Abstract
The earliest tetrapods had hands and feet with up to eight digits but this number was subsequently reduced during evolution. It was assumed that lineages with more than five digits no longer exist but investigations of clawed-frogs now indicate that they posses a rudimentary or atavistic sixth digit in their hindlimb. A recent reevaluation of the stem tetrapod Ichthyostega predicts that its seven digits evolved from two different types of ancestral fin radials, pre-axial and post-axial. In this context we now ask the question, should we consider a pre-axial origin of the thumb as reason for its unique genetic signature?
Highlights
When the first tetrapods emerged from the water around 400 million years ago [1] their hands and feet looked quite different from the ones seen in modern day species
Most interpretations of the relationships between sarcopterygian fins and tetrapod limbs accept that the metapterygial axis runs through the ulna/fibula [11, 12] and that digits correspond to some type of post-axial radial [14, 15] (Fig. 1b)
If we look at the thumb - digit I- in the context of pre-axial versus post-axial digits, it is striking that numerous characters set it apart from the four posterior digits
Summary
When the first tetrapods emerged from the water around 400 million years ago [1] their hands and feet looked quite different from the ones seen in modern day species. For millions of years to follow, tetrapods had six digits until this changed to the canonical pentadactyl Bauplan at the end of the Devonian around 350 MYA [3,4,5] (a period whose tetrapods remain poorly known due to fragmentation of the fossil record [6]). This organization into a limb with five digits has proven extremely stable. Reductions are quite common (as in horses, pigs and birds) but supernumerary digits, beyond the “5”, are exceedingly rare and are only known from mutant or highly inbred domesticated animals [7]
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