No. 60: Nov-Dec 1988
We risk supersaturating our readers with the anomalies of evolution, but we simply cannot bypass an article that is introduced as follows:
"An analysis of the fossil record reveals some unexpected patterns in the origin of major evolutionary innovations, patterns that presumably reflect the operation of different mechanisms."
The most interesting "unexpected pattern" is the gross asymmetry between the diversification of life in the Cambrian explosion (about 440 million years ago) and that following the great endPermian extinction (a little over 200 million years ago). Biological innovation was intense in both instances; both biological explosions burst upon a life-impoverished planet. Many niches were unoccupied. Even so, all existing (and many extinct) phyla arose during the Cambrian explosion and none followed the Permian extinction.
"...why has this burst of evolutionary invention never again been equaled? Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla?"
Some evolutionists blame the asymmetry on the different "adaptive space" available in the two periods. "Adaptive space" was almost empty at the beginning of the Cambrian because multicellular organisms had only begun to evolve; whereas after the Permian extinction the surviving species still represented a diverse group with many adaptations. (Just how the amount of "adaptive space" available was communicated to the "mechanism" doing the innovation is not addressed.) Scientists contemplating these matters, however, seem to concur that microevolution, which supposedly gives rise to new species, cannot manage the bigger task of macroevolution, in particular the creation of new phyla at the beginning of the Cambrian.
(Lewin, Roger; "A Lopsided Look at Evolution," Science, 241:201, 1988.)