I visited the natural history museum this week and found it really interesting. I got confused however when walking along the time line of evolution when mammals evolved. Did mammels evolve seperately from animals which lay eggs, or was there some kind of transition period between an animal laying an egg and then no longer laying eggs, but growing the offspring in a womb?

thank you.

There is a branch of mammalia called monotremes which lay eggs (platypus). They are quite definitely mammals, but a dinstinct grouping from the other two branches, placental mammals (like us) and marsupial mammals (kangaroo, koala).

http://en.wikipedia.org/wiki/Monotreme

Current theory says that mammals evolved from a reptile-like ancestor, which almost certainly laid eggs - virtually all reptiles lay eggs, though some do retain them inside their bodies and give birth to live young.

http://en.wikipedia.org/wiki/Ovoviviparity
http://science.jrank.org/pages/7245/Viviparity.html

Recently, molecular evidence has shown that monotremes are actually an early separate branch, not ancestral, as they have a completely different combination of reptilian-like and avian-like genes then other mammals. If monotremes are in fact an early branch, then it makes sense that our common ancestor probably also laid eggs.

http://news.nationalgeographic.com/news … typus.html

It's hard to say for sure how placental and marsupial mammals evolved our reproductive system, but given that some reptiles do retain their eggs internally, it seems plausible to speculate that our ancestors did this too, then evolved in new and different directions to the placental and and marsupial solutions we see today.

On a side note: the evolution from egg-laying (ovipary) to true live birth (vivipary) has evolved many times in the animal (and similary in the plant) kingdom. (Look up also ovivivipary on wikipedia to get a basic idea) Fish have evolved it separately many times, with the oldest one likely being Materpiscis from the Devonian (380 million years ago) of Western Australia.