Thanks for those, I wasn’t aware that the early venus understanding had changed so much recently. Venusian aerostat floating cities for the win!
From the postings above (as I understand what is being said…!), I understand that the evolutionary process was/is an experimental process: a series of random changes occurs until a structure or process develops that is functionally useful.
Then that particular structure/process is locked in; and other parallel processes are developed.
This continues on a loop until a functional organism, which is an aggregate of the previously developed structures/processes, emerges.
The diversity of life forms occurs as a result of each new structure/process being locked in, and the knowledge of that is passed on to other developing organisms and incorporated into their structure.
So, the “common origin” arises from the fact that each successive organism has used the knowledge of structures/processes developed in its predecessors; and this knowledge is somehow buried in the detail of DNA.
Fine, so far so good. But this raises some other questions which my high school biology and magazine reading does not address (a little knowledge is a dangerous thing):
Referring to the example of the kidney above, a kidney is a survival necessity, as we know what happens to us when our kidneys are removed or nonfunctional.
How does the evolving organism survive without a kidney while it is waiting for one to evolve?
Especially since the life span of the evolving organism will probably be a lot shorter than the time required for a kidney to evolve; I also assume that a physically existing organism would not be evolving a kidney during its life span.
So it either has a kidney, and is alive, or it doesn’t and is dead.
How does/did that work?
It’s not as simple as “kidney alive, no kidney dead”. Plenty of living things do just fine without kidneys. We happen to need at least one kidney, but we’ve evolved into that state. Presumably, at some point there was an organism that didn’t need a kidney, but which evolved some sort of proto-kidney that enabled it to be just a bit better than its fellows at processing waste. As that proto-kidney evolved to be better and better, other systems in the organism also evolved to be more and more dependent on it (maybe, for instance, now that it was better at processing waste, the organism could afford to develop a faster metabolism, letting it move faster). Eventually, you get an organism (like us) that has developed in so many other ways that require having a kidney, that the organism would die without it.
So, are you saying that the evolutionary process is not random, but is directed by necessity?
That is to say, an organism may have evolved to a level of complexity that it needed a more complex/efficient waste processing system, so it booted up the evolution of the kidney?
But then it noticed that it needed a more efficient circulatory system to move the waste through the kidneys, so it then booted up the evolution of the circulatory system?
But then it needed a more efficient oxygen exchange system, so it booted up the development of respiratory system, etc, etc…
I think you’re being deliberately obtuse there. It’s pretty obvious the order of events Chronos is describing do not imply forethought:
Huh?? What???
As a logical extension of the posts made previously, I am asking the question as to what drives evolution. Within any individual system/process, what starts it, what ends it? Why does it go in a particular direction?
Clearly, if the posts noted above are correct, then must be some kind of linkage between the individual biological processes, and in order for them to co-function there must be some communication and co-ordinated direction between them.
So, I don’t understand your comment.
No, it’s more like this… sequestering waste from active metabolism is a good thing whether you’re a single-celled organism or a blue whale. There is a very long and very long continuum of physical structures that can be considered kidney-like, from simple little bubbles inside cells, to specialized cells, to specialized tissues, to whole organs, and everything in between. A little bubble gets to be a little more kidney-like, and so on over millions of years.
There is a certain degree of inevitability about it, I admit, because certain necessities of life are limited to certain designs. Although they took a random path to get there, eyes were never not going to have a hole on one side and an optic nerve on the other.
Looking at the human body, we see that we have two lungs and two kidneys. Removal of one of either does not result in death; obviously, the evolutionary process has provided a non essential redundancy.
So why do we have only one heart? Given the importance of the heart for the maintenance of life, it would seem that of all organs, this is the one that would be a prime candidate for redundancy. Why don’t we have a spare?
The kidneys and lungs (and liver for that matter) are simple enough things that you could put two, three, or ten of them in a series and they would do their jobs. They’re passive and they require no coordination. By contrast, the heart has to coordinate the beating of its 4 separate chambers in just the right sequence. It’s such a delicate balance that sometimes it spontaneously gets out of rhythm and causes cardiac arrest. It would be exponentially harder to get it right between the 8 chambers of 2 hearts.
The proto-kidney appears first, before the need for it does. If an organism without a proto-kidney evolves into something that needs one, it dies, end of story. If, however, an organism that does already have a proto-kidney evolves into something that needs one, it’s fine, because it does in fact have the proto-kidney.
Another answer is: it is a paired organ – the two sides of the heart.
After all, the heart is almost bilaterally symmetrical (and there are functional reasons for the slight asymmetry), and it is near the centre of the body.
But this would just be a hypothesis; I don’t know the evolutionary history of the heart.
Which half of the heart do you think you’d be able to lose yet still live normally?
I guess the first step in evolution is from single-cell to multicellular; cells that normally split after cell division occasionally stick together; if this is occasionally advantageous, the trait becomes dominant. (I.e. makes the little critter too big for bigger cells to eat?)
The step after that is when cells become specialized. Of course, then you need the mechanism by which the cells decide how to specialize, and in what.
Cute.
