Saturday, March 20, 2010

The Cambrian Explosion

On pages 28-29, Jerry Coyne describes the wide span of time across which evolution has occurred, using geologic eras and periods as markers. He states that from 3.5 to 1.5 billion years ago, the only life forms were prokaryotic bacteria, which were followed by the first eukaryotic single-cell organisms. However, around 600 million years ago, there was a so-called "explosion" of multicellular organisms such as worms, jellyfish, and sponges that appear at a very distinct point in the fossil record.
Why did multicellular organisms suddenly develop during this "Cambrian Explosion"? Consider a wide variety of factors in your answer, and be sure to incorporate outside information. In addition, why were bacteria and single-cell organisms able to survive after the Cambrian Explosion?


  1. There are a few theories as to the Cambrian explosion. About 700 million years ago, there was a large increase in atmospheric oxygen, which provided "fuel for movement and the evolution of more complex body structures" ("The Cambrian Explosion"). So once there was enough oxygen in the atmosphere, multicellular creatures were free to evolve and develop.
    Also, some scientists theorize that the Cambrian explosion marked the completion of the evolution of the "genetic tool kit", and once DNA & co. was established, organisms had a much easier time evolving.
    In addition, at the beginning of the Cambrian explosion "the large supercontinent Gondwana, comprising all land on Earth, was breaking up into smaller land masses" (Fossil Museum). The breaking up of land produced lots of new habitats and niches and allowed a large increase in diversity.
    Another theory is that there was no explosion at all, but that species had been evolving at a slow and steady rate all along and we simply lack the fossil evidence to show this.

    As for bacteria, the reason they survived so long is because of their enormous adaptability and incredibly short generation times - one experiment shows 50,000 generations in just 22 years. Natural selection can act on them very quickly and produce sturdy bacteria that survive regardless of any explosion of life or change in conditions.

    fossil museum:

    generation statistic:

  2. The sudden increase in complex organisms during the Cambrian Explosion could be explained by a variety of factors. One possibility is that the End-Ediacaran Extinction (an event that can be seen in the sudden change in surface sea water composition between the Cambrian and Ediacaran periods that is linked to mass-extinction events)opened up a wide variety of ecological roles, and thus, various clades diversified to fill those roles in the wake of the mass- extinction. However, over time competition between organisms in the same niche drove some organisms to extinction and thus brought diversification down again. Coyne mentions this idea of organisms branching out to fill the ecological roles available to them to avoid competition, one example being the diversification of the Hawaiian honeycreeper in beak sizes to adapt to different roles. As time progressed, those with progressively more precisely structured beaks relative to the niche were left alive while the less attuned did not have as high a rate of survival and reproduction. This situation in the Hawaiian islands is a easy to understand analogy, "with species filling niches occupied by very different species on continents or continental islands" (102). Another possible explanation for the event could be the development of certain key traits that would greatly have accelerated the radiation in diversity among organisms. One of these could be the introduction of key genetic material (such as the HOX genes) that brought with it the development of bilateral symmetry, hard shells or bones, and rapid locomotion. With the rise of these genes coupled with multicellular organisms to work with (more possibility for the application of those genes in comparison to the earlier single celled organisms), greater complexity would have emerged and thus resulted in a sudden increase in diversity. Another factor that could also work together with this newfound complexity is the increase in predator-prey dynamics; the introduction of traits like rapid locomotion or hard shells could have driven "an escalating evolutionary process that led to the development of teeth and claws" (Science Daily). This introduction of unfavorable circumstances for prey species as predators became more efficient at killing would have caused greater diversification as many traits were introduced that increased chances of survival and reproduction. Another theory (brought forth in the book "In the Blink of an Eye" by Andrew Parker) surmises that one particular trait, the evolution of a complex eye, was the true driving force behind this sudden increase in diversification and complexity as a result of accelerated predator-prey dynamics. Each of these theories could be a possibility, either individually or in tandem, in explaining the sudden explosion of multicellular organisms during the Cambrian period. In terms of why bactera were able to survive with the introduction of these new complex organisms, the answer would likely lie with their remarkable ability to form mutations quickly (with the uptake of foreign dna etc) coupled with extremely rapid reproduction leading to an increased ability to adapt to changing environmental conditions. Another possibility is that these particular single celled organisms' niches simply were not contested by these new arrivals.

  3. As Brian stated, the Cambrian explosion was precluded by a series of mass extinctions, possibly caused by the mass-melting of glaciers to kill off many species of organisms(A Neoproterozoic Snowball Earth). This opened up several ecological niches for new organisms to fill. One thing that biologists believe could have been a catalyst for the Cambrian explosion, as Nanna said, is the rise in oxygen levels prior to the Cambrian explosion. Higher oxygen levels could have allowed for the formation of adaptations of systems which allowed for metabolic functions using O2(Oxygen-Collagen Priority and the Early Metazoan Fossil Record). While the rise of oxygen levels in the atmosphere doesn't provide evidence for the sudden rapid speciation of organisms, it does show that it allowed for the formation of organisms that used O2 in respiration. A second theory to the origin of the Cambrian explosion is that mutations in the Hox genes of certain organisms were able to cause the relatively rapid formation of many different species in a short time period (Hox genes in brachiopods and priapulids and protostome evolution). Mutations in Hox genes could have changed the way organisms developed, thus producing new types of organisms rapidly compared to a the normal rate of evolution. The Cambrian explosion could have come as a result of all three of these occurrences, as well as other possible factors. Bacteria have been able to survive to this day because they didn't simply evolve into a more complex organism and then not exist anymore; bacteria have evolved themselves, as well as having other species diverge from their lineage. The bacteria today are not the same as bacteria that were our common ancestors back in evolutionary history, but have changed through natural selection and evolution. Another reason that bacteria and prokaryotes haven't become extinct is that they are vital to the earth's ecosystem. As well as having symbiotic relationships with other organisms, prokaryotes also are a major part of the world's primary producers; without the primary production of bacteria and prokaryotes, all other living organisms would have no source of nutrition and energy. The extinction of prokaryotes would mean the collapse of every food web in every ecosystem on earth, making life the way we know it impossible.