Why have one heart but two kidneys?

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 it's really got two answers - 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 the first place. It's not just us that are bilaterally symmetrical. All vertebrates are - 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 we've been locked into this body plan for such a long period of time it's unlikely that it's going to change. I wouldn't 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. I'd 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 it's a combination of midline symmetry inherited from our fishy ancestors, a result of our developmental processes as budding embryos and as with many things, it's like that because it works. In some cases, it's always good to carry a spare.