Contrary to popular misconceptions, the history of life shows no steady Darwinian march of progress (sometimes called survival of the fittest). Instead, we find:

- first, there is the origin of one-celled forms of life, which occurred quite early in our planet's history (3.5 billion years ago). 

- long periods of stasis, during which life forms undergo few changes. For example, the period between the origin of simple forms of life such as cyanobacteria and more complex life forms seems to have lasted about 3 billion years (!). Complex life on Earth is only about 600 million years old.

- sudden "explosions" of new body plans or capabilities, involving vast multiorgan, multisystem changes, which result in many new types of life forms. Many people have heard of the Cambrian explosion, discussed in some detail in The Design of Life, during which

the vast majority of the known animal phyla (over 95 percent) appeared within an exceedingly brief interval of geological time (lasting, according to current estimates), a maximum of 5 to 10 million years. Thereafter, apart from a few exceptions, new animal phyla stop appearing throughout the geological record. (P. 63)

- mass extinctions. The trilobite - so numerous that it was the signature fossil of the Cambrian period (from about 524 mya) - went extinct, along with about 90% of Earth's species, in the massive Permian extinction. The once dominant trilobites are last found in the record at 251 mya. Their closest living relative is the horseshoe crab (which is not actually a crab).

Why is the Ediacaran explosion a puzzle?

Before the Cambrian, there was another much less well known explosion, the Ediacaran (575 mya to 542 mya). In general,

The Ediacaran life forms differ substantially from those of the Cambrian (see figure 3.6). Many of them are so strange that it is not clear if they can properly be regarded as animals. Ediacaran fossils consist of disc- and frond-like organisms. Some paleontologists think they are closer to lichens than to animals. (p. 65, The Design of Life)

Because the Ediacaran creatures are so little known, the significance of their sudden appearance and disappearance is often overlooked: Many scientists have been hoping to find a smooth, orderly transition from the earliest cyanobacteria to the Cambrian creatures, precisely the sort of transition that Darwin's theory of evolution predicts. But the Ediacarans are not only no help to their theory, they are actually quite a setback. An entire complex fauna came into existence quite suddenly (in terms of geological time), and just as suddenly disappeared. Worse, the Ediacarans are NOT ancestors of the Cambrians.

These weird and wonderful Ediacarans were in the news recently. They were called the "Avalon explosion", after a peninsula in Canada's province of Newfoundland, whose Mistaken Point has provided a rich trove of Ediacaran fossils.

Researchers Bing Shen, Lin Dong, Shuhai Xiao, and Michal Kowalewski analyzed Ediacaran fossils and discovered yet another remarkable fact. They had started with a conventional assumption, which Charles Darwin would certainly applaud, that the earliest Ediacaran fossils, the Avalon group, would be simpler and less diverse than the later ones, the White Sea and the Nama group. (These fossil groups are named for the areas where they are found.)

But the researchers' assumption turned out to be mistaken. As recounted in Science News Daily,

Surprisingly, however, as shown by Shen and colleagues, these earliest Ediacara life forms already occupied a full morphological range of body plans that would ever be realized through the entire history of Ediacara organisms. "In other words, major types of Ediacara organisms appeared at the dawn of their history, during the Avalon Explosion," Dong said. "Subsequently, Ediacara organisms diversified in White Sea time and then declined in Nama time. But, despite this notable waxing and waning in the number of species, the morphological range of the Avalon organisms were never exceeded through the subsequent history of Ediacara."

In other words, no Darwinian evolution had occurred. To their credit, the researchers highlight the problem rather than talking around it:

"The explosive evolutionary pattern was a concern to Charles Darwin, because he expected that evolution happens at a slow and constant pace," said Shuhai Xiao, associate professor of geobiology at Virginia Tech. "Darwin's perception could be represented by an inverted cone with ever expanding morphological range, but the fossil record of the Cambrian Explosion and since is better represented by a cylinder with a morphological radiation at the base and morphological constraint afterwards."

Darwin reckoned that there should be long and hidden periods of animal evolution before the Cambrian Explosion, Xiao said.

But paleontologists have not found such evidence, and recently scientists have learned that biological evolution has not been moving on a smooth road.

Actually, it's worse than that. There was no road between Avalon and Cambria. The most remarkable thing about Avalon life is that it strutted its strange stuff a while and then, as far as we know, just disappeared, as did the trilobite and the dinosaur.

It is becoming increasingly apparent that a serious theory of evolution must account for the massive input of complex information into life forms at specific times and places - due to causes we don't yet know.

See also "The Avalon explosion of Ediacaran morphospace" for British physicist David Tyler's view, with accompanying diagrams. Here's the original paper's citation and abstract: The Avalon Explosion: Evolution of Ediacara Morphospace Bing Shen, Lin Dong, Shuhai Xiao, Michal Kowalewski Science 319, 4 January 2008: 81-84. Abstract: Ediacara fossils [575 to 542 million years ago (Ma)] represent Earth's oldest known complex macroscopic life forms, but their morphological history is poorly understood. A comprehensive quantitative analysis of these fossils indicates that the oldest Ediacara assemblage - the Avalon assemblage (575 to 565 Ma) - already encompassed the full range of Ediacara morphospace. A comparable morphospace range was occupied by the subsequent White Sea (560 to 550 Ma) and Nama (550 to 542 Ma) assemblages, although it was populated differently. In contrast, taxonomic richness increased in the White Sea assemblage and declined in the Nama assemblage. These diversity changes, occurring while morphospace range remained relatively constant, led to inverse shifts in morphological variance. The Avalon morphospace expansion mirrors the Cambrian explosion, and both events may reflect similar underlying mechanisms.