Evolutionary baggage

Evolutionary baggage is the part of the genome of a population that was advantageous in past individuals but is disadvantageous under the pressures exerted by natural selection today.[1]

Origin

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.[2] 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.[2] In the case of humans, 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.[3] Other hypotheses link evolutionary baggage to genetic diseases. Since all organisms descended from a common ancestor, evolutionary baggage can be seen universally among organisms.[2]

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.[4] 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.[4] The condition has been linked to variation in the HLA genes.[4] Furthermore, there is genetic evidence of a high prevalence of the condition among Scandinavian individuals,[5] 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.

Diet

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.[6] 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.[6]

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.[6] 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.[6] 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.[7]

Sickle-cell and malaria

To understand the interaction between sickle-cell disease and malaria, here is a little background.

As a recessive gene, Sickle-cell disease is only present if homozygous, with no dominant gene to beat them out. Sickle-cell disease, originating in people living in tropical areas where malaria is prevalent, is a hereditary blood disorder characterized by rigid, sickle-shaped red blood cells.. The unusual shape and rigidity of these altered red blood cells reduces a cell’s ability to effectively travel with regular blood flow, occasionally blocking veins and preventing proper blood flow. Life expectancy is shortened for people with sickle-cell disease, though modern medicine has significantly lengthened the life expectancy of someone with this disease. As detrimental as the effects of sickle-cell disease seem, it also offers an unforeseen benefit; humans with the sickle-cell gene show less severe symptoms when infected with malaria, as the abnormal shape of blood cells caused by the disease hinder the malaria parasite’s ability to invade and replicate within these cells.[8] It is possible to have the sickle-cell allele, but not have the disease, for example if heterozygous.

Malaria, a mosquito-borne infectious disease of humans and other animals, is a potentially deadly disease that causes fever, fatigue, nausea, muscular pain, coughing, and, in extreme cases, coma and death. Malaria is caused by parasitic protozoans transferred through mosquito saliva into a person’s circulatory system, where they travel to the liver to mature. Though eliminated in the U.S., there were an estimated 219 million documented cases of malaria in 2010 according to the World Health Organization.[9]

Now, the correlation between sickle-cell disease and malaria is a double-edged sword. On the one hand, having a sickle-cell allele does limit the life expectancy of a person. On the other, the presence of sickle-cell genes reduces the detrimental effects of malaria should it be contracted. Natural selection allowed for the spreading of the sickle-cell gene in areas of high numbers of mosquitoes carrying malaria; those that weren't as susceptible to malaria were much more likely to live than those that were. However, as malaria is not as prevalent as it once was, the benefits of sickle-cell have eroded, leaving behind the detrimental effects of the disease.

Implications

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.

Notes

  1. ↑ Appenzeller, T. 1999. "Test tube evolution catches time in a bottle." Science. 284: 2108-2110
  2. ↑ 2.0 2.1 2.2 Thanukos, A. 2008. "Views from understanding evolution: parasites and pathogens take the leap." Evolution:Education and Outreach 1:25-28
  3. ↑ Spotorno, A.E. 2005. "Evolutionary medicine: an emergent basic science." Rev. Med. Chile. 133:231-240.
  4. ↑ 4.0 4.1 4.2 Moalem, S., K.B. Storey, M.E. Percy, M.C. Peros, and D.P. Perl. 2005. "The sweet thing about type 1 diabetes: a cryoprotective evolutionary adaptation." Med. Hypotheses. 65: 8-16.
  5. ↑ Wendorf, M.A. 1999. "Diabetes and enterovirus autoimmunity in glacial Europe." Med.Hypotheses. 52: 423-429.
  6. ↑ 6.0 6.1 6.2 6.3 O'Keefe, J.H. and L. Cordain. 2004. "Cardiovascular disease resulting from a diet and lifestyle at odds with our paleolithic genome: how to become a 21st-century hunter-gatherer." Mayo. Clin. Proc. 79: 101-108.
  7. ↑ Frassetto, L A; Schloetter, M; Mietus-Synder, M; Morris, R C; Sebastian, A (2009). "Metabolic and physiologic improvements from consuming a paleolithic, hunter-gatherer type diet". European Journal of Clinical Nutrition 63 (8): 947–955. doi:10.1038/ejcn.2009.4. PMID 19209185.
  8. ↑ Wellems TE, Hayton K, Fairhurst RM (September 2009). "The impact of malaria parasitism: from corpuscles to communities". J. Clin. Invest.119 (9): 2496–505.
  9. ↑ Nayyar GML, Breman JG, Newton PN, Herrington J (2012). "Poor-quality antimalarial drugs in southeast Asia and sub-Saharan Africa".Lancet Infectious Diseases 12 (6): 488–96.