animals themselves, and some of the key regulators of bilateral animal development exist in Cnidarians, which don't share that body plan. Other work has revealed that genetic networks of regulatory genes that are used in appendage and body plan specification probably predate the origins of either limbs or a body plan.
In short, the genetic tools were in place were in place for millions of years before the Cambrian, but it took the Cambrian's unique combination of environmental challenges and opportunities to force organisms to deploy them in new adaptive combinations.
Vanishing gaps in the vertebrate invasion of land
A key event in the origin of modern humans occurred in swamps nearly 400 million years ago. Prior to that time, vertebrates made do with fins and life in the water. Ted Daeschler of the Academy of Natural Sciences in Philadelphia reviewed our latest understanding of how those fish wound up on land, with limbs to propel them.
Which is a fish, which is a tetrapod? Trick question—they're all tetrapods.
Daeschler pointed out that a few decades ago, we had two species that don't even appear on the diagram here, Eusthenopteron and Icthyostega, and a big gap in between them. Although scientists always want to know more, the gap wasn't a huge problem for them, as they could recognize subtle features of skeletons that the lobe-finned fish shared with the earliest tetrapods. But it was a problem in terms of public relations, as the public had a hard time tracking these subtleties, allowing opponents of evolution to focus on the gap and declare it unbridgeable.
In the 1980s and 90s, other species, such as Pandericthys and Acanthostega, began to fill this gap. Daeschler indicated that a clear pattern emerged, one that linked the appearance of these species with a specific environment and one that represented a new ecological opportunity. All of the fossils were found on a band that, given the then-current arrangement of the continents, was equatorial. The specific environment, however, was one that hadn't existed previously: broad, alluvial valleys and flood plains that were transformed in the wake of the origin of trees. The recognition that this environment spurred tetrapod evolution has led directly to the discovery of Gogonasus and, perhaps the most famous transitional species ever, Tiktaalik.
In this environment, many of the features of the transitional species were preadaptive. Lobed fins aided maneuvering in a complex environment in the same way that limbs later did, in water or out. Muscle attachment sites in the bones of the fins worked equally well when used in legs. For paleontologists, the discoveries that filled the gaps revealed that this major transition occurred through a series of forms that were mosaic, with features added to the tetrapod repertoire in an order that's essentially random. Tiktaalik has a broad, fish-like snout, but the far end of its skull has been reordered to allow the first flexible neck seen in a tetrapod.
Two messages were clear from Daeschler's summary. The first is that there's so much left to discover; we don't know which gap will wind up filled next, just that gaps continue to be filled with rich information. He'll continue chasing road crews as they dig through Pennsylvania for as long as they'll let him. The second message is that it's time to let go of the false distinctions that are left over from Linnean times and only serve to confuse the public. In Daeschler's view, all of these animals, on both sides of the former gaps, are tetrapods. Some have limbs, some have fins, but they're clearly all part of a boundary- and gap-less transition.