The egg that let backboned animals leave the water for good
Frogs must return to ponds to breed, but lizards, birds and people never have to. The difference traces back to amniotes, a group whose embryos float inside a protective sac of fluid. Clawed footprints found in Australia in 2025 suggest this group arose earlier than anyone had assumed.
Amniota is defined as the smallest group containing humans, the Greek tortoise and the Nile crocodile, plus every descendant of their last common ancestor. What unites them is an embryo wrapped in three extra membranes: the amnion for protection, the chorion for gas exchange and the allantois for storing waste. Add internal fertilisation, tough keratinised skin, breathing by moving the rib cage and an ankle bone called the astragalus, and these animals could breed far from water, either by laying shelled eggs or by carrying young inside the mother. Amphibians, by contrast, keep a larval stage, metamorphosis and gills. The name itself comes from a Greek word for the membrane round a fetus, which originally meant a bowl for catching sacrificial blood and ultimately derives from the word for lamb.
The egg did not appear in one leap. Early ancestors swapped the jelly coat of an amphibian egg for a fibrous shell membrane, which allowed bigger eggs and livelier embryos. Without internal breathing and waste structures, though, such eggs probably could not exceed about 1 cm across, which would have kept adults small; modern lizards laying eggs that tiny rarely exceed 10 cm in body length. Many early amniotes still laid eggs in damp hollows under logs rather than in dry places.
Body fossils long pointed to an origin in the mid to late Carboniferous, with the oldest known being Hylonomus and Asaphestera from Nova Scotia, around 318 million years old. The Snowy Plains footprints, dated 358.9 to 354 million years ago, push the split between the reptile-and-bird line and the mammal line back further, probably into the Devonian.
Skull openings behind the eye still sort the lineages: none in anapsids, one in synapsids such as mammals, and two in diapsids like birds and crocodiles. Ernst Haeckel coined the name Amniota in 1866.
Source: Amniote
