Milestones in the evolution of eggs

ALTHOUGH reptiles “conceived” the eggshell, it was birds that grasped the challenge and took it to the next level.

Interestingly, birds are the only animals which practise this mode of reproduction 100 per cent uniformly, whereas other animals may have representatives of both egg-laying and live birth. Almost certainly, birds never evolved viviparity (giving birth to live young that have developed inside the parent), because of the vast energy expenditure requirements it would take in flying while carrying a reproductive system full of developing young.

“The addition of those extra membranes allowed the egg