The evolution likely occurred in two stages as one of the animal’s neck vertebrae stretched first toward the head and then toward the tail a few million years later, according to a new study of extinct giraffids’ cervical vertebrae.

The study, published online in the journal Royal Society Open Science, shows the specifics of the evolutionary transformation in extinct species within the giraffe family.

“First, only the front portion of the C3 vertebra lengthened in one group of species,” explained study senior author Dr Nikos Solounias from the New York Institute of Technology College of Osteopathic Medicine.

“The second stage was the elongation of the back portion of the C3 neck vertebra.”

“The modern giraffe (Giraffa camelopardalis) is the only species that underwent both stages, which is why it has a remarkably long neck,” he said.

Dr Solounias and his colleagues at the New York Institute of Technology College of Osteopathic Medicine studied 71 fossils of nine extinct and two living species in the giraffe family.

“We also found that the most primitive giraffe already started off with a slightly elongated neck,” said co-author Melinda Danowitz.

“The lengthening started before the giraffe family was even created 16 million years ago.”

But the main discovery came after the scientists analyzed anatomical features of the various fossils and compared them to the evolutionary tree.

They found the cranial end of the vertebra stretched initially around 7 million years ago in an extinct relative of giraffe known as Samotherium.

That was followed by a second stage of elongation on the back or caudal portion around one million years ago.

As the modern day giraffe’s neck was getting longer, the neck of another member of the giraffe family, the okapi (Okapia johnstoni), was shortening. The species is the only other living member of the giraffe family.

“Yet, rather than evolving a long neck, this species is one of four with a secondarily shortened neck, placing it on a different evolutionary pathway,” Dr Danowitz said.

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Melinda Danowitz et al. Fossil evidence and stages of elongation of the Giraffa camelopardalis neck. Royal Society Open Science, published online October 7, 2015; doi: 10.1098/rsos.150393