This issue is often quoted as an example of a question that is impossible to answer. But evolutionary theory tells us that the short answer to the question is 'the chicken'. Although the first chicken to lay an egg almost certainly looked very different from the chickens we are familiar with today. The egg evolved as a good way to give birth to offspring that could survive until the chick was mature enough to 'stand on its own two feet' (so to speak). Before egg-laying evolved, chickens would have 'carried' their young for longer and given birth to chicks that did not have the benefit of the nutrition and protection provided by the egg. Laying eggs was simply a better solution to the multiple needs of survival/ development in the environment of the species. These needs include; ability to get through the birth canal, continuing nutrition, warmth and protection from harm. In chickens, the best compromise was to lay eggs with 'in-house' nutrition and to keep them warm outside the body. In humans, it was to detach from the internal womb/ placenta and feed outside the body with milk. Evolution always 'chooses' the best compromise when there are multiple considerations.