1 In every cell
Your power stations
Inherited from The first complex cells · more than 1.5 billion years ago
Almost every cell in your body contains hundreds of mitochondria, tiny parts that turn food into usable energy. They were once free-living bacteria. Well over a billion and a half years ago, perhaps closer to two billion, a bacterium came to live inside another cell and never left. That partnership led to all complex life: plants, fungi, animals and you.
How we know
Mitochondria still have their own small, ring-shaped DNA, like bacteria. They divide by splitting, like bacteria. Their genes are closest to a group of living bacteria.
Careful: When exactly this happened, and what the host cell was like, is still debated.
2 Eye
Your blind spot
Inherited from The first vertebrates · about 563 million years ago
In your eye, the nerve fibres lie in front of the light-sensing cells and leave the eye through a single hole. That hole has no sensors, so each eye has a small blind spot. Your brain fills it in, so you never notice. Every animal with a backbone has eyes built this way.
How we know
Lampreys, whose line split from ours more than 500 million years ago, already have the same layout. Octopus eyes evolved separately: their wiring lies behind the light cells, and they have no blind spot.
Careful: This layout is not simply a mistake. It keeps the light cells next to the tissue that feeds them, and the brain hides the gap.
3 Neck and chest
The nerve that takes a detour
Inherited from Early jawed fish · about 463 million years ago
The nerve to the left side of your voice box leaves the brain, runs down into your chest, loops under the main artery near the heart and climbs back up to the throat. In fish, the same nerve runs a short, direct path to the gills. As our ancestors grew necks and the heart moved down, the loop was dragged along. In a giraffe the detour is several metres long.
How we know
In embryos, the nerve forms next to the arteries of the pharyngeal arches, the same structures that become gills in fish, and it is pulled down with them as the heart descends. On the right side, the nerve loops lower in the neck instead.
Careful: The detour works well enough. It shows how evolution reshapes what already exists, rather than starting from scratch.
4 Chest
Lungs from a fish
Inherited from Early bony fish · about 435 million years ago
Your lungs began as a pouch off the gut in early bony fish, which could gulp air when the water held little oxygen. Lungfish and bichirs still breathe this way. In most other fish, a gas-filled pouch became a swim bladder that keeps them afloat.
How we know
Lungs and swim bladders are related, but they grow from opposite sides of the gut in embryos: lungs from the underside, swim bladders from the top. Comparing living fish suggests lungs came first.
Careful: Exactly how the swim bladder arose from the ancestral air pouch is still being worked out.
5 Hand and arm
A fin you write with
Inherited from Lobe-finned fish · about 402 million years ago
Your arm has one bone, then two, then a cluster of small wrist bones, then fingers. The same pattern, without fingers, sits inside the fleshy fins of fossil lobe-finned fish. Your ancestors among them began to use these fins to prop themselves up in shallow water.
How we know
Fossils such as Eusthenopteron and Tiktaalik, about 385 and 375 million years old, show fins with a shoulder, an elbow and wrist-like bones. Many of the same genes shape fins in fish embryos and hands in ours.
Careful: Tiktaalik was a relative of our ancestors, not necessarily an ancestor itself.
6 Chest
Your hiccups
Idea, not yet proven
Inherited from Possibly early four-limbed animals · more than 352 million years ago
A hiccup is a sudden pull of the breathing muscle, followed by a snap shut of the airway. One idea is that it is left over from a reflex that tadpoles and some fish use to push water over their gills while keeping it out of the lungs. Another idea is that it helps baby mammals bring up air swallowed while feeding.
How we know
Supporters of the first idea point out that the nerve circuits for hiccups resemble those that control gill breathing in tadpoles. Supporters of the second note that hiccups are most common in babies, and even before birth.
Careful: Both are hypotheses. Nobody knows yet why we hiccup.
7 Skin
Goosebumps
Inherited from The first mammals · about 181 million years ago
When you are cold, or moved by music, tiny muscles pull your body hairs upright. In a furry animal this fluffs up the coat, which traps warm air or makes the animal look bigger. Our body hair is too thin for that to help much, but the reflex remains.
How we know
All mammals have hair, and the same small muscles sit beside each hair in a cat, a mouse and you.
Careful: These muscles are not useless. In mice, they and their nerve help to wake the stem cells that grow new hair.
8 Ear
Jaw bones that became ear bones
Inherited from The ancestors of mammals · about 181 million years ago
Two of the three tiny bones in your middle ear, the hammer and the anvil, were once part of the jaw joint of your ancestors. Over tens of millions of years they shrank, moved backwards and became part of a sensitive ear.
How we know
A series of fossils shows these bones changing step by step. In mammal embryos they still start out next to the lower jaw before moving into the ear.
Careful: The fossils suggest this happened more than once: separately in the line of the platypus and in ours.
9 Belly
Your belly button
Inherited from Placental mammals · about 97 million years ago
Your navel is the scar left by the umbilical cord, which joined you to the placenta before you were born. A placenta that feeds the young for a long time inside the mother is the way of placental mammals, from mice to whales to you.
How we know
Comparing the placentas of living mammals on the family tree of mammals suggests what the placenta of their common ancestor was like.
Careful: Marsupials such as kangaroos also make a placenta, but only briefly, and many mammals have a navel too, often hidden by fur.
10 Mouth
Wisdom teeth
Inherited from Placental mammals, with a shorter face in our species · about 97 million years ago
Like most placental mammals, your ancestors had three molars on each side of each jaw. The third molar, the wisdom tooth, often has no room in your mouth, because human jaws have become shorter. Jaws seem to grow smaller when children eat soft food, which may be why crowded teeth became common after farming began, mostly in the last ten thousand years.
How we know
Skulls of hunter-gatherers usually have more room for teeth than skulls of farming populations.
Careful: Some people never grow wisdom teeth, and for many they come in without trouble.
11 Eye
Seeing red and green
Inherited from The shared ancestors of apes and Old World monkeys · about 29 million years ago
You can tell red from green because a gene for a light-sensing protein was copied in the ancestors you share with the monkeys of Africa and Asia. Most other mammals see fewer colours, a bit like a person with red-green colour blindness.
How we know
The two genes sit side by side on the X chromosome and are almost identical, a typical sign of a copied gene.
Careful: Why it spread is less certain. One idea is that it helped find ripe fruit or young red leaves. Because the two genes are so alike, they can get mixed up, which is why red-green colour blindness is common.
12 Lower back
Your tailbone
Inherited from The first apes · about 25 million years ago
At the bottom of your spine, a few small fused vertebrae are all that is left of a tail. Apes lost the tail after their line split from the Old World monkeys (the monkeys of Africa and Asia). In 2024, researchers found a change in a gene called TBXT that may have helped.
How we know
Human embryos have a short tail for a few weeks before it is absorbed. Mice given a similar genetic change were born with shorter tails or none.
Careful: The tailbone is not useless: muscles and ligaments of the pelvis attach to it.