Obviously I’m providing an alternative answer to why it came to be that we have paired lungs, kidneys, ovaries etc, but not two hearts. I’m making no claim about redundancy.
But actually, now you bring it up, I’m skeptical about your hypothesis. I doubt that we have a bilaterally symmetric ancestor that only had one of any of these organs, for instance.
So the redundancy did not win out because it was selected for; having the tissues/organ was selected for and they were paired because mutations tend to affect both sides of the body in a symmetrical organism.
My mistake; since you were answering a question about redundancy, I assumed your answer concerned redundancy.
My hypothesis has no need of that hypothesis. My comments only dealt with why the cost/benefit equation doesn’t favor dual hearts.
Not buying it. First, it doesn’t explain while dual hearts didn’t arise. Second, A complex organ like kidneys and lungs are energetically expensive to build and maintain, not to mention a huge disease vector. Huge expenditures/liabilities like that do not persist over deep time unless they provide a big benefit. Third, why aren’t the spleen or pancreas or gallbladder bilateral?
A better answer is that there is benefit in having copies of critical organs if it is feasible, and multiple vertebrate hearts are just infeasible.
Yes, your mistake. The questions were “Why do we have only one heart? Why don’t we have a spare?”.
My answer was “One hypothesis is we don’t have a spare because the two sides of the heart began as paired organs”.
Then you retorted “Which half is spare?”. :rolleyes:
In this case though, it’s pretty irrelevant. It’s not like both setups have been tried, like-for-like.
Consider the question: “Why don’t animals communicate via radio waves?”. We could talk about the cost/benefit of doing this, but the fact is it is an idea which nature hasn’t tried. Evolution appropriates whatever is already there and makes incremental changes.
I’m not saying that there is no benefit to having two lungs, say, I’m talking about how / why it originated.
wrt the heart, it’s rather like asking “Why do we only have one brain? Why don’t we have a pair of brains?” and the answer obviously is that the brain is composed of paired units.
Actually, look what other organs we have paired. Hands! Knees! Eyes! It’s almost like the default is for organs to be paired in a bilaterally symmetrical organism, unless they’re along the lateral line (in which case they’re often composed of two hemispheres).
I don’t see that this is an issue at all: a circulatory system could evolve in which one heart feeds the top half of the body, while a second heart could feed the lower half. Each heart can be a copy of the other and beat independently. Any spontaneous, but temporary, arrhythmia in one or the other could be absorbed by the natural slack in the circulatory system. This already happens now with our existing circulatory system in which transient arrhythmias are relatively common.
The more significant question would be how a twin heart system would evolve in the first place; both hearts and the attached plumbing system would need pretty precise co-ordination and evolve in precise lock step in order to produce a functioning system.
I think there is a flaw in this argument: taken to its logical extreme we could see a body with all kinds of peculiar, but non functioning, tissues or organs waiting for their day to come.
However, this would require that the organism expend all kinds of energy and nutrients to develop and support currently useless tissue, which may or may not die off, or ultimately serve some useful purpose.
This would be of great detriment to the survival of the host organism.
On the other hand, look at cancer: is cancer really only evolution in progress? Are tumors only the spontaneous creation of future new organs, moving their way up the evolutionary chain?
Organs develop in step with the requirements, and developing organs improve efficiency, and are thus selected for.
Start with a sponge - basically a bunch of identical cells with no structure. They extract food from water, and use energy, and excrete salts (the byproducts of cellular respiration). As the sponge grows, cells divide and replicate, sometimes with errors. Most of these erroneous cells die. But maybe one of those cells is extra good at moving salts around. It helps the whole sponge by getting rid of the waste from surrounding cells. So that modified cell replicates, and that sponge survives and replicates till all sponges have some salt filtering cells because it gives them an advantage. At the same time, the sponge is developing structural differentiation, because it improves water flow, and thus feeding. And maybe those salt-filtering cells are better at the bottom of a long tube, to get rid of the salts produced in all the other cells. So the best surviving sponges are the ones with the salt cells at the end of the tube. And, as evolution proceeds, those cells are getting better at filtering salt, but maybe are relying on surrounding cells for energy, because they are specialising. And the sponge isn’t just a sponge any more, it’s a worm, and those salt removing cells are starting to be structurally isolated from the rest of the worm and there are other structures developing to feed waste products to the excretory cells for better efficiency and survivability. And it is starting to look like a kidney, but it isn’t fully structural yet, but as the worm develops more complexity, so the semi-kidney is getting more complex and the worm is getting more developed and the kidney is now encapsulated and the fish is able to swim faster due to better waste removal and …
It happens all at the same time.
No, not at all. Cancers are just unregulated cell growth, and evolutionary pressures have suppressed the ability for cells to to do this, because it reduces species survival. But most cancers do not restrict the ability of the species to reproduce and succeed , so cancer still exists.
Si
That is not the implication at all. The proto-kidney need not be useless. It may serve the organism very well. Look at the coxal gland of the lobster for example, a single unpaired blob that filters urine. It seems crude and primitive to us, but to the lobster it’s a state-of-the-art kidney. An organ like that was probably the forerunner to the modern kidney. Don’t fall for the bogus argument of so-called “irreducible complexity”.