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Diseases and Human Evolution

Ethne Barnes

Early human groups would not have helped vector-borne parasites. Most insect-carried parasites need a constant supply of young hosts with immature immune systems. Early nomads didn't provide this reservoir of infection. Small human groups with few offspring, relatively short lifespans, constantly moving.

Old World where diseases evolved. Firstly a large number of domesticated animals, from which microbes crossed over. Secondly trade routes right across continents, allowing exchange and mix of microbes.

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As agriculture spread, those unwilling to settle down were forced to the margins. But each new innovation expanded the area that could be farmed. Such as ox-driven plough and markets in the new towns which encouraged surpluses for trade. Metal technology provided better tools (farm more land) and better weapons (take more land).

After the original disruption of humans catching animal diseases, eventually reach an equilibrium where microbe and host populations tolerate each other. The most susceptible die quickly, don't pass on their genes, and the most virulent form of the disease kills its host too quickly for the microbe to be passed on. So less susceptible hosts and less virulent diseases evolve in tandem.

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Malaria parasite reproduces by capturing hemoglobin in the blood. In response, different human groups have evolved 400 hemoglobin variants:
- where malaria doesn't occur, get no hemo variants.
- because very heavy evolutionary pressure, many variants appear, and inevitably mutations are either very dangerous (kill the baby) or partly dangerous (bad consequences if both mother and father carry gene variant)
- partial protection from the mutated version, which blocks malaria, and normal version produces hemoglobin you need.
- sickle cell anaemia mutation changes shape of hemo cell and shortens life span of each cell so parasite can't complete development
- but if have 2 copies of mutant gene, short lifespan means you don't get enough blood oxygen, which causes anemia, and unusual shape blocks small capillaries blocking oxygen and causing tissue damage
- classic Mendelian punnet square distribution: 25% severe hemo deficiency; 50% partial deficiency + protection; 25% normal hemo, no protection.
- 5 different mutations each produce different versions of the sickle cell, meaning this has evolved in 5 different populations, mostly in Africa.

A different mutation evolved in the Levant, and is carried by 50% of Kurds. This mutation blocks an enzyme called G6PD. Normally G6PD protects hemo molecule from oxidants which remove oxygen it is carrying. The G6PD deficient populations relax hemo molecule so malaria parasite can't attach. Downside is that makes carriers vulnerable to any oxidants, so they can't use standard anti-malaria drugs. And they are extremely allergic to food like fava beans, with even the pollen being enough to trigger massive self-destruction of red blood cells.

Most of today's human diseases originated from our livestock. We are the descendants of the survivors - those who evolved resistance, or at least tolerance, to the animal diseases. But unprotected New World populations devastated by new diseases when settlers arrived.

Tb or tuberculosis, probably evolved in voles (on basis that they survive infection unaffected). Cattle eat grass contaminated with vole droppings. Infects their lungs, they cough, and the aerosol form spreads easily to other cows when they are herded together for night protection. And then spreads to humans who share same sleeping area. Humans then spread disease the same way, by coughing. Greater the concentration of people/animals, more transmission.

Not everyone who is exposed, sickens. If 20 healthy people exposed, 10 will catch the infection, but only 1 will develop disease (genetically varied resistance). But if have a compromised immune system - if very, old, very young, underfed, or by AIDS - then trouble. Lies dormant for years, waiting for immune system to let down its guard, and then kills off lung cells - the disease that used to be called consumption.

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