The Cambrian explodes and fish get limbs
The Cambrian explosion
A Halwaxiid, poised to explode into
three branches of animal life
Douglas Erwin of the Smithsonian then made the jump to the origin of modern animal life during the Cambrian explosion—as he put it, "3 billion years of boredom later..." Erwin presented the Cambrian explosion as a matter of three big ideas as well: biological challenges, ecological opportunities, and developmental potential. The biological challenges of the era are pretty obvious: in his view, the global glaciations that left glacial sediments in the tropics and are likely to have shut down or severely limited the global carbon cycle.
These changes, however, were accompanied by two events that, in Erwin's view, were essential enablers of a broad radiation of species. Both had the common feature of allowing many organisms access to a resource that was essentially unlimited, and thus free from competition. The first of these was oxygen, which reached unusually high levels in the Cambrian atmosphere. Results published while this article was in preparation reveal that the radiation of animal life closely tracked oxygenation of the ancient ocean. The second was nutrient-rich sediments. Tracks from the first burrowing animals appear in sediments just prior to the Cambrian, and Erwin argues that these animals kept resources in the sediments circulating within the biological community long after they would have otherwise settled out and been buried.
This Cambrian jellyfish probably shared many genes with the first bilateral animals
Erwin also described animal life that was poised to explode. The prior era was filled with the Ediacaran Fauna, which he described as, "no eyes, no appendages, lots of fronds, and maybe some guts." But that era also generated fossil embryos that suggest that bilateral animals predated the Cambrian. More telling, however, has been the findings of modern genomics and evo-devo. Genomic studies reveal that many of the genes involved in producing complex animals predate 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 existe