When children learn about different animals and how to classify them, they are often taught that three features unite mammals: hair, milk, and live birth. This last trait is likely taught to contrast mammals with the many other vertebrates that lay eggs.
But this last point is not correct. Nearly all mammals give birth to live young, but a handful do lay eggs. These are known as the monotremes, which encompass the platypus and several species of echidnas or spiny anteaters, all of which live in Australia, Tasmania and New Guinea.
The fact that a few mammals lay eggs, plus that most other land-dwelling vertebrates do as well, points to the idea that the rest of mammals descended from egg-layers. So how might one test this hypothesis? David Brawand and his colleagues  had the idea of looking at the genomes of mammals to see if they have any remnants of egg yolk genes.
Besides monotremes, the remaining mammals can be divided into two general groups: (1) marsupials, whose offspring are born early in development and then finish developing in a pouch (marsupium), and (2) placental mammals, which develop with the help of a placenta connecting the mother to the fetus. When Brawand and his colleagues looked at the genomes of three placental mammals (human, dog, armadillo), they found remnants of two egg-yolk genes (VIT1, VIT3), both of which possessed loss-of-function mutations. These mammals share some loss-of-function mutations in the genes, suggesting that the genes were inactivated in a common ancestor. Similarly, the researchers found remnants of three egg-yolk genes (VIT1, VIT2, VIT3) in the marsupials they studied (opossum, wallabies), with shared loss-of-function mutations in each (Figure 1), which, again, imply loss in a common ancestor.
Figure 1. DNA sequence alignment of egg-yolk genes . Highlighted portions indicate loss-of-function mutations. Gallus gallus = chicken; Monodelphis domestica = opossum; Macropus eugenii and Wallabia bicolor = wallabies.
By contrast, at least one egg yolk gene is intact in the egg-laying platypus. Together, these data suggest that the inactivation of egg yolk genes in placental and marsupial mammals is connected with the loss of their egg-laying ability through evolutionary time.
Questions for Creationists
Why do mammals that do not lay eggs have non-functional egg yolk genes in their genomes? If these species do not lay eggs and didn’t evolve from ancestors that lay eggs, why would God have put these in their genomes? Why do some mammals share some identical loss-of-function mutations in their egg yolk genes genes?