How has evolution created our blood, lungs and the heart?

We can't exist without blood, which transports the oxygen to all areas of our body. However, the blood needs a lung, which gives it the oxygen to transport. The blood also needs something which lets it flow through the whole body, which are our veins. And in order to allow the blood to flow through our veins, an organ is needed to pump the blood, which is our heart. We also need a brain which controls all that, and the brain in turn needs the blood in order to function right.

Evolution makes very slow steps....."it just doesn't jump". So, How did evolution manage to create all that?

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    $\begingroup$ The systems you mention, the blood, heart, vessels, and lungs exist in partial forms in other organisms. For example, insects have open circulatory systems with primitive blood vessels and a simple heart. Instead of lungs, air is moved through a system of tubes. Earthworms are even simpler, they get their oxygen by diffusion across their skin, but they still have a simple heart and blood vessels. The systems did not simply appear fully formed, but evolved one piece at a time over many millions of years. $\endgroup$ – user137 Jul 28 '16 at 12:51
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    $\begingroup$ But even for those little creatures, with less organs and simpler structure, applies the same thing. For example the earthworms: in order to allow blood flowing, a heart and a oxygen input (by diffusion across their skin) is needed. But how did that evolve by one piece after another, when no piece can exist without the other. $\endgroup$ – asparagus Jul 28 '16 at 13:27
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    $\begingroup$ Aspagus, you contradict yourself: you say that all these systems are interdependent, then you gloss over the fact that worms don't have lungs, and continue to claim that all these systems are interdependent and can't exist without each other. The fact that worms don't have lungs is evidence that the circulatory system does not exhibit irreducible complexity. $\endgroup$ – user151841 Jul 28 '16 at 15:52
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    $\begingroup$ user151841, concerning lungs you are right, but I didn't just mean the organ, but the process or the function, which makes all that interdependent. Because, the system of the earthform still can't be without getting oxygen through the skin. So, the earthworm can only live, when the blood is flowing. In order to allow bloodflowing, there have to be a heart, vessels, and, to let the blood carry the oxygen, a function of getting in oxygen. But now I know a little bit about how this has happened, and that this is clearly possible. $\endgroup$ – asparagus Jul 28 '16 at 16:11
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    $\begingroup$ @asparagus A good course in comparative anatomy will show you all the intermediate forms you are looking for. Try Kardong's Vertebrates. $\endgroup$ – kmm Jul 28 '16 at 16:43

While others have addressed the big picture aspects of your question, I think it would be useful to look at the specifics.

Have a look at the heart (or more accurately, the hearts) of the earthworm: enter image description here

They're nothing more than veins with some pumping muscles wrapped around them. It seems almost a stretch to call them hearts, they are shaped so different from what we think of as a heart proper.

Also, note the earthworm's lungs, or rather, lack of them. It doesn't have any! Why not? It doesn't need them. It gets enough oxygen through its skin via osmosis. It's only larger organisms that need dedicated systems to concentrate oxygen from the surrounding environment.

So, the worm has a simpler system (no chambered heart, no lungs) that works.

All vertebrates descended from a common ancestor that was very similar to this earthworm. It had simple hearts, and no lungs. You can follow the evolution of the human heart through fish heart: enter image description here

which is a more sophisticated pumping vessel with two chambers.

Amphibians evolved from fish, reptiles from amphibians, and mammals from reptiles. In this diagram, you will find that the heart becomes more sophisticated and efficient in each: enter image description here

So, this should give you a good idea of the evolution of the human heart from simpler, working system. I won't take the time to draw out the evolution of blood vessels or lungs; maybe someone else will, or you can google them yourself, the information is readily out there. But they all follow the same pattern: gradual, incremental improvements on working, simpler systems.

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    $\begingroup$ the evolution of the human heart from simpler, working system How did this simpler system appeared? the information is readily out there Can you please share some links to that pages? $\endgroup$ – A.L Jul 28 '16 at 16:28
  • $\begingroup$ @A.L Sure! Here is a graphic of the evolution of lungs: images.slideplayer.com/13/4156687/slides/slide_2.jpg To get this, I just did a google image search for "evolution of the lungs". For the evolution of actual bloodtubes from non-tube systems, it's a little more tricky. You could start at this page: en.wikipedia.org/wiki/Flatworm#Distinguishing_features see the "more advanced bilaterans" column $\endgroup$ – user151841 Jul 28 '16 at 16:40

This kind of question was raised in a book called "Darwin's Black Box" by Michael Behe, who is a biochemistry professor in the U.S. - he calls this 'irreducible complexity' (IC). For example, the blood clotting cascade system where you have a large number of components that are all apparently essential for the process.

