Evolution, 4th Edition

“In my possession are two little embryos in spirit [alcohol], whose names I

have omitted to attach, and at present I am quite unable to say to what class

they belong. They may be lizards or small birds, or very young mammalia, so

complete is the similarity in the mode of formation... in these animals.”

In this passage in On the Origin of Species, Darwin quoted the world’s lead-

ing authority on the embryonic development of animals. Karl Ernst von Baer had

found that the early embryonic stages of the most diverse vertebrates had fea-

tures in common that they would lose later in development, and that the special

characteristics that distinguish mammals from reptiles, or primates from rodents,

developed only at later stages. Early in development, a human is virtually indis-

tinguishable from an alligator, much less any other mammal (FIGURE 15.1). This

astonishing fact is one of countless examples of features displayed early in

development that added to Darwin’s mountain of evidence for the evolution of

diverse species from common ancestors. The embryos of birds, anteaters, and

baleen whales develop incipient teeth that are resorbed and are lacking in the

adults. Human embryos have a tail that usually stops growing and persists only as

three fused vertebrae at the end of our spine; but infants are occasionally born

with an anatomically complete tail. As in other primates, human fetuses develop

a dense coat of hair, although most humans shed it about a month before birth.

Clearly, the developmental processes by which a fertilized egg becomes a

differentiated organism, with features such as toes and a tail, are shared among

species; and these processes result in yet other puzzling similarities. Some plants

The flowers of most angiosperms bear distinct sepals, petals, stamens, and carpels that differentiate be-

cause of the action of combinations of specific regulatory genes. Water lilies, such as Nymphaea lotus,

are one of several phylogenetically basal angiosperm lineages in which sepals and petals are not well

differentiated, and in which the stamens can have a petaloid form. These plants suggest that the gene

regulatory networks that determine the identity of organs have evolved gradually.

Ethics, religion, and evolution

Development

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