The Cambrian Explosion - Essay Example

The beginning of the Cambrian saw an explosion of new life forms on Earth. The diversity of life that appears in the fossil record during this time is unique in several ways. First the number of unique species that appeared is unparallel in geologic time. Over the space of a few tens of millions of years, more species appeared that at nay other time in the history of the Earth. Another unique feature of the early Cambrian deals with the diversity within each existing family and indeed, within the species themselves. The Cambrian radiation is truly a singular event in our planets history, which helps to explain why there is so much interest in studying it. Several questions are currently perplexing scientists concerning the Cambrian “explosion” as it is often referred to. Much recent study has occurred in an effort to discover the engine or engines that drove such rapid evolutionary change for animals all over the Earth. Some theories seem to look at the big picture, while others attempt to look for pinpoint explanations or even causation for the evolutionary change. One big picture idea centers on the sigmoid curve that is associated with much evolutionary change throughout history. Indeed, the sigmoid curve has been associated with everything from business practices to learning within the classroom. When someone refers to a “learning curve”, they are actually referring to a sigmoid curve. In essence, a sigmoid curve indicates the passage of time from left to right. The line on the chart measures the amount of achievement. At the beginning, the line has a low slope, but then reaches a point where the run of the slope increases rapidly, only to level off again. Some feel that the Precambrian represents the low-slope of the evolutionary sigmoid curve. No fossils exist from the Precambrian, but we know there must have been biological materials present. The only explanation for the Cambrian “explosion” that does not involve antecedent biota would contain the spontaneous creation of life by a supernatural power. This is unacceptable from an evolutionary standpoint, so the prior existence of biological life in the Precambrian must be a given. The Cambrian therefore is represented on the sigmoid curve as the steep-sloped line that eventually levels off. Very little time actually elapses compared to the areas of flat slope, but much change occurs. This theory holds that, in essence, it took all of the Precambrian for biota to “learn” how to evolve. Once these preconditions were accomplished, a rapid time of evolutionary change occurred that leveled off again when conditions changed and the evolutionary “lessons” learned by the animals of the earth no longer applied to the new environment or conditions. The weakness of this argument is the fact that the Cambrian “explosion” has no equal. There is no evidence of a series of sigmoid type curves in the fossil record. Hot on the heels of the Cambrian radiation is the Ordovician. During this time fossils continued to show increasing levels of specialization and diversity. Although all of the basic components were established during the Cambrian, the Ordovician may serve to complicate the sigmoid type of explanation for the Cambrian “explosion”. The question of concerning the Ordovician centers on determining if it is a separate event from the Cambrian radiation or if it is an extension of the Cambrian. If it can be determined that it is a separate event, then the sigmoid pattern does not hold. We would have had two time periods of rapid growth that sere not separated by the appropriate length of low-slope development. If the Ordovician is, however, simply and extension of the Cambrian then it is actually a part of the same steep-slope on the sigmoid curve and the theory remains intact. Aside from the big picture sigmoid explanation of why the Cambrian “explosion” occurred, there are many more specific questions that need to be answered. Scientists are concerned with finding out not just why so many species developed during the Cambrian, but also why the disparity between species was so great. Disparity refers to the morphological changes present between species. The time of onset is also a concern for many scientists. The sigmoid design explains this simply by saying that enough time had lagged for the animals to “learn” evolution. But still questions of time exist. Why did the Cambrian “explosion” start when it did and last as long as it did? Questions also center on explaining why the Cambrian “explosion” has not been duplicated again in geologic time, even though it appears as though certain events such as mass extinctions may have provided ideal situations for further evolutionary “explosions”. The missing fossils linking the early Cambrian to their antecedents in the Precambrian are also missing. In an effort to answer some of these questions, a series of hypothesis have been proposed over the years. Some of these proposals are stronger than others given our current understanding of the science. These explanations are roughly divided into two categories. One is a series of environmental explanations while the other centers on developmental abilities of the animals involved in the evolution. The leading environmental theory centers on the level of available oxygen in the atmosphere. The argument is made that once oxygen levels reached a critical threshold, then rapid evolution could occur. The problem with this theory is that the level of oxygen necessary to support life on land appears to have been available long before the Cambrian. Why didn’t evolution happen sooner? Other major geologic events such as a period of “snowball earths” and methane burps caused by true polar wandering appear to be linked to the Precambrian/Cambrian boundary. No one is sure, however, why these global geological phenomenon might have triggered rapid evolution. Developmentally, some point to the idea of canalization as the reason for the Cambrian “explosion”. Following the idea of the sigmoid curve, this idea suggests that with each morphological change, the animal lessens the breadth of possible future morphological changes. As a result, the changes become for focused and at a certain point, rapid evolutionary change is possible because many of the evolutionary possibilities have been eliminated. In essence, the Precambrian was a time when too many evolutionary possibilities were possible, but as time went on, the possibilities were narrowed, resulting in rapid evolutionary changes during the Cambrian. Many of the current questions about the Cambrian “explosion” do not have ready answers. Other theoretical and ecological theories are being postulated and investigated. What is known is that the Cambrian was an exciting time of rapid evolution. The causes of this evolution are not entirely understood. Bibliography Droser, M. L. "The Ordovician Radiation: A Follow-up to the Cambrian Explosion?" Integrative and Comparative Biology 43.1 (2003): 178-84. Print. Marshall, Charles R. "Explaining The Cambrian "Explosion" Of Animals." Annual Review of Earth and Planetary Sciences 34.1 (2006): 355-84. Print. Read More