Just 21 days after you were conceived, the flat, round embryo that went on to become the person you are today started to fold over.
The place where the two edges came together formed your spinal cord, and your heart – no bigger than a grain of sand – began to beat. This is just some of the fascinating information I gleaned from the first part of BBC2's Countdown to Life: the Extraordinary Making of You.
Part one of three took us through the first eight weeks after conception – a time, as presenter Michael Mosley pointed out, when some women may not realise they are pregnant. Yet while there may be no obvious external signs, inside the body hundreds of changes and processes that could determine the rest of our lives are already under way.
Take identical quads Holly, Jessica, Georgie and Ellie, who were the result of a 64-million-to one chance. Their single egg divided to create four genetically identical sisters – a process that happened just five days after conception.
We met Randy, whose life changed for ever just 19 days after he was conceived. All of his organs are on the wrong sides – he is a perfect mirror image of most human beings. This was because of some tiny hairline structures called cilia that come to life on day 19, spinning clockwise to create a leftward current to activate genes that will tell the organs where to go.
In Randy's case, his cilia didn't spin and the genes that should have been activated on the left were switched on on the right. And for Brazil's De Silva family, a quirk relating to the delightfully named sonic hedgehog protein, has resulted in 14 of the family's 26 members having six digits on each hand.
Every case study was mesmerising. Mosley was both charming and compelling. He described a body scan as "a work of art", and when he heard the heartbeat of an unborn child at five weeks, he seemed even more excited than the expectant parents.
If ever there was a man who loves his job, this was he. And if ever there was a time to rally against proposed cuts to the BBC, it's on the strength of programmes like this that the battle should be fought.
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