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In fact, the 'ocelloid' within the planktonic predator looks so much like a complex eye that it was originally mistaken for the eye of an animal that the plankton had eaten."It's an amazingly complex structure for a single-celled organism to have evolved," said lead author Greg Gavelis, a zoology PhD student at UBC. "It contains a collection of sub-cellular organelles that look very much like the lens, cornea, iris and retina of multicellular eyes found in humans and other larger animals." [Emphasis added.]
It is perhaps the most extraordinary eye in the living world -- soextraordinary that no one believed the biologist who first described it more than a century ago.Now it appears that the tiny owner of this eye uses it to catch invisible prey by detecting polarised light. This suggestion is also likely to be greeted with disbelief, for the eye belongs to a single-celled organism called Erythropsidinium. It has no nerves, let alone a brain. So how could it "see" its prey?
"The internal organization of the retinal body is reminiscent of the polarizing filters on the lenses of cameras and sunglasses," Leander says. "Hundreds of closely packed membranes lined up in parallel."
Scientists still don't know exactly how warnowiids use the eye-like structure, but clues about the way they live have fuelled compelling speculation. warnowiids hunt other dinoflagellates, many of which are transparent. They have large nematocysts, which Leander describes as "little harpoons," for catching prey. And some have a piston -- a tentacle that can extend and retract very quickly -- with an unknown function that might be used for escape or feeding.
Nevertheless, the genomic and detailed ultrastructural data presented here have resolved the basic components of the ocelloid and their origins, and demonstrate how evolutionary plasticity of mitochondria and plastids can generate an extreme level of subcellular complexity.
These examples demonstrate the wealth of subcellular structures and associated light-receptor proteins across diverse microbial groups. Indeed, all of these examples represent distinct evolutionary branches in separate major groups of eukaryotes. Even the plastid-associated eyespots are unlikely to be the product of direct vertical evolution, because the Chlamydomonasplastid is derived from a primary endosymbiosis and assimilation of a cyanobacterium, whereas the Guillardia plastid is derived from a secondary endosymbiosis in which the plastid was acquired 'second-hand' by intracellular incorporation of a red alga. Using gene sequences recovered from the warnowiid retinal body, Gavelis et al. investigated the ancestry of this organelle by building phylogenetic trees for the plastid-derived genes. Their analysis demonstrated that this modified plastid is also of secondary endosymbiotic origin from a red alga.Although derived independently, there are common themes in theevolution of these eye-like structures. Many of them involve thereconfiguration of cellular membrane systems to produce anopaque body proximal to a sensory surface, a surface that in four of the five examples probably involves type 1 rhodopsins. Given the evolutionary derivation of these systems, this represents a complex case of convergent evolution, in which photo-responsive subcellular systems are built up separately from similar components to achieve similar functions. The ocelloid example is striking because it demonstrates a peak in subcellular complexity achieved through repurposing multiple components. Collectively, these findings show that evolution has stumbled on similar solutions to perceiving light time and time again.
The work sheds shed new light on how very different organisms can evolve similar traits in response to their environments, a process known as convergent evolution. Eye-like structures haveevolved independently many times in different kinds of animals and algae with varying abilities to detect the intensity of light, its direction, or objects."When we see such similar structural complexity atfundamentally different levels of organization in lineages that are very distantly related to each other, in this case warnowiids and animals, then you get a much deeper understanding of convergence," Leander says.
Typically, in the rush to present the major features of the scientific landscape, most of the arguments required to achieve such knowledge are excised. Consequently, science can appear to its students as a monolith of facts, an authoritative discourse where the discursive exploration of ideas, their implications, and their importance is absent (7). Students then emerge with naïve ideas or misconceptions about the nature of science itself -- a state of affairs that exists even though the National Research Council; the American Association for the Advancement of Science; and a large body of research, major aspects of which are presented here, all emphasize the value of argumentation for learning science (8-10).
You're innocently walking down the street when aliens zap away the sensory neurons in your legs. What happens?
"We usually get lots of vigorous debate on this one," Leupen said. (For curious readers, the answer is d).
- Your walking movements show no significant change.
- You can no longer walk.
- You can walk, but the pace changes.
- You can walk, but clumsily.
What Is a Suggested Plan for Teaching a Unit on Neo-Darwinian Evolution?
Objective education means that students must be allowed to form and express their own opinions. An objective unit covering neo-Darwinian evolution might look something like this:
Most public school curricula stop after step 1, missing out on the benefits from steps 2 and 3. Some might claim those extra steps would take too much time. But teaching the modern neo-Darwinian theory of evolution in an objective fashion need not take any more time than the 2-3 weeks typically spent on an evolution unit.
- First, cover the required curriculum by teaching the material in the textbook. Ensure that students understand the scientific arguments for neo-Darwinian evolution. (1-2 weeks)
- Next, spend a few days discussing scientific criticisms of neo-Darwinian evolution. The supplementary textbook Explore Evolution, the DVD Investigating Evolution, and the Icons of Evolution Study Guide are potential resources. Encourage students to think critically. (2-3 days)
- Finally, consider allowing students to spend a couple days wrestling with the data and forming their own opinions. This could include in-class debates, or an assignment where students write a position statement on neo-Darwinian evolution. In these exercises, students may defend whatever position they wish, but must justify it using only scientific evidence and scientific arguments. (1-2 days)
More importantly, any extra time taken to teach this topic objectively is not wasted -- it will help students better understand the evidence, better appreciate scientific reasoning, and fulfill standards requiring critical thinking and use of the inquiry method. Finally, this approach will be welcomed by students who find this topic engages their interest in science.
They share a basic body plan characterized by a tentacleless and octaradial body with an oral-aboral axis, eight rigid struts (termed here "spokes") radiating from the aboral end and arched to converge to the oral end, eight soft-bodied flaps or lobes supported by the spokes, eight pairs of ctene rows, a conspicuous apical (or aboral) organ walled by eight rigid platesand housing a spheroidal or ellipsoidal statolith, and an oral region surrounded by eight apiculate lappets...The eight arcuate spokes [in one species] bear robust spines (Fig. 2, L to N, and figs. S4 to S6) and retain their structural integrity even when disarticulated, suggesting a remarkable degree of sclerotization. [Emphasis added.]
The occurrence of sclerotized and armored skeletons inCambrian representatives of several animal groups -- including entoprocts, phoronids, lobopods, scalidophorans, and now ctenophores that are exclusively soft-bodied among modern survivors -- is a remarkable phenomenon. The independent skeletonization among these diverse Cambrian animals provides indirect evidence for an intensified level of ecological interactions (for example, arms race) and also highlights the importance of paleontological data in illuminating theevolutionary legacy that would be otherwise inaccessible by studying living animals alone. The widespread occurrence of skeletonization echoes Stephen Jay Gould's view of the striking morphological disparity of many animal phyla during their Cambrian debut, and the contrasting evolutionary trajectories of skeletonized cnidarians and ctenophores also elucidate the contingent fate of evolutionary innovations such as skeletonization.
Here, we report several sclerotized and armored ctenophore species, based on new material and reinterpretation of previously published material from the early Cambrian Chengjiang biota (ca. 520 Ma). Along with armored Cambrian entoprocts, phoronids, lobopods, and scalidophorans, the new fossils suggest a vanished Cambrian history of skeletonization in multiple animal groups, imply the ecological importance of skeletonization in theCambrian explosion, and highlight the remarkable morphological disparity in certain Cambrian animal clades relative to their modern survivors.