Lab 8: Darwin's Dilemma

One of the most spectacular events in the history of life is the Cambrian explosion, which is also one the great mysteries of science. This was a moment of geological time when complex animals first appeared on earth fully formed, without evidence of any evolutionary ancestors. Charles Darwin viewed this as an inexplicable mystery. He envisioned the evolution of life through a multitude of small, undirected steps. Yet, the fossil record reveals no such pattern of gradual development. Instead, early in the Cambrian period compound eyes, articulated limbs, sophisticated sensory organs, and skeletons burst into existence seemingly out of nowhere.

Hallucigenia

The fossils of the Burgess Shale, more than half a billion years old, preserve an intriguing glimpse of early life on Earth. These fossils merit special interest for several reasons: they were buried in an underwater avalanche of fine mud that preserved exceptionally fine details of the structure of their soft parts. Only hard parts are preserved in most other Cambrian deposits, limiting information within the geologic record; they represent an early snapshot of the complexity of evolving life systems. The Burgess Shale fossils as a group have already developed into a variety of sizes and shapes from the much simpler, pre-Cambrian life forms; many of them appear to be early ancestors of higher forms - from algae to the chordates (a major group of animals that includes human primates). Other appear unrelated to any living forms and their later disappearance presents an intriguing mystery.

The Burgess Shale Site 510 Million Years Ago

From this picture it is easy to imagine how an avalanche of fine mud sliding down from the submerged reef top would have carried off any animals living in the shallow reef waters above. This avalanche could have caught some animals in mid-water and certainly would have overwhelmed and buried any creatures living at its base. The hard parts of all these animals caught in the mudslide were preserved as fossils, like the process at any other Cambrian site. However, here the fine mud also penetrated and filled all available spaces and crevices within the animals, thus preserving the shapes and locations of all the soft parts. 

A very busy place!

Cambrian Map

Take a careful look at this world map. Does China (3) lie due north of Africa? Is China really that small!? Why is Africa northeast of Europe? When is Europe way below the equator, two-thirds of the way to the South Pole? According to current interpretations of the paleogeographic record this is how the continental blocks were arranged about one-half billion years ago - before plate tectonics (which are still active) rearranged them into today's familiar positions.