The amniotic egg
An egg with its own private pond inside it frees vertebrates from water for good.
- When
- Known to
- ± 10 million years
- Era
- Carboniferous, Paleozoic
The amniotic egg encloses the embryo in fluid-filled membranes with a yolk for food and a chamber for waste, all within a shell that resists drying. Reproduction no longer requires returning to water, and vertebrates can colonise dry uplands and continental interiors. Every reptile, bird and mammal — including those, like us, that later re-internalised the egg — depends on this structure.