| IB 104 - Lecture 3 - Sponges to Cnidaria - 8/29/05 Evolutionary origins of animals Animals appeared on earth during the Ediacaran period , with all of the major phyla established by the Cambrian period The first question requires us to make the leap from single-celled eukaryotes to multicelled eukaryotes. The hypothesis most widely accepted today, although by no means proved, is that the ancestors of animals were colonies of protistans. First, there are alive today protistans that form colonies. These protistans are called choanoflagellates (collared flagellates, for their microvilli surrounding their single flagellum). Second, the choanoflagellates closely resemble cells found in one simple type of modern day animal, the sponges. To make this scenario work, we need to envision a floating hollow ball of cells that, over evolutionary time, gradually became more complex by developing different types of cells. ItÂÃôs worth noting that we believe plants evolved multicellularity independently of animals, hence the vast differences between plant and animal cell biology, physiology, and embryology (weÂÃôre not sure about fungi). Which also partly explains why botany and zoology have long been separate subjects, and why we still have separate plant and animal biology courses. The second question, did the transition from protistan ancestors occur only once, is another way of asking whether or not all animals alive today share a relatively recent common ancestor. The two hypotheses considered most seriously today are 1. one common ancestor and 2. two common ancestors. If one is more impressed by the overall similarities of animals, even those with very simple body designs, one might be more likely to envision that the protoanimal arose only once, and we are all descendants of that animal. If one reflects more strongly on the fundamental differences between the radially symmetric animals and the bilaterally symmetric animals, one might favor the two ancestor hypothesis. Most comparative biochemical studies today support the one ancestor hypothesis (see handout). | |
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