Now I have to say I find the idea that this is a problem very unconvincing, to say the least. However, it's a reasonable question to ask; how does a system of interdependent elements evolve, if we assume that no part can change gradually without the whole system breaking?

There are - at least - two major problems with this. Firstly, the assumption that you can't change any part of such a system has mostly turned out to be false. Secondly, systems would obviously evolve from other, simpler systems which are just as effective.

Say I start out with three elements in my system (three proteins, for example). They are all essential as each requires the other to function properly. Now I introduce another protein to the system and make it dependent on only one of the existing proteins. Is this system IC? No, we can remove the new protein and the whole thing still works. Gradually, we make the other parts of the system dependent on the new protein and suddenly we have an 'IC' system.

In other words, the 'problem' lies in imagining that you have to go from nothing to a complete working mousetrap. What seems more likely is that elements of a system are changed one by one, and that the system evolves through a series of states where you could point to some element and claim that it is essential.

One final point to note is that no multicellular organism is born whole in one step. The processes that an embryo goes through are conceptually similar (though not exactly ) to evolution in that you can have different organs developing at different times, or simpler versions of them that can work together as a simpler system.

To make this a little less abstract, consider the earthworm example given in the top answer. It has just a simple heart(s) and blood vessels - it doesn't seem that difficult, therefore to add in some lungs. Here's a trivial diagram:

heart, blood, and lung interaction

The lines here are interactions between the organs - the heart pumps blood through the vessels, and the lungs (if any) oxygenate the blood. We evolve from the simpler system (1) to the more complex system (2) just by adding another element.

However, the difficulty with some systems is that the interactions between the parts are dependencies. A very simple example could be proteins that activate/deactivate other proteins (by phosphorylation, say). Then we could theoretically get a situation like this:

protein activation systems

Here, the final system (4) looks like it is 'irreducibly' complex because you can't remove any of (A, B, C, D) without breaking the cycle. However, at each step, we only added or removed one dependency. This also shows the importance of redundancy in biological systems. If you knock out either C or D from system (3) then it still works.

  • $\begingroup$ This nearly makes the point of the most convincing argument I've ever heard for evolution. That is, the intermediate steps for the "mousetrap" were simply to make a better paperweight, and the resulting random configuration accidentally worked as a mousetrap. But it still ignores the "required functionality" aspect of the original question. Each of the functions in an IC system is separately required, but each structure is collectively interdependent. $\endgroup$ – Jed Schaaf Jul 28 '16 at 16:11
  • $\begingroup$ @JedSchaaf Yeah, the mousetrap is a pretty standard metaphor. It may even have been used in Behe's book. I think the key idea is that elements of the system and dependencies within the system can be introduced independently. I see it like a network or graph, where elements are vertices (eg: proteins) and dependencies are edges. I can add a new vertex, then add a new edge, then another edge, and suddenly I have an 'irreducible' system where I can't delete that vertex! Perhaps I should add this to the answer... $\endgroup$ – gilleain Jul 28 '16 at 16:42
  • $\begingroup$ If we have a system where A depends on B and C, B depends on A and C, and C depends on A and B, then we could add D that depends on A, but we cannot then make A also depend on D. A might incorporate D into its functionality, but it doesn't depend on it. D could be removed and A would still work. $\endgroup$ – Jed Schaaf Jul 28 '16 at 16:52
  • $\begingroup$ @JedSchaaf Hmmm. Unfortunately my model is too simple to say what 'depend' really means. I just have some association between two elements (say protein-protein binding) like an undirected edge. $\endgroup$ – gilleain Jul 28 '16 at 16:58
  • $\begingroup$ This doesn't answer the question except in the most general, abstracted sense. To someone who doesn't "get it", a mousetrap probably doesn't seem all that complex, compared to the vertebrate cardiovascular system. $\endgroup$ – user151841 Jul 28 '16 at 18:55

This is a good question, but it has a vast scope, as you're talking about the progression of millions of different living animals over hundreds of millions of years, none of which are still alive, so we have to make inferences based on what we observe in their surviving offspring.

