The long neck of modern-day giraffes evolved in several stages as one of the animal's neck vertebrae stretched first towards the head and then towards the tail a few million years later, scientists have found.
Scientists have long theorised that the long neck of modern-day giraffes evolved to enable them to find more vegetation or to develop a specialised method of fighting.
The new study of fossil cervical vertebrae shows the evolution likely occurred in several stages as one of the animal's neck vertebrae stretched first toward the head and then toward the tail a few million years later.
The study shows, for the first time, the specifics of the evolutionary transformation in extinct species within the giraffe family, researchers said.
"It's interesting to note that that the lengthening was not consistent," said Nikos Solounias, a giraffe anatomy expert and paleontologist at New York Institute of Technology College of Osteopathic Medicine.
"First, only the front portion of the C3 vertebra lengthened in one group of species. The second stage was the elongation of the back portion of the C3 neck vertebra.
"The modern giraffe is the only species that underwent both stages, which is why it has a remarkably long neck," Solounias said.
Solounias and Melinda Danowitz, a medical student in the Academic Medicine Scholars programme, studied 71 fossils of nine extinct and two living species in the giraffe family.
The bones, discovered in the late 1800s and early 1900s, were housed at museums around the world, including those in England, Austria, Germany, Sweden, Kenya, and Greece.
"We also found that the most primitive giraffe already started off with a slightly elongated neck," said Danowitz.
"The lengthening started before the giraffe family was even created 16 million years ago," she said.
The researchers analysed anatomical features of the various fossils and compared them to the evolutionary tree.
"That's when we saw the stages playing out," said Danowitz.
Solounias and Danowitz found the cranial end of the vertebra stretched initially around 7 million years ago in the species known as Samotherium, an extinct relative of today's modern giraffe.
That was followed by a second stage of elongation on the back or caudal portion around one million years ago. The C3 vertebra of today's giraffe is nine times longer than its width - about as long as an adult human's humerus bone, which stretches from the shoulder to the elbow.
As the modern day giraffe's neck was getting longer, the neck of another member of the giraffe family was shortening. The okapi, found in central Africa, is the only other living member of the giraffe family.
Yet, rather than evolving a long neck, Danowitz said this species is one of four with a "secondarily shortened neck," placing it on a different evolutionary pathway.
The study is published in Royal Society Open Science.