Mid-term exam take-home question

Due 8:00 am Tuesday, October 16

Biology 303-11010

You will be able to answer this question satisfactorily based on good lecture notes alone. You may if you wish, however, look for additional sources of information. Be careful to use reasonably up-to-date scientifically-valid literature.

The fossil record indicates that most phyla arose between 600 and 500 million years ago, many at approximately the Precambrian-Cambrian boundary. It is also late in the Precambrian that we find our first organisms with mineralized skeletons, and such skeletal material quickly becomes more abundant at the beginning of the Cambrian Period.

Some genetic evidence, using “molecular clocks,” suggests that the origin of most animal phyla occurred more than 600 million years ago; in fact, some studies suggest it occurred closer to a billion years ago. Molecular clocks, however, involve some assumptions about constancy of genetic change that may not hold true.

There are two main points of view:

1) The animal phyla really did originate late in the Precambrian and early in the Cambrian. The fossil record is good enough to tell us that animal phyla had not diversified before the latest Precambrian.

2) The animal phyla arose long before the Precambrian-Cambrian boundary, but all phyla were small and soft-bodied for several hundred million years, leaving little or no fossil record. Perhaps something about ocean chemistry allowed mineralized skeletons to be created more easily by the Cambrian Period, and the potential for skeletonization created a better fossil record of certain groups and perhaps provided a stimulus for diversification of body form with these phyla.

There is not a strong scientific consensus for either point of view.

To do: Use a bulleted table of pros and cons of your own construction for these two different points of view, using what you know about the preservation of fossils and the diversification of early animal life. Based on your list, which hypothesis do you favor?

Please type your answer and turn it in at the start of your mid-term exam on Tuesday, October 16. Don’t forget your name!