First Life
3.8 billion years ago, in water, something happened that had never happened before in the observable universe. Chemistry became biology. Non-living molecules began making copies of themselves. Life had started.
The impossible question
Nobody knows exactly how life started. It is one of science's great unsolved questions. What we know is that 3.8 billion years ago, single-celled organisms called prokaryotes existed on Earth. They were simple — no nucleus, no complex internal structure — but they were alive. They could metabolise energy, respond to their environment, and reproduce.
Where it may have happened
Leading candidates for where life first emerged include hydrothermal vents on the ocean floor — hot, chemically rich environments shielded from the radiation that bathed early Earth's surface. These vents create steep chemical gradients between the acidic ocean water and alkaline vent fluid, which could have driven the first chemical reactions that resembled metabolism.
The greatest partnership in history
For 2 billion years, single-celled prokaryotes were all that existed. Then, about 1.5 billion years ago, something extraordinary happened: one cell was engulfed by another and, instead of being digested, it survived inside. It became a permanent resident — a cell within a cell. The inner cell specialised in producing energy. We call those structures mitochondria. This partnership created complex cells, and eventually everything else — plants, animals, fungi, and you.
The oxygen revolution
Early life didn't need oxygen — the first atmosphere had almost none. Then cyanobacteria evolved photosynthesis, using sunlight to split water and producing oxygen as a waste product. Over hundreds of millions of years they pumped enough oxygen into the atmosphere to fundamentally change Earth's chemistry — the Great Oxidation Event. For most existing life at the time, this was a catastrophe. For the life that evolved to use oxygen, it was a superpower.
The RNA World hypothesis proposes that RNA — not DNA — was the original molecule of life. RNA can both store genetic information and catalyse chemical reactions, meaning a single molecule could theoretically both carry instructions and execute them. DNA, which can only store information, and proteins, which can only act, may have evolved later as specialist tools from this original all-in-one molecule.