetes was advantageous during the extreme cold temperatures of the Ice Age 14,000 years ago. Type I diabetes is deleterious now, but would have conferred an advantage for individuals to survive extremely cold temperatures; this condition prevents production of insulin, keeping blood sugar levels high in the blood, which lowered the freezing point of the blood and protected the individuals with the condition. The condition has been linked to variation in the HLA genes. Furthermore, there is genetic evidence of a high prevalence of the condition among Scandinavian individuals, who were the direct descendants of the Northern European populations that endured the most extreme conditions during the Ice Age. This genetic evidence suggests that type I diabetes was a favorable condition in the past, and can be considered evolutionary baggage in individuals today because it can cause severe health problems.
It is a fundamental biological concept that organisms are most fit if they eat the same diet which their evolution has adapted them to. Throughout history, humans adapted to a diet by living off the land. Today, humans maintain the same genes that have been evolved from our ancestors and adapted to the diet they consumed. This has created a mismatch between the modern diet and lifestyle (which has emerged in evolutionary terms very recently), and the diet and lifestyle to which the human body has slowly adapted to over thousands of years. This phenomenon can be considered evolutionary baggage because the deviation from the Paleolithic diet and lifestyle has led to “diseases of civilization” such as obesity, hypertension, and type II diabetes.
The human body adapted over time to crave calorie-dense foods and to store energy efficiently because our ancestors did not have abundant food; this is called the thrifty gene hypothesis. These adaptations are genetic baggage today, causing humans to store fat efficiently despite abundant food availability. This leads to obesity, which is an important risk factor for cardiovascular disease. However, studies have shown that consuming a Paleolithic-type diet rich in unprocessed foods and lean protein decreased blood pressure and cholesterol level while increasing glucose tolerance.
Evolutionary baggage from our ancestors may be causing disease in individuals today. In our ancestors, these phenotypes conferred selective advantage against harsh climate, infectious disease and scarce food. However, since these conditions are less common in today’s world, individuals with these phenotypes are at a selective disadvantage. This disadvantage has major implications in the area of health care because some of the diseases caused by evolutionary baggage are major contributors to the rising cost of health care in developed countries.
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