Understanding Evolution - a one-stop source for information on evolution.

2016-01-11. Ancient Start of Animal Evolution Wasn't Delayed by Low Oxygen. By Cody Sullivan, Earth & Space Science News (EoS; AGU).

2010 July 26. Translating Stories of Life Forms Etched in Stone. By Sean B. Carroll, The New York Times. Excerpt: …The difficulty posed by the Cambrian Explosion was that in Darwin’s day (and for many years after), no fossils were known in the enormous, older rock formations below those of the Cambrian. This was an extremely unsettling fact for his theory of evolution because complex animals should have been preceded in the fossil record by simpler forms… but we now have fossil records from the time immediately preceding the Cambrian. The rocks reveal a world whose oceans were teeming with a variety of life forms, including primitive animals, which is certainly good news for Darwin.
…This once-worrisome gap in the fossil record is a period of intense interest to geologists as well as paleontologists. The former have even given it its own division in the geological timescale. The Ediacaran Period, from 635 to 542 million years ago, is the first new geological period to be named in more than a century. Moreover, geologists have developed some intriguing theories about how dramatic changes in the Earth’s climate and chemistry during the Ediacaran may have allowed for the evolution of animals
…But finding these fossils has posed many new mysteries. Many of the creatures are so unlike modern forms that deciphering what they are and how they lived continues to challenge paleontologists. Prof. Andrew Knoll of Harvard University has likened the Ediacaran forms to a paleontological “Rorschach” test because different scientists often interpret the same fossil very differently.
…The kinds of animals that paleontologists have been especially eager to identify in the Ediacaran are those with bilateral body symmetry, the feature characteristic of the majority of modern animal groups, including ourselves.
…Recent chemical analyses of Ediacaran sediments reveal that the deep ocean lacked oxygen before and during that ice age, then became much richer in oxygen and stayed that way after the glaciers melted. That sharp rise in oxygen may have been the catalyst to the evolution of animals, including our ancestors.

2008 October 17. Evolution Primers. National Center for Science Education - articles on evolution.