In cases where a single species has both egg-laying and live-bearing populations, we can infer live birth has evolved very recently. One Australian species, Bougainville’s skink, has a live-bearing form on Kangaroo Island, off South Australia, but had an egg-laying form found on the mainland in South Australia, Victoria and New South Wales.
Live birth has evolved more recently in reptiles than in mammals. So researchers have to rely on systems where evolutionary transitions have occurred more recently. Understanding why evolutionary transitions that happened so long ago occurred can be difficult to infer. A couple of these egg-laying ancestors are still around today – the platypus and echidna – and both are found in Australia.įrom fossil evidence we know that today’s live-bearing mammals had a live-bearing ancestor that roamed the earth 160 million years ago, so live birth in mammals must have evolved before this. Evolution is a slow process, which means that many adaptations we see in animals today arose a long time ago.