Parading across many an African skyline is the iconic giraffe with its long neck, a sturdy extension that lets it munch on treetop foliage. Despite the extraordinary length of its neck, the giraffe has only seven vertebrae in its neck, the same number as other mammals. The giraffe's cervical vertebrae articulate with ball-and-socket joints, making the giraffe's neck extraordinarily flexible. In the giraffe each cervical vertebra is supersized—10–11 inches long in an adult. The whole bony chain of huge cervical vertebrae is anchored by extra-thick ligaments to bony extensions on the thoracic vertebrae near the shoulders. That ligamentous support and the fact that the front legs are taller than the rear enable a giraffe to effortlessly balance an 8-foot tall, 500-pound neck and hold its head high. The giraffe's comparably supersized heart generates a blood pressure 2.5 times that of humans to push blood all the way up to the animal’s brain. Yet when the giraffe dips its neck to drink or graze on groundcover, the sudden onrush of high-pressure blood is diverted into a marvelous network of vessels to keep it from flooding the giraffe’s head. Meanwhile, tight skin around the legs and thickened vasculature throughout the giraffe’s body prevents its high blood pressure from damaging structures below the neck. Everything about the giraffe is designed to work together to support its high-flying head.
Genetic Underpinning of a Grand Design
Geneticists have recently sequenced the giraffe genome and that of its short cousin, the okapi, and compared them to other mammals. Having identified genes that differ dramatically in giraffes, scientists believe they've located the genetic blueprint responsible for many of the giraffe's unique features.
Despite the extraordinary length of its neck, the giraffe has only seven vertebrae in its neck, the same number as other mammals. Photo courtesy of Taylor Pool. Evolutionary scientists believe these differences, the genetic underpinning of what makes a giraffe a giraffe, are the result of random mutations accumulated over millions of years. They believe the giraffe evolved from a common ancestor shared with cattle by way of these mutations. Commenting on the study, David Haussler of the Genomics Institute at the University of California, Santa Cruz, says, "This is one more wonderful demonstration of the power of comparative genomics to connect the evolution of animal species on this planet to molecular events that we know must underpin the extraordinary diversity of life on this planet."
But did random molecular events generate the diversity of life on earth? Have scientists now discovered the evolutionary path giraffes followed in their supposed divergence from a common ancestry with cattle? Is a giraffe just an evolutionary modification of a primitive cow? Let's have a look.
The researchers found that, compared to other mammals, the giraffe has multiple distinctive differences in 70 of its genes. Two-thirds of these are regulatory genes—genes that govern the expression of other genes. These regulatory genes govern skeletal, cardiovascular, and nervous system development as well as physiologic functions. "All of these genes in the giraffe—we have them ourselves," says Douglas Cavener, who led the study. "What made giraffes unique is just to tinker with them a bit and alter them in subtle ways."