Evolution? The Sickle Cell Enigma
In 1954 Dr. Anthony Allison published that people who have one gene for a mutated form of hemoglobin are protected from getting malaria, a disease that kills a million people every year. Therefore, this change in the DNA, which leads to sickle cell trait, has been touted as a positive mutation, particularly for people who live in areas where malaria is endemic. It is thought that this beneficial effect is why 40% of Africans and 10% of African Americans have a defective hemoglobin gene. Of course, children born to those individuals, otherwise known as sickle cell carriers, have a one in four chance of having two genes coding for the disease. This means that they will suffer from full blown sickle cell anemia, which is an extremely debilitating disease that decreases average life expectancy to 45 years of age. The mutation somehow renders red blood cells “sticky” so they clog up the blood vessels, leading to pain, breathlessness, fever, stroke, and organ failure. Seems to be a no win situation. If a person has the mutation, they are protected from malaria, but they also run the risk of having offspring who will suffer from a painful and ultimately deadly disease.
An article in the Nature magazine reports that researchers have now discovered how the hemoglobin mutation prevents malaria. A bit of cell biology is necessary at this point–please refer to the diagram, which illustrates one method whereby cells transport their products from the place where they are manufactured (ER) and refined (Golgi) to the cell surface. Basically, the cell builds bridges made of actin, puts the product in a vesicle, and moves the vesicle to the cell surface, where it is released in response to a signal. A simple analogy is a country. Products for export are manufactured and refined inside the country. They may then be loaded into railway cars and trains; the boxcars run along railway tracks to the borders using diesel as fuel. Of course, completion of the export process may require other communication.
So, how does this apply to malaria and sickle cell trait? Apparently, the usual course of infective action is that the parasite will enter a red blood cell and cause it to build a bridge (made out of actin) to transport a sticky substance or protein to the surface of the cells. But, in people with sickle cell trait, the mutant hemoglobin prevents the building of the bridge, so that the vesicles that normally carry the protein to the cell surface float freely within the cell. They never get to the surface. The product is not exported because the railway lines could not be built; there was a pile of rubble in the way. This prevents the red blood cells from becoming sticky and clogging the vessels. Thus, sickle cell carriers are resistant to malaria.
Some would assert that this is an example of evolution in action. After all, the organism has mutated to increase fitness for its particular environment. But, does being a sickle cell carrier, which can be symptomatic in some circumstances and gives a 25% chance of having sick offspring, really increase fitness? And why, in the many, many years that man has been plagued by the malarial parasite, have we not evolved a type of resistance that does not cause illness or has the parasite not evolved a way to overcome these mutations? For an excellent analysis of these and other questions, read The Edge of Evolution by Dr. Michael Behe who, by the way, compared positive mutations to blowing up bridges way back in 2007. Almost prophetic!