Web Bit 17-1: Are Humans Still Evolving?
By Mary K. Miller
Will future generations look at the archaeological evidence and think 21st-century humans as strange looking as we consider the Neanderthals to be? Are human bodies and brains still evolving?
The evolution question boils down to this: Is our genetic makeup changing over time? Some experts on human evolution say that our culture and technology have overwhelmed biology and stabilized the gene pool. In essence, they say we've overcome the pressures of natural selection and environmental influences and brought evolution to a grinding halt. Other biologists say that the environmental and technological changes we've created are actually increasing the forces that drive human evolution, creating genetic change from generation to generation at a rate unprecedented in human history.
The problem is that evolution is devilishly hard to measure in the short term. It also depends on whether you view the process as ANY genetic change over time ("microevolution") or ONLY the creation of a new species ("speciation" or "macroevolution"). All agree that we are almost certainly not headed toward the creation of a new human species any time soon. The last time that happened was more than 100,000 years ago, when our ancestors underwent a major genetic upheaval, and anatomically modern humans, known as Homo sapiens, appeared on the scene.
How does Darwin's rulebook apply to Homo sapiens? According to Darwin, evolutionary success means producing offspring who themselves reproduce. Natural selection favors individuals that are genetically best adapted to their environment. These individuals pass along their genes to future generations. Whether you're a pea plant or a human, you're an evolutionary dead end if you fail to reproduce.
Genetics are, however, often subtle. Particular traits, such as the genes responsible for sickle cell disease or other inherited diseases, may give an advantage to individuals who inherit only one copy of the gene. If a genetic trait provided no benefit to the individual, it would eventually disappear from the gene pool.
What about survival of the fittest? If you die before reaching adulthood, you cannot reproduce. In the past, one of the biggest killers of children was infectious disease (still a leading cause of death in many developing countries). Those that made it to reproductive age had robust immune systems or genetic resistance to pathogens. In many cases, these pathogens can drive human evolution as they evolve along with us.
One of the most interesting case histories involves malaria and sickle cell disease. Sickle cell is a genetic disease, caused by a single mutation of a gene involved in red blood cell production. It's relatively common in African-Americans; about 10 percent of the population carry the sickle cell gene and 1 in every 400 babies will inherit two copies of the gene and develop sickle cell disease. When oxygen levels drop in the blood stream of a patient, their red blood cells change from round and plump to sharp and sickle-shaped. These sickle-shaped blood cells get tangled up, forming clumps in the spleen and blood vessels and causing intense pain and tissue damage.
Sickle cell disease is a serious, sometimes deadly, condition that should have disappeared from the gene pool under natural selection. Yet it's very common, even more so in Africa than in the U.S. Why should the gene be preserved in such high proportions?
Here's where the malaria pathogen enters the picture. Malaria is passed to humans by the bite of a female Anopheles mosquito, which delivers a tiny infectious microbe known as a plasmodium. Malaria infects red blood cells and is one of the leading causes of death in many developing countries, claiming more than one million lives a year.
Before the advent of agriculture in Africa, some 10,000 years ago, much of the continent was covered with dense forests, and the Anopheles mosquito was rare. When early farmers began clearing forests for planting crops or grazing cattle, however, they created the perfect environment for mosquitoes. In the open fields, pools of standing water allowed Anopheles to breed. The mosquito population exploded, went looking for a blood supply, and found it in nearby farmers and their families.
Along with mosquitoes came the malaria-causing plasmodium. Its presence began to exert a selective force on humans, favoring those with resistance to the disease. Humans with at least one copy of the sickle cell gene have such protection from malaria. When the malaria parasite enters a blood cell with the sickle trait, it depletes the cell's oxygen reserves and causes the blood cell to distort into its characteristic shape. The sickle-shaped (and infected) cell is more easily filtered out from the blood stream by the spleen, which keeps the infected cell from bursting open and spreading to other blood cells. The spleen is also an area where higher concentrations of white blood cells can identify malaria-infected cells and destroy them.
Within a few thousand years after agriculture arrived in Africa, the sickle cell trait had spread among farmers there. In the present population of West Africans, 20 percent are carriers of the sickle-cell gene, a case where culture and a pathogen have combined to change the proportions of different genes—the essence of microevolution.
In general, natural immunity from infectious disease may be on the decline, at least in the industrial world. Modern medicine, in the form of vaccines and antibiotics, has done much to subdue many human pathogens such as smallpox, meningitis, and polio. That means, however, that we're increasingly dependent on medical technology to prevent deadly infections. With medicine neutralizing natural selection, we're no longer selecting for disease-resistance traits. That means that natural resistance from pathogens could eventually disappear from the human genome; another example that our species is still undergoing microevolution, if not always in the direction we would choose.