[Arnab]

Nasif posed the following question:

3. Why does all animal species appear all of a sudden in Cambrian period? "The beginning of the Cambrian period, some 545 million years ago, saw the sudden appearance in the fossil record of almost all the main types of animals (phyla) that still dominate the biota today". (from Science). What happened to gradual evolution?

Responses:

1. Chris Nedin of the department of Geology, Adelaide University in Australia writes in talk.origins:

What most people think of as the "Cambrian Explosion" is, in actual fact, not a sudden burst of life, but a rapid increase in the number of fossils found in the fossil record. This is because around this time organisms started to mineralize their exoskeletons using the abundant calcium and carbonate from the surrounding seawater. Previously to this, organisms had entirely organic exoskeletons similar to many insects today. This type of exoskeleton is not easily preserved and usually decays too rapidly to survive as a fossil. The much tougher mineralized exoskeletons preserved far better, resulting in a large increase in the number of fossils.

The second claim usually made about the "Cambrian Explosion" is that most if not all of the major animal groups came into existance at this time. This claim is not correct. It is almost certain that such major groups as annelids (worms), cnidarian (corals and jellyfish), gastropods (snails) and probably arthropods have a pre-Cambrian history.

It should be pointed out that almost all pre-Cambrian fossils have no hard parts such as mineralized exoskeletons, and as such they are very unlikely to be preserved.

While there was a rapid (over a 5-10 million year period) diversification of animal life during the "Cambrian Explosion", this was a diversification from an already existing stem stock of organisms, which were soft bodied and thus underrepresented in the fossil record.

What we see in the fossil record are representatives of all the major groups which possess mineralized body parts. This record funnels back to the Early Cambrian where most of the groups apparently disappear. This disappearance does not represent the origin of the group, as some would suggest, but the origin of mineralized hard parts. The groups continue to exist below their occurrence in the fossil record, but they appear to be absent because they have no hard parts and are not fossilised.

The "Cambrian Explosion" represents what we call a taphanomic boundary, that is, it represents a large increase in the chances of organisms to be fossilised (by having hard parts) and hence appear in the fossil record. It does not represent the origin of those groups.

Source: www.talkorigins.org/origins/feedback/jun97.html


2. An editorial in Takorigins:

The reader asserts that "macroevolution by any known naturalistic processes is impossible." That's quite an unsupported assertion, but unfortunately for the reader, the scientific evidence proves him wrong. Presumably the reader is aware of the fact that the geological record shows widespread and significant change in the types of organisms that have existed throughout the planet's history. For over three billion years, the planet was entirely populated by single celled organisms. There were no mammals, no fish, and no trees. Then, during the Cambrian explosion approximately 550 million years ago, dozens of new forms appeared, some with fossilizable hard body parts that allowed us to witness the birth of some of the Earth's earliest multicellular organisms. With a few exceptions, these pioneering invertebrates that appeared in the Cambrian were bizarre by today's standards. For instance, the giant Anomalocaris was a strange-looking creature vaguely reminiscent of a squid, but with two odd feeding arms protruding from the bottom of its head.

Anomalocaris, like many of the other Cambrian organisms, does not fit into any of the broad categories, known as phyla, that characterize modern organisms. Nevertheless, while today we wouldn't recognize most of the organisms from the Cambrian period, it is believed that all but one modern phylum had an early representative in that period.

Later on came a further diversification of life, which Stephen Jay Gould has described as "variation on set anatomical themes" established during the Cambrian explosion. Fish emerge 500 million years ago; primitive sharks emerge 375 million years ago; amphibians, insects and ferns emerge 350 million years ago; reptiles emerge 300 million years ago; primitive crocodiles and mammals emerge 200 million years ago; dinosaurs go extinct 65 million years ago; many modern mammal species emerge between 65 and 20 million years ago; primitive bipedal hominids with small brain capacities emerge 4 million years ago; the first primitive humans emerge 1.6 million years ago. Life has obviously done a lot of changing over time.

If the reader was taught that the fossil record proves Darwinism true because it presents a progression from simple to complex, then he was taught wrong, and he misunderstands Darwinism. The fossil record does not progress from simple to complex, nor is Darwinian selection a theory of such progress. Darwinian natural selection is simply a progress-neutral mechanism for adaptive change.

When I pointed out earlier that evolution predicts that organisms with recent common ancestors will share a large amount of their genetic material, the reader responded that "intelligent design" also predicts genetic similarity.

But why does intelligent design predict this? Couldn't an omnipotent creator just as easily have chosen to make all his animals with completely divergent genetic makeups? Unlike evolutionary theory, "intelligent design" would explain divergent genomes just as easily as it explains similar genomes.

Concepts like creationism that can explain everything explain nothing. Evolution, which is supported by numerous and independent lines of evidence, is a robust scientific theory that is falsifiable and makes predictions.

Source: www.talkorigins.org/origins/feedback/nov96.html

3. From “Introduction to Evolutionary Biology,” Version 2, Copyright © 1996-1997 by Chris Colby:

Animals start appearing prior to the Cambrian, about 600 million years ago. The first animals dating from just before the Cambrian were found in rocks near Adelaide, Australia. They are called the Ediacarian fauna and have subsequently been found in other locales as well. It is unclear if these forms have any surviving descendants. Some look a bit like Cnidarians (jellyfish, sea anemones and the like); others resemble annelids (earthworms). All the phyla (the second highest taxonomic category) of animals appeared around the Cambrian. The Cambrian 'explosion' may have been a result of higher oxygen concentrations enabling larger organisms with higher metabolisms to evolve. Or it might be due to the spreading of shallow seas at that time providing a variety of new niches. In any case, the radiation produced a wide variety of animals.

Some paleontologists think more animal phyla were present then than now. The animals of the Burgess shale are an example of Cambrian animal fossils. These fossils, from Canada, show a bizarre array of creatures, some which appear to have unique body plans unlike those seen in any living animals.

The extent of the Cambrian explosion is often overstated. Although quick, the Cambrian explosion is not instantaneous in geologic time. Also, there is evidence of animal life prior to the Cambrian. In addition, although all the phyla of animals came into being, these were not the modern forms we see today. Our own phylum (which we share with other mammals, reptiles, birds, amphibians and fish) was represented by a small, sliver-like thing called Pikaia. Plants were not yet present. Photosynthetic protists and algae were the bottom of the food chain. Following the Cambrian, the number of marine families leveled off at a little less than 200.

The Ordovician explosion, around 500 million years ago, followed. This 'explosion', larger than the Cambrian, introduced numerous families of the Paleozoic fauna (including crinoids, articulate brachiopods, cephalopods and corals). The Cambrian fauna, (trilobites, inarticulate brachiopods, etc.) declined slowly during this time. By the end of the Ordovician, the Cambrian fauna had mostly given way to the Paleozoic fauna and the number of marine families was just over 400. It stayed at this level until the end of the Permian period.

Source: www.talkorigins.org/faqs/faq-intro-to-biology.html