Trying to think of things to write on this blog I asked for help from my friend’s son Ben. He wants to know: “Why does a platypus lay eggs?”  Well, here goes nothing and I will try to make this as painless as possible.

The simplest answer to that question is because there were once many egg laying mammals around which have all gone extinct.

Scientists break mammals into three large groups:

Prototherians - which lay eggs (eg. platypuses and echidnas)

Metatherians - young develop in pouches (marsupials such as kangaroos and koalas)

Eutherians – ‘modern mammals’ which give birth to well-developed offspring (rabbits, elephants, horses, and humans, ect.)

All three of these groups of animals share  three of the characteristics which are unique to all mammals; hair, three inner ear bones, and modified sweat glands which produce milk (mammary glands). 

Members of the Prototherians (platypus and spiny anteater) represent a very primitive form of mammals and still have some of the characteristics of their ancestors, for example the ability to lay eggs. If we look at Date-A-Clade, which shows how organisms are grouped together based on common characteristics, we see that the line for the platypus branches off from that of other mammals 166 million years ago.  All of the other primitive mammals below where the platypus and other mammals branch off were egg layers too. All of the egg laying mammals above the split evolved over millions and millions and millions giving us marsupials (kangaroos) and placental mammals (cats, dogs, and humans).

Want to know more about the Protherians check out these pages:
http://www.scientificamerican.com/article.cfm?id=extreme-monotremes
http://www.mammalsrus.com/prototheria/prototheria.html