Help Me Understand

[TheRocketSurgeon]

TRS,


Thank you for the links.  I have seen similar things before, and did I mention, that I hate drawings presented as evidence.  It does seem to be more of the same.  When similar things are found, and they agree with the common descent model, it shows evolution.  When similar things appear, and they do not agree with the common descent model, then it shows evolution.  I do understand that this is not an inconsistency as there are other factors in the making of the determination, however for one who is skeptical of common descent; I think the second claim takes away from the first.  I also do not think it has been demonstrated, that because these ridges which turn into gills in one embryo look like these ridges which become something completely different in humans and other mammals is evidence of common descent.  It may fit with the story, but similarity as we have seen, does not show common descent necessarily.  Also in the early stages of embryo development, considering what you are going from, to what the end result is, I think it is reasonable to expect a great number of things, to have a similar appearance at certain points.   The fact is that in humans, these are never gills, and do not have anything more than a similar appearance at a certain stage of development.

With respect, I think you have their reasons for 1) illustrating what can only otherwise be known by data, since you can't easily (or at all) photograph some of these things that warrant visual representation-- in other words, something is not fake simply because I draw it to show how all the pieces come together; it's why we draw the center of the earth, to show the core, mantle, etc.-- and 2) their reasons for making the comparison to development, both very confused.

DNA does not represent a blueprint for an animal. What it represents are a set of "dividing, differentiating, and folding instructions" as the embryo goes from being a ball of stem cells (by simple division) to a hollow sphere and then folds in on itself to form the layers which begin to differentiate into new types of cell. At every stage, it keeps folding and copying and making new types of tissue in order to form that system. We can now "read" those instructions in the DNA, but we already knew they were there from embryology, because we could watch each stage and see the same processes  at work, and only later  processes turned those embryos through the necessary additional folds and changes to become something further down the chain of evolutionary development. We can SEE it, even though it's hard represent visually because in real life, tissues are hard to differentiate because everything's nearly the same color. So we make nice, pretty drawkings; bird and fish (etc) identification books are often done by artistic renderings rather than photographs, to help highlight some of the features that identify the real thing.

So you're right, the gill slits "never become gills"; what they do is start forming the same way that tissues in fishes, which have the DNA to make it keep developing into gills, form. They later will take a different path in the "switching and folding game", and turn into the structures we have in our throats to make sound. Evolution often works like that, taking sets of instructions and turning them in a new direction.

In other words, we share (from our common ancestry) the developmental Steps 1-58, say, but at Step 59 our DNA says to do something different, and the switch-and-fold process turns in a new direction that continues via the "updated" set of instructions that developed in one of our ancestors, after the split from the creature that doesn't have that set of instructions. There may even be other aspects of development that remain mostly unchanged from the set inherited equally-intact by our relatives; that's why Neil Shubin's book is called Your Inner Fish.

You're quite right, that just because similar things appear it does not mean they arrived by common descent; that's what convergent evolution is, period. Closer examination can tell you, via other traits, whether or not the similar design emerged because the creatures share common gene pools that passed the trait down, or whether it is the result of convergence-- an organ or system that works so well that it will be heavily selected for, in gene pools that produce that structure. One of the most famous ways we can show this is to look at the eyes of octopi and those of humans. They seem extremely similar, at first glace, but a closer examination shows that they develop in completely different ways, especially with regard to how the light-sensitive nerves grow and develop; ours run from the retina to the inside of the eyeball and back out through the "blind spot" in our vision, which our brains correct for so we do not see it without covering one eye, while the nerves from the retina of an octopus go straight back to the brain, and thus produce no blind spot.

Finally, common descent can also be demonstrated by looking at the genomes, in which non-coding sections (ones that don't do active things that can be acted upon by Natural Selection pressure) are simply passed down relatively intact, generation to generation, and thus contain "markers" which can be used to track family-tree patterns and see if species are related in that way. It is something that would have shown evolution via common ancestry to be 100% false, if it had been false; instead, it validated the Theory to a degree that was not possible, before. There is no question in anyone's mind that we know how this works.

It's not a "story". It's not something they're throwing out evidence to confirm as true. It simply is the way things are, and all signs point to it.