That means if you want to learn how 'intermediate' (say, not-quite-lungs, not-quite-heart, not-quite-brain) body systems could function, you'd first need to learn about the biology of lots of other animals. Not all animals have lungs or hearts or nervous systems. Not all animals have blood.

More to the point, though, the key factor is that several hundred million years is a really, really, really long time. It's such a long time that it's well outside any typical human scale of comprehension. Consider the entirety of your life experience thus far and everything you've seen change. In comparison to how long the evolutionary process has been operating, your life's span has been on the order of a millisecond out of a day.

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    $\begingroup$ Thank you for your great answer! What I don't understand is, how the evolution managed it to create that all. Because, the evolution makes very slow steps, so "it just doesn't jump". $\endgroup$ – asparagus Jul 28 '16 at 12:59
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    $\begingroup$ The amount of time is irrelevant if there is no method that can show how the changes would have occurred. The fact that other organisms exist with structures that could be called 'intermediate' is also irrelevant for the same reason. Remember, these other organisms also need to have evolved through other 'pre-intermediate' "stages". $\endgroup$ – Jed Schaaf Jul 28 '16 at 16:05
  • $\begingroup$ I would argue that this is a poor answer for someone who lacks a fundamental understanding of biology and evolution. Truthfully, one can talk about how the cardiovascular system at a simplified, generalized level without learning the biology of lots of other animals. $\endgroup$ – user151841 Jul 28 '16 at 19:02

Simpler forms developed to handle simpler requirements. Take planarians, for instance, which are thin and small enough that they can receive their oxygen supply by diffusion straight through their surface. Now imagine a slightly bigger animal that needs a slightly more sophisticated system to oxygenate their internal regions well. A muscle with an aberrant, autonomous twitch would be enough to stir/circulate more oxygenated fluids through the body. Past that, any little accident that facilitates this (e.g. some cells bind to oxygen a little better, the muscle twitches a little stronger or more regularly, etc.) is another form closer to what we see today.


A good question, indeed, and not easy to answer (or grasp). I'll give a very simplified answer. Bear in mind that the processess I'll describe are REALLY complex.

You have to think way before blood, brains, etc. Billions of years ago, organic molecules were formed in the planet. These organic molecules started to """join""". Millions of years latter, simple cells which didn't even have a nucleus were formed. Some millions of years latter, cells with nucleus started to form. Latter, these cells started to agreggate, turning into colonies of unicellular individuals. In time, these colonies turned into multicellular individuals, but with all cells being equal to one another. After that, cells in one organism started to differentiate into some functions (digestive and neural, for example). Slowly, more complex organisms were formed, as the cells which formed these organisms started to differentiate and form various kinds of tissues, which, through millions of years, developed into more and more complex organisms. Think of Cnidarians, for example. They are very "simple" (I use "simple" as a proxy for "not complex") beings. They have no circulatory system. A circulatory system developed in latter groups: the first, "simple" circulatory system appeared in Nematodeans (if I'm not mistaken). But it was really "simple". With time, other, more complex, circulatory systems started to originate due to various evolutionary pressures. The same applies to every type of cell, tissue or organ you can think about in any organism: complex organisms are the result of millions of years of simpler organisms which generated more complex organisms, by really little steps.

I hope you get the idea of what I'm trying to say. For really grasping all this, you have to study a lot of evolution, because it is a hard concept for us to deeply understand.

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    $\begingroup$ you have to study a lot of evolution That's a strange answer, like this is a secret kept by biologists. Can someone that studied evolution write explanations of this concept for everyone? $\endgroup$ – A.L Jul 28 '16 at 16:26
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    $\begingroup$ Well, I didn't express myself correctly. When I said "a lot" I didn't mean that one must have a major in biology. What I think is something like this: suppose one has a class about this topic ("How does evolution creates complexity"). When the class is finished, one might think "Ok, I get the idea", and maybe that person gets the idea, but to really grasp the concept of how evolution creates complexity from simplicity is harder to understand than just by watching a couple of classes. It takes reading and critically thinking. I just wanted to emphasize that. $\endgroup$ – Lfppfs Jul 28 '16 at 17:32

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