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The Evolution of Organs

Post of the Month: May 2006


Subject:    Re: How?
Date:       29 May 2006

Windy wrote:
> 'Rev Dr' Lenny Flank wrote:
> > Windy wrote:
> > > 'Rev Dr' Lenny Flank wrote:
> > > > wrote:
> > > > > Hello. I am new to talk origins (but I've lurked for a month or so). I
> > > > > was wondering how internal organs could have evolved. What models are
> > > > > there for this?
> > > >
> > > > Basically, internal organs are all just various bags branching off from
> > > > tubes.
> > >
> > > But it's interesting why they would be bags.
> >
> > What else COULD they be? They have to be enclosed, so they can do
> > whatever it is they do to whatever they do it too. But they have to
> > have an opening, so stuff can get inside and out.
> > And that is a "bag".
> But why have one opening in and out? Why not lots of little openings
> along the way? I think the amphioxus shows a lot more metamerism in its
> organs, so assuming our ancestor was similar to that, why have just one
> or two bags instead of "decentralized" organs?
> > Well, since there are openings for stuff to go in AND go out, it's
> > really just a fat tube.
> I guess even fish kidneys are a "fat tube" since they are contained
> from the rest of the body... but I was wondering why in mammals most
> organs are roundish blobs with few openings.

This one-time "professor" has been on vacation and out-of-touch for a few days. So I missed the entire professorial debate and obligatory pun cascade only to then discover some strange musings about the real question, the origin of internal organs.

There is an enormous prejudice being shown here, as if humans, mammals, vertebrates, and chordates are the only kind of organism with differentiated tissues and organisms that exist. It is true that somebody way at the start mentioned sponges and that is the correct place to begin.

The notion of organ development is quite intimately tied to the notion of multicellularity. How did that begin and how did cell differentiation begin? There are bacteria that form clusters and strings and the prokaryotic blue-greens (cyanobacteria) even have some form of differentiation in the heterocyst cells that are specialized for the fixation of nitrogen. But prokaryotes never developed true multicellularity and complexity of cell types. Eukaryotes figured out how to control the genetic material to produce multiple cell types from one genome and also how to send signals between cells so that events occuring in one cell could modify the activity of another cell. All the multicellular eukaryotes including protistan brown and red algae and the fungi do have specialized differentiated cell types that work together to produce the functioning organism. The green algae clearly show several examples (google 'Volvocine line') showing closely related but distinct organisms with a single cells, a few cells, a dozen or more cells through the Volvox with hundreds of cells. Plants show a clear development from mosses, which have well differentiated tissues each with different cell structure and function to the vascular plants with distinct organs: root, stem, and leaf. Similarly animals show a similar range. Sponges contain a few types of specialized cells that function together in a coordinated way for the survival of the organism. The Cnidaria (Hydra, coral and anemones, jellyfish) have rather clearly defined tissues (a cluster of similar cell types with a distinct function) organised to perform nervous, muscular, digestive, and reproductive functions. The flatworms (Platyhelminths) show well defined organs (a structure formed of more than one tissue type, organized to function in a coordinated way to perfom some function) -- digestive, nervous, muscular, excretory, reproductive. These animals do neither the tubular form of organization nor the coelom that was mentioned earlier. Organs do not depend on these. (Note, the flatworms may be degenerate forms derived from a coelemic ancestor -- in that case look to the roundworms and rotifers as well as a bunch of lesser known phyla for animals with true organs but no coelom).

Tubes and sacs are simply highly convenient shapes for performing a variety of functions. Cells can readily form into sheets, filaments, and solid masses as is seen in the protista, fungi, plant and animal kingdom. Solid masses are extremely difficult to maintain because diffusion cannot supply nutrients and respiratory gases nor remove wastes from the interior. So sheets and filaments are the main structures formed. Sheets curved around to form tubes and sacs allow for fluids carrying essential chemicals to be moved around from one part of the body to another. That way, cells that synthesize and excrete chemicals can have their secretions act in other locations where cells can be specialized to make use of or respond to those secretions. And, since cells are basically biochemical machines, that is the only really good way to build a complex body.

Incidentally, digestive and reproductive systems are the first and most important for complex animals, with sensory/motor and excretory (urinary, not fecal) systems close behind. Respiratory and circulatory systems are relative newcomers only needed when body size grew to such an extent that structures are no longer close enough to the body surface for or to the distributed digestive system for diffusion to do its work. Skeletal structures are quite unnecessary for aquatic types. When enough animals of a variety of complex types developed and started becoming effective predators, then a series of specialized protective (and fossil forming) protective devices became necessary and we have the Cambrian radiation (explosion?). However there was a rather long period of development of complex animals that preceded that time.

