The transformation from fish fins to feet is thought so difficult to achieve that it only happened once.[1]  Estimates for the time it took to complete the transition, start to finish, are limited to 15 million years or less.  This is because 15 million years is the distance from the earliest Frasnian to the latest Famennian, i.e. – from 378-363 million years ago.  Rich fossil beds display abundant flesh-finned fish such as Eusthenopteron and Panderichthys in the earliest Frasnian, but no legged-animals; yet 15 million years later in the late Famennian, fully developed legged-animals are abundant.  Leading tetrapod expert Clack estimated that the transition happened in less than 5-10 million years, because footprints and trace fossils of legged animals Obruchevichthys and Elginerpeton appear in the late Frasnian, 5 million years before the well-defined forms Acanthostega and Ichthyostega become plentiful.[2]  Others see a slightly longer transition, from 9 million years[3] to 12-15 million years.[4]  Hence, by all estimates, fish with fins transformed into land-walking creatures with well-defined and fully functional legs in 5 to 15 million years or less.  The possibility that it happened in an even shorter time is still open, as a flood of freshly discovered fossil tetrapods are still turning up.  The first important transformation from fins to feet is noticed in Panderichthys, the most primitive fossil form to adapt its arm bone into an immobile platform to support the body.[5]

     This 5 to 15 million years is a short period of time when compared to the amount of time it took for natural selection to perfect legs.  From the first appearance of legs in the fossil record, until the time legs evolved into efficient running machines, is about 135 million years.  The first animals with legs were extremely slow and cumbersome on land.  This was because legs were not positioned directly under the weight of the body, but were flanged out to the sides.  Thus, the earliest four-legged animals could only waddle.

     Also, legs were rather short for a long period of time, failing to lift the head much above the ground.  If a species could have evolved the ability to grow long legs, it would have proven a distinct advantage, for long legs would have enabled that species to eat herbs forbidden to animals of a lower height.  As it was, legged-vertebrates did not even evolve the ability to eat plants at all until the latter Carboniferous.  One would think, since they grew legs in less than 15 million years, they should have evolved a digestive system to cope with plants in less than 40 million years, but such was not the case.  Height could have also protected animals' necks from the bite of predators.  Under these selective pressures, we might expect that tall animals should have evolved fairly quickly.  After all, if it only took 5 or 15 million years for evolution to turn a fish fin into a leg, it should not take much longer for evolution to make the leg more efficient.  Yet this did not happen.  Land animals stayed low to the ground, even the largest of them barely able to lift their heads much more than a couple feet off the forest floor.  Their legs flanged out to the sides, inefficiently supporting their body weight, and slowing them down. 

     These inefficiencies were retained for about 80 million years, at which time modest improvements were made by the mammal-like therapsids.  Yet the therapsids' legs were still flanged out to the side, and they failed to reach the height necessary to eat herbs high in the trees.

     The archosaurs achieved a breakthrough about 250 million years ago when they attained the ability to place their weight directly under their legs, rather than having their legs splayed out to the sides.  The descendents of the archosaurs, the dinosaurs, perfected this ability with the evolution of a socket joint in the hip capable of both swift speeds and the ability to stand on just two feet.  They also added length to their legs for greater height.  Hence, it was not until the time of the dinosaurs that legs finally reached their full potential in terms of speed, strength, height, and efficiency.  The first dinosaurs evolved about 230 million years ago – roughly 135 million years after legs first evolved.  Thus, despite their comparatively rapid evolution from fish fins, legs retained grave inefficiencies for 135 million years.

          The paradox is inconsistency in the rates of evolutionary progress.  It took only 5 to 15 million years for fish fins to turn into legs, yet it took another 135 million years for unsteady waddling legs to become sure-footed fast running legs.  The greater morphological change occurred in the shorter period of time, and the lesser morphological change occurred in the longer period of time.  If natural selection required a whopping 135 million years to make adjustments to an existing form, then why did it require only 5 or 15 million years to invent a radically different form?  It is a matter of a strange discrepancy in the speed of evolutionary change.  If natural selection works only slowly over time, as genetics suggests, then it must be asked what force besides natural selection causes accelerations in the amount of evolutionary change?