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Sometimes, I hold on to a link just because I find it interesting. Here's one such, from ars technica: Why are vertebrate eyes so different from those of other animals? A new hypothesis proposes that our ancestors lost their eyes, then rebuilt them. Okay, well, they didn't "rebuild" them, but I'll allow for some poetic license. Still, lots of people misunderstand evolution: there's no agent behind it, and "rebuild" smacks of agency. After losing its original eyes, one of our distant ancestors may have done what evolution does best: tinkered with what was available, reshaping a single central visual organ into two new eyes. I'll also emphasize that this is a hypothesis. It's not a certainty. That means there may be some evidence to support it, but the evidence could mean something else entirely. So, you know, if you're the kind of person who gets invited to cocktail parties (I am not), it's best not to present this as settled science. According to the data considered by its authors—a team from the University of Sussex (UK) and Lund University (Sweden)—vertebrate eyes, ours included, may not descend directly from the paired eyes of early bilaterian animals. Instead, they may have been “reinvented” from what was once a single light-sensitive organ that survived an evolutionary detour. I think most people have at least a vague idea of what a vertebrate is: an animal with a body plan that features a spine, which includes true fish, amphibians, reptiles, birds, marsupials, and mammals; and excludes ones like insects, octopuses, jellyfish, and politicians. “Vertebrate eyes are so fundamentally different from the lateral eyes of other animal groups,” explains Dan-Eric Nilsson, senior author of the study from Lund University and a leading expert in eye evolution. We think spider eyes are weird and scary. But to the spider, we're the weird, scary ones. It always amused me as a kid to see the "elephant is scared of mouse" trope in cartoons, when humans being scared shitless of spiders is about the same thing. “The key difference is the identity of the main photoreceptor, which is of ciliary nature in the vertebrate eye but rhabdomeric in other animal groups, such as arthropods and cephalopods,” he adds. Oh, yeah, that clears things right up, like Visine after a good night. Look, I don't claim to be a genius or anything (except when I'm being smug), but I'm not completely stupid. I just don't know everything, and a lot of that is specialized biology lingo. To understand what Nilsson is getting at, we need to unpack a few key concepts. Fortunately, the article recognizes that and is about to 'splain things. There are two major classes of light-sensitive photoreceptor cells—rhabdomeric and ciliary—that differ in shape, in the visual pigments (opsins) they contain, and in their electrical responses to light. Somehow, I don't remember that being covered in the few biology classes I took. But it tracks with what I've learned since. Most invertebrates rely on rhabdomeric photoreceptor cells for vision, while ciliary cells mediate light sensing but not vision—they generally help regulate internal biological clocks. Vertebrates, however, brought both types of photoreceptors into the same organ. So, even the expert's explanation with all the lingo was incomplete. The authors argue that the invertebrate, rhabdomeric-based arrangement represents the ancestral state of eyes, inherited from the common bilaterian ancestor and shared by present invertebrates. I don't think the article explains "bilaterian." By the word, I'd think it would mean something like "midline symmetrical;" in the case of vertebrates, the spine runs along the symmetry line. And I'd be mostly right. After the bilaterian lineage split—one branch giving rise to insects, crustaceans, and mollusks, the other leading to a group called deuterostomes that includes chordates and vertebrates—one of our distant ancestors appears to have become more sedentary. That's right, that's why I really saved the article: so I could joke about how I'm an evolutionary throwback with my sedentary nature. “The ancestral deuterostome adopted a burrowing lifestyle, either living sessile on the seafloor or partially burrowed, with only parts of its body protruding,” says George Kafetzis, research fellow at the University of Sussex. Under those conditions, two lateral eyes may have become more of a liability than an advantage. “Neural tissue in general is very expensive to maintain and function,” Kafetzis explains. As a result—an idea already proposed in the literature—the lineage may have gradually lost its paired eyes. I'm not 100% sure, but I think this is analogous to how certain species who adapted to caves lost their vision. “We think that in this early deuterostome, the median eye contained both ciliary and rhabdomeric cells,” Kafetzis explains. As a result, both cellular lineages were incorporated into a single, ancient, cyclopean eye, which later evolved into the vertebrate eyes. I mean, okay, it's a hypothesis. What nonscientists might call a "theory," but it's not a theory in the scientific sense. A trace of this transformation may still survive in the pineal complex at the base of the brain—often referred to as a vertebrate “third eye.” Mystics like to make a big deal out of this "third eye" thing. Unfortunately, the band name is already claimed. Scientists have long recognized striking similarities between the retina and the pineal organ, leading many to suspect that the two evolved from a single ancestral structure, with the pineal representing a more rudimentary version. Kafetzis and his colleagues see it differently. Oh-HO! I "see" what you did there. The article explains things further; then: Though grounded in existing ideas and data, the new proposal offers a potentially far-reaching synthesis. Several aspects still require firmer evidence. The idea that the ancestral chordate adopted a burrowing lifestyle remains debated, and the claim that early bilaterians already possessed paired lateral eyes is still speculative. Additionally, I didn't see any proposals for how the hypothesized single cyclopean eye might have evolved into the kind of bilaterally symmetric eyes most of us see, and see with, every day. The article does emphasize the hypothetical nature of this claim, and it's not something that we can "do our own research" on. We'll just have to let the slowly grinding gears of science to do their thing: either falsify it, or support it enough to turn it into a theory. In other words, we just have to wait... and see. |
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