How Giraffes Became Winners by a Neck
POSTED WED, 10/7/2015
Giraffes have taught generations of students how evolution works. Not directly, of course. Communicating throughnocturnal humming is a barrier to classroom instruction. But the modern giraffe – Giraffa camelopardalis – is often used as the textbook example of why Darwin and Wallace were right and Lamarck was wrong.
The setup goes something like this. Think of a little protogiraffe gazing hungrily at some tasty leaves high up on a tree. Someone from the Lamarckian school of evolution, the argument goes, might assume that the little giraffoid would stretch its neck to grab the lowest of those high leaves and, through exertion, develop a longer neck that it would then pass on to its offspring. Repeat for best results. A Darwinian, on the other hand, would expect the protogiraffes to vary in neck length and those that just happened to have slightly longer necks would be able to reach more food, survive longer, and mate often enough to pass on that variation to the next generation, who would play out the scenario over again.
While the scenario is a bit of a caricature of what Lamarck actually thought, it’s still useful in getting at the basic evolutionary equation that Darwin and Wallace independently distilled. Yet, despite the thought experiment’s popularity, we’ve known little of how the giraffe actually got its neck. Today’s tall browsers definitely evolved from shorter-necked ancestors, but how? A new study by New York Institute of Technology’s College of Osteopathic Medicine anatomist Melinda Danowitz and colleagues now provides an answer. [...]
Truly long-necked giraffes didn’t evolve until about 7.5 million years ago. Samotherium, Palaeotragus, Bohlinia, the extinct Giraffa sivalensis and the living Giraffa camelopardalis preserve enough transitional features to let Danowitz and colleagues reconstruct how this stretching occurred. It wasn’t simply a matter of drawing out their vertebrae as if they were in some sort of anatomical taffy pull. The front half of the neck vertebrae became elongated in Samotherium and Palaeotragus, generating forms intermediate between today’s Giraffa and their foreshortened predecessors. Then, within the last two millions years or so, the lineage leading up to the modern Giraffa elongated the back half of their neck vertebrae, giving them even more reach and making them literally at the top of their class.
If you could assemble all these fossil bits and pieces into a short film replaying giraffe evolution, you wouldn’t end up with the smooth transformation of a small-statured herbivore into a towering, checkered browser. There’d be starts and stops and side stories, the ending not being a goal but a happenstance..
Reference:
Danowitz, M., Vasilyev, A., Kortlandt, V., Solounias, N. 2015.Fossil evidence and stages of elongation of the Giraffa camelopardalis neck.
