Science Questions

Why have one heart but two kidneys?

Sat, 25th Jun 2011

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Rebecca Ferris asked:

I was just wondering why we have 2 of some body parts and only one of others. Why do we have 2 kidneys when we can get on perfectly well with just one? And why don't we have 2 hearts when it's so common for the one we have got to go wrong? Could this change in the future as we evolve, or has it already changed?

Becca Ferris



We posed this question to Dr. Sebastian Shimeld from the University of Oxford and Dr. Robert Whitaker from the University of Cambridge.

Sebastian - Now that's a really interesting question and itís really got two answers Ė one of which is how we develop in the womb, but there's also an evolutionary explanation to this which is how we got to be in this situation in Heart and Lungsthe first place.  Itís not just us that are bilaterally symmetrical.  All vertebrates are Ė be they birds, reptiles, frogs, or fish.  In fact, not just vertebrates but almost all other animals are bilaterally symmetrical as well.  This includes worms and flies.  This is because bilateral symmetry evolved a very long time ago, at least 500 or 600 million years ago. Our body plan has been locked into bilateral symmetry since that point. 

This brings me onto the last part of the question which is - could it change?  I think given that weíve been locked into this body plan for such a long period of time itís unlikely that itís going to change. I wouldnít say completely impossible because there are one or two organisms, or one or two animals, which have managed to change this.  A really good example of this is the octopus which not only has a major heart but has managed to evolve two ancillary hearts as well to help its blood flow.  So, unlikely to change I think, but perhaps not completely impossible given enough time and the right selection.

Diana -   And the developmental point of view from Dr. Robert Whitaker at Cambridge University.

Robert -   The obvious first reaction of many people would be to just suggest that multiple identical organs are simply there for spare parts, but I do not believe that this is the correct explanation.  Iíd like to look at the conundrum from a developmental point of view. 

The early embryo has an outer layer, a single midline tube passing from mouth to anus to become the gut.  From the single and simple midline tube, is developed the intestines. However many other organs develop from it by a system of budding from the tube.  Such organs include the lungs, the liver, the pancreas... and whether these become a single organ or two organs depends on whether the bud that grows from tube stays as a single bud or divides to grow more than one.  The liver for instance is a single organ whereas the lung comes from two buds to give the organs that we see in the developed child.  So what about the kidneys I hear you ask?  Well, they develop not as a single tube, as with the gut, but on either side of the body quite separately. 

There's a fundamental difference between there being two parts to a single organ, for example the lungs and the brain which all develop from a single outgrowth, as opposed to two separate organs with identical functions such as the kidneys, the ovaries, the testicles, which all develop on separate sides of the body.

Diana -   So itís a combination of midline symmetry inherited from our fishy ancestors, a result of our developmental processes as budding embryos and as with many things, itís like that because it works.  In some cases, itís always good to carry a spare. 


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2 hearts would have to work together harmoniosly & if the coordinated  pumping went out of whack, they would interfere with each other? CZARCAR, Sun, 19th Jun 2011

Do any animals naturally have two hearts?
chris, Sun, 19th Jun 2011

Cephalopods have one central
systemic heart and two branchial hearts,
which are clearly shown after injection
with acrylic paint.

Other examples of multiple hearts
include the oyster, Ostea gigas4 with one
systemic and two accessory hearts, the
latter has a diameter of 1 mm during
systole, and 5 mm in diastole. The
earthworm, Phylum oligochaeta, have
two aoratae and multiple hearts
arranged by segments. The hagfish,
Myxine glutnosa, has five hearts.5 There
is a three-chambered systemic heart,
two accessory, one portal, and one
caudal heart.
The Lancet Volume 352, Issue 9129, Pages 665-750 (29 August 1998) CZARCAR, Mon, 20th Jun 2011

(oops, looks like we had the same source)

As you know, mammalian hearts have right and left heart chambers which pump blood independently, but are combined into a single heart because the flow to the lungs and to the rest of the body must be balanced perfectly, most easily done by pumping both chambers simultaneously.

I'm seeing notes that the earthworm has the equivalent of multiple hearts which likely has the evolutionary advantage of regeneration after damage.

Cephalopods (squid, octopi) are also supposed to have multiple hearts.


The Hagfish is supposed to have 4 or 5 independent hearts.

There is a lot of discussion on how the Brachiosaurus was able to supply blood to its head and brain.  There are some suggestions (without evidence) of multiple hearts.  However, I don't believe there is any indication of a heterogeneous neck vertebrae, thus discounting the idea of neck hearts.  However, perhaps you could solve the problem by adding muscular arteries somewhat like an esophagus that would be able to propel a bolus of blood upward.  Perhaps with valves like heart valves, or vein valves.

Note, with humans, we have a single heat, however, our deep leg veins can be pumped by muscle contraction while walking or running. CliffordK, Mon, 20th Jun 2011

I remember the consternation in first year anatomy when a very distinguished surgeon told us that we have one kidney but two livers!!  There were mutterings along the lines of "has the old prof lost it?" 

If you look at blood supply, innervation and, visceral support the two kidneys can be profitably be thought off as the two extreme ends of a horse-shoe shaped single organ.  Conversely the two lobes of the single organ liver can be better treated as two separate entities as they have a far greater distinction than the actually separate kidneys.  This was all a long time ago but aI believe there are developmental/genetic routes that can be traced that confirm this surgical idea.

AS we do have a double circulatory system then it is possible that this was from two hearts - I don't really remember the genetic heritage imatfaal, Mon, 20th Jun 2011

Hey all.
Great answers on the show! It was nice to hear the answer from two different viewpoits. But, I was surprised that no one mentioned vestigial body parts.

For example, whales, being mammals, have vestigial legs within their body. It seems that it is much easier to evolve something new, or to evolve a new use dor something old, than it is to remove something through natural selection.

Another great example is the human brain. When our primate ancestors moved from relying on their noses to relying  on their eyes their brains grew in size to afford greater optical processing rather than just losing the parts of the brain that were specialized for olfactory processing. In fact, the entire history of big leaps in our brains has been one lump growing atop another. Perhaps this is why we supposedly only "use 10% of our brain." kenhikage, Wed, 29th Jun 2011

If that were true JFK would have had a longer career ... RD, Tue, 5th Jul 2011

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