The Bar-Headed Goose
Thomas H. Jukes
MILLIONS OF YEARS before George Leigh Mallory attempted to ascend Everest “because it was there,” a remarkable bird had found its way over Everest’s summit, thanks to winning a prize in evolution’s lottery. The bird was the bar-headed goose, truly a Reinhold Messner of the avian world.
High-altitude climbers depend absolutely on the oxygen supply to their muscles. This, in turn, depends on a substance that is called hemoglobin that is carried by the red blood cells. The evolution of hemoglobin is one of the remarkable stories of biology.
Our human hemoglobin started out about 500 million years ago, long before we appeared on the scene, as a single molecule of a protein containing about 140 amino acids. About 450 million years ago, a great event in evolution took place. This was when the hemoglobin gene duplicated and the two duplicates went in different directions. One of them became alpha hemoglobin and the other beta. Alpha and beta form a pair held loosely together, and this pair joins to another alpha-beta pair. The quartet of these four molecules has remained in the blood of all vertebrates ever since. Transition from a single molecule to a quartet changed the properties of hemoglobin so that it could more efficiently pick up oxygen in the lungs and release it in the tissues where it is needed for the production of energy in the muscular movement. The process was characterized by the Bohr effect. Oxygen combines readily with hemoglobin in the lungs where the acidity, or pH, is slightly higher than average. The oxygenated hemoglobin, abbreviated as HbO2, travels to the capillaries in the tissues where the acidity is slightly higher, which means that the pH is lower. Under these conditions, the oxygen is readily given up, and the HbO2 becomes Hb, or reduced hemoglobin, which is why our veins are blue. At high altitudes, oxygen in the air is lessened because of the lower barometric pressure, and the difficulty in obtaining enough oxygen stimulates the bone marrow to make extra quantities of red cells. The stimulus is provided by a hormone called erythropoietin, abbreviated as EPO, which has made a fortune in the stock market for its producer, the AMGEN Corporation, in the last year or two.
The amino acids in the hemoglobin molecule can become changed by mutations in DNA. Hundreds of hemoglobin mutants have been identified and described. Some of them make no difference, and others are harmful, producing genetic diseases.
This story is about a hemoglobin mutant that made a goose into a mountaineer several million years ago. I am writing about this because in August 1991 some German scientists reported that they had transformed human hemoglobin to be like the hemoglobin of the bar-headed goose which breeds around lakes in Tibet at an elevation of 4000 to 6000 meters. The goose migrates to the Plains of India, across the Himalaya and flocks have been known to fly over Mount Everest.
The fascinating part of the story is that, although most mutations in hemoglobin in humans and other animals are either harmless or deleterious, the mutations that took place in the bar-headed goose actually improved its hemoglobin as compared with that of its close relative, the non-mountaineering greylag goose, so that the mutant hemoglobin had a greater capacity for oxygen. When this happened to the ancestor of the bar-headed goose, the bird found it could fly higher and further than before, so that it could live in places where ordinary geese could not compete with it. The key mutation was changing from proline to alanine at the 119th position in the alpha hemoglobin chain. Several other mutations took place in other locations, but these had no effect on oxygen transport; they were so-called “neutral” mutations.
This sounds like an unusual and unique event, but, very remarkably, a similar but not identical mutation took place in the Andean goose, thus making it possible for this goose to fly at 9000 meters, far above the usual enemies of geese. This time, the mutation was from leucine to serine at position 55 of the beta chain.
At this stage of our story, the two unusual geese were merely scientific curiosities. Enter a new scientific discovery called site-directed mutations. This remarkable procedure makes it possible to remove an amino acid from its place in a protein and replace it with another. It is carried out through the genetic code. As an example, the sequence CCG in the hemoglobin gene can be replaced by GCG. This changes proline to alanine. The new genetically engineered human hemoglobin artificial mutants have been studied for their oxygen-carrying capacity, and, like those of the high geese, it is greater than regular hemoglobin.
Somewhere down the line, in the future, an expedition of Himalayan climbers will receive tranfusions of the new mutant hemoglobin to prepare them for ascending to 8000 meters, Perhaps, far above them, they will see a wedge-shaped flock of birds. These will be bar-headed geese.