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Why do some of our organs come in pairs, such as kidneys, whereas we only have one heart, one liver etc?

Richard Swifte
Darmstadt, Germany

Unlike rocket engineers, who design in 鈥渞edundancy鈥� (spare parts for use if key components fail), evolution doesn鈥檛 waste energy producing spare body parts unless they have a definite survival value, either for the individual or the species. Most creatures in the wild have their work cut out finding enough food to survive, let alone growing unused parts. So it is fair to expect that having two organs fulfils a definite need.

For example: two ears enable better location of sounds; two eyes provide easier estimation of distance and motion if they face forward, and all-round vision if they are on the sides of the head. We have two lungs, but there would be little difference in energy requirements if we instead had one double-size lung. Arguably, having two lungs enables space for the heart more easily.

More problematic is why we have two kidneys, since a person can function adequately with just one. These organs come in pairs in the large majority of land animals, amphibians and fish. It is likely that our primeval fish ancestors developed pairs too, although since soft organs rarely fossilise, it is currently impossible to be certain.

Norah Fogarty
Developmental biologist King鈥檚 College London, UK

Fourteen days after fertilisation, a groove called the primitive streak forms down the midline of the human embryo. Blocks of cells called somites are arranged in pairs on either side of the groove. This sets the body plan of bilateral symmetry with a left and right side. This is different from other organisms like starfish and sea urchins, which have radial symmetry.

Organs form singly along the streak or in pairs on either side of it. For example, the spinal cord and brain form along the streak, so we have one central nervous system. We only have one liver, stomach and thyroid because these develop from the gut tube, which also runs along the streak.

Organs that are formed from the somites, such as the kidneys, ovaries, testes and eyes, appear as pairs. While we have only one heart, it begins as two organs that later fuse together. In fact, the heart retains bilateral symmetry, as it has a left side and right side.

Of course, the interesting thing is that there are always exceptions to the rule. The lungs develop from the gut tube, yet we have two. It is thought that the lung evolved to split in two to increase lung capacity as animals got bigger. In contrast, while the spleen comes from the somites, we only have one. The reason behind this is unknown.

Craig Johnson,
Lecturer in anatomy University of Bristol, UK

Questions about why our organs look the way they do can normally be answered by looking at the earliest stages of our development.

Many of our organs emerge as buds from a single, primitive gut tube. The lungs, liver and pancreas all develop in this way. How these organs bud from the tube dictates their eventual fate. Paired lung buds emerge, projecting into each side of the ribcage, while several liver buds emerge, fusing together into a single organ. You can still see lobes of the liver 鈥� remnants of the buds from early development 鈥� with the naked eye.

Simultaneously, the kidneys develop in their own way. They don鈥檛 emerge from this primitive tube, but form bilaterally on either side of the body from a paired precursor found near the pelvis. The testes and ovaries develop from this precursor too. The kidneys eventually ascend to sit beneath the ribs. However, if the kidneys come into contact with each other during this ascent, they can fuse, forming a single midline organ known as a horseshoe kidney.

When considering why we don鈥檛 have two hearts or a second brain, evolution shows us that what we already have works so well that there has never been a need to grow another.

@seamusmchugh
via Twitter

Doctor Who has two hearts. You鈥檙e welcome.

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