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On the outside, we look symmetrical, but that stops with our internal organs. Even organs that look symmetrical at first tip one way or the other -- the right lung is divided into three lobes while the left only has two, for instance. 00:12
We all have the same internal asymmetry, except for the 1 in 20,000 people who have a condition known as situs inversus, where the internal organs are inverted left to right. This usually causes no negative effects, except for rare emergency cases when it can keep doctors from identifying which organ is in pain. 01:11
Researchers looking at a special bunch of cells in a central region of an embryo found that its hair-like cilia began to beat in unison toward what would later become the left side. This beating washes a current over the outside of the embryo that activate pressure sensors, which turn on a gene that determines which side the embryo's organs will live on. 03:07