L'origine de vos doigts

 

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In science, however, knowledge is not written in stone. It is subject to change in light of fresh evidence. New discoveries can necessitate revision of the textbooks. Our recently described Elpistostege fossil, which was unearthed in 2010 at the UNESCO World Heritage Site of Miguasha in Quebec, is one such find. It is not a new species of elpistostegalian. Rather it is the original founding member of the band. But this time we have a complete, perfect specimen. And it has led us to propose a different theory of how fingers evolved and gave rise to the vertebrate hand structure that persists in the more than 33,800 species of tetrapods alive today, including humans.

(...) When Clement eventually began segmenting out the pectoral fin, we were at the edges of our seats. As the first complete elpistostegalian pectoral fin ever discovered, it was certain to contain vital clues to the transition from fins to limbs. The preliminary results did not disappoint. They not only confirmed Béchard's initial CT results suggesting the presence of extra bones in Elpistostege's fin but also showed those bones in far greater detail. Now we could see that the fossil included an unexpected series of many small, tightly packed bones. Typically the end of the pectoral fin skeleton contains small bones called radials that support the rodlike fin rays. The bones evident in this part of Elpistostege's fin were in the right place to be radials, but the large number of bones and the way some of them were arranged in discrete rows suggested that they were something else. We strongly suspected that these never-before-seen bones hidden in the pectoral fin of this ancient fish were actually digit bones similar to the ones found in the fingers of tetrapods. We identified two digits that were each composed of multiple, articulated bones, as well as three possible digits each composed of a single bone.

(...) Why might a fish benefit from being able to maneuver in this way? The skull of Elpistostege contains a clue: at the back of the head is a pair of large holes called spiracles. Some modern-day fishes that breathe air have similarly large spiracles. For a long time, the function of these holes was uncertain. In a study published in 2014, Long worked with a team of ichthyologists at Scripps Institute of Oceanography in La Jolla, Calif., led by the late Jeff Graham, to nail it down. Analyzing the spiracles of the living bichir Polypterus, we showed that they are instrumental in breathing air. Assuming the spiracles served the same purpose in Elpistostege, the ability to employ the fins to do a push-up in the shallow rivers and estuary this fish inhabited—and thereby get the head out of water for a breath of fresh air—could have been advantageous.

Elpistostege was not necessarily restricted to the aquatic realm, however. Today's lungfish and some catfishes can propel themselves along land for short periods with their fins. With its far more powerfully built fins, Elpistostege was probably that much more capable of venturing ashore.