Why Humans and Most Mammals Don’t Lay Eggs

Niba, from the KQED Deep Look series Big Ideas, quite knowledgeably explained why humans and other mammals gestate rather than lay eggs, noting that there are a few exceptions to this rule.

Of the more than 5,000 known species of mammals, only five lay eggs. And to add to that, there are different reptiles, fish and invertebrates and other non-mammals that also do live birth. One size doesn’t fit all.

She also examined the variety of eggs laid by other land and sea creatures, noting that humans were once born from eggs.

The vast majority of animal species on this planet lay eggs, most insects, most fish, most amphibians, most reptiles,all birds, and even a few mammals lay eggs to reproduce. 
And if you go back far enough, you can see that our ancestors laid eggs for millions of years too. So what happened to us?