The Human Body Series: From a Single Cell to Old Age

The Human Body series follows one body from a single cell to old age. Each timeline works at its own scale: days for the embryo, weeks for the fetus, months for the first years, and years and decades for everything after.

The embryo: the first eight weeks

The Human Embryo: The First Eight Weeks starts at fertilisation, when a single cell forms with 46 chromosomes, 23 from each parent. On day 22 a simple tube of heart tissue begins to contract. The heart is the first organ to work. By day 27 the neural tube has closed at both ends, and if it fails to close, the result is spina bifida. By day 56 the embryo is about 3 cm long, has all its major organs, and is now called a fetus.

The fetus: week 8 to birth

The Human Fetus: Week 8 to Birth counts weeks from fertilisation, so add two for the usual pregnancy count. The first twitches come at week 9, though they are far too small to feel. At the halfway point, week 18, the fetus weighs about 300 grams and the mother starts to feel kicks. Week 22 marks the threshold of viability: with intensive care, about half of babies born then survive. Birth comes at about 38 weeks, when the baby is around 50 cm long and weighs 3.5 kg.

The first three years

The First Three Years: From Newborn to Toddler begins with a newborn who sees best at 20–30 cm, about the distance to a face during feeding. First steps and first words both come around the first birthday. After about 50 words, a vocabulary spurt follows at around 18 months. By two, a child is roughly half as tall as they will be as an adult.

Puberty

Puberty: From Child to Adult Body gives average ages with their normal ranges. Girls start about a year earlier than boys and have their fastest growth at about 11, compared with 13 for boys. The average age of a girl's first period is now 12½, against around 15 in the 19th century. The prefrontal cortex is still maturing into the mid-twenties.

A whole life

A Human Life: The Body from Conception to Old Age ties the series together. A newborn has around 300 bones and cartilage pieces, and many of them fuse to leave 206 in an adult. The collarbone is the first bone to start forming, on day 47 of the embryo, and the inner end of the collarbone holds the last growth plate to close, at about 25. Bone and muscle mass peak around 30. Almost everyone needs reading glasses by 50.