A new paper from the New York Institute of Technology reports on fossil studies of the giraffe’s neck vertebrae which show that (press release):
…. 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’s authors say the research shows, for the first time, the specifics of the evolutionary transformation in extinct species within the giraffe family. ..
“It’s interesting to note that that the lengthening was not consistent,” said Nikos Solounias, a giraffe anatomy expert and paleontologist at NYIT 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.”
…. “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.” ….
….. 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.
Clearly one evolutionary pathway has given us the modern giraffe with its ridiculously long neck, but there was also a pathway which led to the shortening of the neck
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.
But I find the story that the elongation of the giraffes neck was due to natural selection, as a consequence of a survival advantage for longer necked individuals in time of drought, somewhat unsatisfactory. The idea that it was sexual selection at play (a longer neck providing the males with better fighting ability) is even more unsatisfactory.
Let us suppose that a prolonged drought led to all middle size shrubs dying out or at least becoming unavailable to ancestral giraffes through competition. It would have had to have been a very selective drought that allowed only grasses and very low shrubs to survive along with the leaves of taller bushes and trees. We need to remember also that for such a natural selection to work, all the shorter necked giraffes needed to die out and thus be de-selected. It is not impossible that the shorter necked animals were just crowded out by more efficient herbivores which led to the longer necks opening up a niche not available to the other herbivores. But I find the arguments for the extinction of shorter necked giraffes while other herbivores prospered somewhat unconvincing.
The evolutionary pathway to the longer neck is still a mystery.
Why not longer legs? I like this homage to Gary Larson by evolution-outreach.com
Brian Switek reviewed the various theories a few years ago and pointed out
… significant neck elongation began around 14 million years ago during the Late Miocene — after the lineage to which the relatively short-necked okapi split off — and by about 5 million years ago giraffes of modern proportions had evolved. …….. it appears that the elongation of giraffe necks occurred during a global pattern of aridification in which grasslands replaced forests.
For the moment, the question of “How did the giraffe get its long neck?” must be answered with “We do not yet know”
Switek wrote that five years ago – but it still applies.
We still don’t know.
The k2p blog 