The 250-million-year-old egg that rewrites our family tree

A curled-up embryo the size of a walnut, hidden in a South African rock for two decades, has finally answered one of evolution's oldest questions — did our mammal ancestors lay eggs?

By Helen Cooper

Picture a squat, pig-sized creature with a turtle's beak, two downward-pointing tusks and bare, leathery skin. Now picture it waddling across a parched, broken landscape where nine out of every ten species on Earth have just been wiped out.

That was Lystrosaurus — the improbable survivor of the worst day in life's history, and, as we've now learned, one of our own distant relatives. A fossil discovery announced this month finally reveals how it pulled off the trick: it laid eggs.

For more than 150 years, palaeontologists have hunted for proof that the therapsids — the sprawling group of proto-mammals that eventually gave rise to us — produced eggs like their reptilian cousins, or whether they had already switched to live birth. The platypus and the echidna, the oddballs of the modern mammal family, still lay eggs today, so the suspicion was always there. The evidence was not.