What is the human body made of?
The human body is made of six main elements by mass: oxygen (65%), carbon (18.5%), hydrogen (9.5%), nitrogen (3.5%), calcium (1.5%), and iron (0.7%). Trace elements like phosphorus, potassium, and gold make up the remaining 1.3%.
Every atom in your body was forged long before you existed. The hydrogen — 9.5% of you — was made in the first three minutes after the Big Bang, 13.8 billion years ago. It has never been anything else.
Oxygen — 65% of your body
Oxygen is the most abundant element in you by mass. Most of it is in water molecules, inside every cell. The oxygen atoms in your body have been cycling through the Earth for billions of years — as rain, ocean, glacier, and river. They were originally made inside massive stars that fused helium into oxygen, then scattered it into space when they exploded as supernovae.
Carbon — 18.5% of your body
Carbon is the backbone of every molecule in your body — DNA, proteins, fat, sugars. It enters your body through food and was recently a plant that pulled carbon dioxide from the air using sunlight. Carbon is made when three helium atoms fuse together inside a star, a process so unlikely it requires a very specific nuclear resonance to work.
Hydrogen — 9.5% of your body
Hydrogen is the simplest and oldest atom inside you. It makes up the two small arms on every water molecule. Almost all hydrogen was made in the first three minutes after the Big Bang, before a single star existed. It has been hydrogen for 13.8 billion years.
Nitrogen — 3.5% of your body
Nitrogen is in every strand of your DNA and every amino acid. You get it by eating protein. About half the nitrogen in your body probably came from the Haber process — a factory that pulls nitrogen from the air to make fertiliser. Before that invention, all life depended on lightning and soil bacteria for its nitrogen. The atoms themselves were made inside stars through the CNO cycle.
Calcium — 1.5% of your body
Calcium is your skeleton — about a kilogram of it in your bones and teeth. It cycles through the Earth over huge timescales, as limestone, chalk, coral reef, and soil. Calcium is made in the final stages of a massive star's life, in the silicon-burning shell just before it collapses and explodes.
Iron — 0.7% of your body
There are about three grams of iron in you. Most of it is in your blood, inside haemoglobin, where it carries oxygen to every cell. Iron is where a star's furnace stops paying for itself — fusing it takes more energy than it releases. When a massive star's core fills with iron, it collapses and explodes, scattering the iron into space.
Where did the atoms in your body come from?
Every element heavier than hydrogen was made inside a star through nuclear fusion and scattered into space by supernova explosions. Over billions of years, this debris mixed in giant molecular clouds. About 4.6 billion years ago, part of one such cloud collapsed into a spinning disc — the solar nebula — and formed our Sun and planets, including Earth. The atoms in your body have been rock, ocean, atmosphere, and countless living things before becoming you.