In any event, there are abundant examples of organisms, not just vertebrates and not just animals, that show different stages of complexity in cell type and organization illustrating how evolution might have produced the internal organs we find in modern animals. (Note: I am not suggesting that we vertebrates actually went through a "sponge" stage, then a "cnidarian" and a "flatworm" stage in our own evolution. Merely that sponges and cnidarians and flatworms illustrate levels of development that might have been stepping stones in that evolution.)

On a completely different approach, consider the development of a multicellular organism with complex organs and organ systems. It starts as a single cell, then a mass of relatively undifferentiated cells that gradually develops differentiation and structure in the process of organogenesis. And it has to function fully as a living, metabolizing individual all the while. The regulatory genes that control such a process are the key to understanding how it works and those regulatory genes are widespread in the animal kingdom. Clearly, the development of those genes in early animal-like protists was the key to the evolution of multicullar organ-bearing animals.

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Honor Your Family

Post of the Month Editor's Pick: May 2006


Subject:    Do you want to be descended from a monkey? Why yes, yes I do
Date:       5 May 2006

First of all, I think an introduction is in order, as this is my first post to this group. I think it's actually my first ever Usenet post as well, although I've been a regular on BBSs and Fidonet and, later on, WWW forums.

I'm 27 and male. I have an M.Sc. in computer science and am working on a Ph.D. in computer engineering. My main research topics are data mining and computer ethics. No, they don't have a whole lot to do with each other, but that's just the way I am. I minored in mathematics and philosophy.

I've been a hopeless science buff all my life. For years my attention was dominated by physics and astronomy (besides CS and math, of course), but my recent acquisition of a David Attenborough DVD set rekindled the dormant biology nut in me. That, in turn, led me to Talk.Origins, which I've been enthusiastically browsing for the last few days. I've particularly enjoyed the POTM archive; I've found there some genuine insights - and I'm not using the term lightly - into evolution, into the essence of science and into the human condition.

So, I came across the "argument" addressed on the subject line of my post in the list of creationist claims. It is, of course, not strictly an argument in the classic dialectical sense and, arguably (no pun intended), not even meant as such. It's easy to see that it's designed to appeal to your emotions rather than your reason, so from a logical standpoint it is easily refuted. The response in Mark Isaak's list even takes a stab at explaining why it doesn't have to be such a bad thing to be descended from a monkey, but it's inevitably brief and superficial. No offence at all to the editor or the author, I see very well why it has to be that way.

However, I think I may have something meaningful to contribute - a personal experience, something that occurred to me just a couple of hours ago as a matter of fact. Maybe it will help somebody who is genuinely troubled by this "monkey business" to understand why a lot of people appear to be perfectly "zen" with it. Maybe not. Maybe there are no such people reading this group. In any event, I wanted to share this with somebody, and I figured the people here would be the most likely to appreciate what I have in mind.

So, do I want to be descended from a monkey (an ape, whatever - let's not pick nits here)? Obviously I don't get to choose, but if I did, I wouldn't say no. Why? Because it's cool to be descended from a monkey. It's even cooler to be descended from a little furry thing that scurried around under the cover of night back in the reign of the dinosaurs, blissfully unaware that it would soon find itself on the winning side of a great revolution. Still cooler to be descended from a creature of the ancient oceans aspiring to see if there's any chance you could survive without being constantly wrapped in the comfortable blanket of water. It keeps getting cooler and cooler, all the way up to the ultimate ancestor, whatever sort of critter that may have been. I can't imagine anything that would make me feel more fundamentally connected with the world around me, both in space and in time. The idea of having been just thrown smack in the middle of a ready-made world certainly doesn't do the trick for me, even if the world was specially prepared for the arrival of my kind by a benevolent creator.

And this brings me to one possible way of looking at the notion that belief in evolution leads to rejection of morality. Again, whether or not this is true has no bearing on whether evolution happens, but I find it an intriguing point to analyse nevertheless. If the theory of common descent delivers the goods, so to speak, then I can look upon every single living organism as a kind of remote cousin. The cactus on my desk? He's family. The bananas in the fruit bowl in my kitchen? Family. The birch trees in front of the building I live in? Family. The squirrels and magpies I occasionally see in those trees? Family. The bacteria in the gastric systems of those squirrels and magpies? Family. So, if we're to love and honour our family, surely then we must love and honour everything alive?


P.S. You'll note that I have a tendency to express myself somewhat poetically at times. Minor scientific inaccuracies may be safely attributed to artistic licence.

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