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Perhaps because of this bizarre cycle of creation and destruction, cultures throughout the world have developed practices, rituals, and myths around the placenta, to account both for its importance and its impermanence. Many animals – including some humans – eat it. Indeed, there are numerous recipes online for anyone who wants to savour one, including such delights as roast placenta (with bay leaves, a tomato sauce, and peppers), placenta cocktail (chilled, with vegetable juice), and placenta lasagne, or bolognaise. You can even dehydrate and use it much like a chorizo.

In some cultures, the placenta is buried with great ceremony after the birth of a child. In Hawaii, this tradition was briefly made illegal, until a law came into force in 2006 that guarantees a woman’s right to take her placenta home from the hospital so that she can perform the rite. In Malaysia, the placenta is considered a baby’s sibling; in Mexico, its friend, el compañero – a good description since, for humans, the placenta truly is indispensable. Without it, humans could not give birth to live babies; it supplies all the things that a foetus in the womb cannot get for itself.

The appearance of mammals, as well as snakes, birds, and lizards from a common ancestor back in the Jurassic period – about a hundred and fifty million to two hundred million years ago – was dependent on the evolution of this remarkable organ. From elephants to elephant shrews, and from dolphins to flying lemurs, the overwhelming majority of the 4600-odd species of mammals alive today develop a true placenta, allowing offspring to emerge from a mother’s body with well-developed organs after an extended period germinating in the womb.

Some mammals, specifically marsupials such as the kangaroo and koala, have only a pseudo-placenta, which means that they must give birth at a very early stage of gestation. After that point, the embryo – looking something like squirming larva – crawls from the womb and finds its way to the pouch covering the mother’s nipples, where it can suckle milk. The joey continues to develop there, technically outside of the mother’s body, for many months. The pseudo-placenta in marsupials does not last long, nor is it very sophisticated.

The strange and complex approach to making babies in marsupial mammals reflects the strange and complex evolutionary history of the organ. In fact, the placenta has actually been ‘invented’ many times, in different families of animals. Fish have a version, in varying forms, and some sharks have a very advanced placenta, but no species have placentas quite like those found among mammals. Mammalian placentas are extremely complex and structurally diverse, with up to six layers that connect the mother with the developing embryo. Genes have arisen to adapt the mammalian placenta to a range of reproductive environments, to cater for situations as diverse as the twelve offspring expected after a mouse’s twenty-day gestation, to the lone calf that results from an elephant’s two-year pregnancy. How the placenta evolved to meet these needs, and everything in between, remains a mystery.

The last common ancestor of mammals, birds, and reptiles is likely to have had only the chorion, the outermost of a true placenta’s four layers, and a very basic one at that. Today, you can see a chorion when you peel a hard-boiled egg; it is the delicate layer that you find sticking to the shell, just inside. In modern birds and most reptiles, this thin, translucent membrane allows gases to be exchanged between the egg and the outside air. Our last common ancestor would also have had a yolk sac, which in modern birds, reptiles, marsupials, and some fish circulates nutrients to the developing embryo; it would also have had a primitive allantois, another sac-like tissue that gives rise to the blood vessels of the mammalian placenta as well as the umbilical cord. In chickens, the chorion and part of the allantois fuse together, forming a layer thick with blood vessels that pulls calcium from the eggshell to help nourish the embryo. In most marsupials, the allantois develops no blood vessels, since the embryo remains in the pseudo-placenta for only four or five weeks; instead it serves to store waste from the foetus’s kidneys, a critical job ensuring that the embryo doesn’t abort from toxicity. In other mammals, though, the allantois serves this purpose but is wound up in the wiring of the umbilical cord. Somehow it is these basic layers, the chorion and the allantois, that in mammals became the complex placenta.

Early in human development, after sperm meets egg and the DNA from father and mother have fused, the fertilized egg divides into two cells, as we saw in the last chapter. But the story is not as simple as that: the two cells are not equal. One of the cells is larger, and will continually and rapidly outgrow the other, as it divides over and over again. As the larger cell masses into a solid ball, it is surrounded by a flattened ring of tissue, constructed through the labour of the smaller, more slowly dividing cell. In nine months, it is the solid ball of cells that will be pulled screaming from the womb as a newborn child. The outer ring, called the trophoblast, will never become a part of the baby. It is the precursor of the placenta.

As the trophoblast grows into the life-support system for the embryo, in humans it will twice invade and destroy the lining of the mother’s womb. The first time happens within a week of conception. Fifteen weeks after conception, the second incursion occurs. This time the cells penetrate far deeper into the uterus, boring one-third of the way into the womb’s walls and entwining itself there in a structure that resembles a labyrinth, ensuring that nutrients can be leached from the mother in order