Cambrian Fossils -Part 2

Started by bstratton, 01/18/2003 03:01PM
Posted 01/18/2003 03:01PM Opening Post
The sudden appearance of animal structural types in the Cambrian period, and the lack of new types since 500 million years since the Cambrian is discussed in the book -What Evolution is, (2001), by Ernst Mayr, who is the professor Emeritus in the Museum of Comparative Zoology at Harvard University. he wrote on page 279:

" How can we explain the two great puzzles in the phylogeny of animals?

The first puzzle is the sudden appearance of 60 to 80 different structural tyes ( body plans) of animals in the early Cambrian, and the second puzzle is why no major new types originated in the 500 million years since the Cambrian.

It is now clear that the seemingly sudden origin ( within 10-20 million years) since the Cambrian ( beginning 544 million years ago) is an artifact of preservation. By use of the molecular clock, the origin of the animal types can be placed at about 670 million years ago, but the animals living between 670 and 544 million years ago are not preserved as fossils because they were very small and without skeletons.

The reason why no major new types originated in the ensuing 500 million years is more complex and only partly understood. However, molecular geneticics has led to an explanatory suggestion. Development is tightly controlled in the now living organisms by very precise "working teams" of regulatory genes. In the Precambrian, there were apparently only a few such genes, which did not control development as tightly as later on. This allowed a frequent occurrence of rapid major restructuring of the structural types. By the end of the Cambrian, the dominance of these regulatory genes had been fully established and the origin of completely new structural types had become difficult, if not impossible. One must always remember that the changes prior to the Cambrian did not occur suddenly, but over a period of several hundred million years , even though not documented in the fossil record."

Brien
Posted 01/18/2003 03:09PM #1
Part 2 appears to me to nearly answer part I, but here's more from pbs's Evolution Library on The Cambrian Explosion:

For most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and multi-celled algae. But beginning about 600 million years ago in the Precambrian, the fossil record speaks of more rapid change...

Then, between about 570 and 530 million years ago, another burst of diversification occurred, with the eventual appearance of the lineages of almost all animals living today. This stunning and unique evolutionary flowering is termed the "Cambrian explosion," taking the name of the geological age in whose early part it occurred. But it was not as rapid as an explosion: the changes seems to have happened in a range of about 30 million years, and some stages took 5 to 10 million years.

It's important to remember that what we call "the fossil record" is only the available fossil record. In order to be available to us, the remains of ancient plants and animals have to be preserved first, and this means that they need to have fossilizable parts and to be buried in an environment that will not destroy them.

...Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Once the body plans that proved most successful came to dominate the biosphere, evolution never had such a free hand again, and evolutionary change was limited to relatively minor tinkering with the body plans that already existed.