Biology Now, 2e

Whale Hunting ■ 205

hind limbs. “We know, from fossil evidence, that

early whales lose their hind limbs,” says Thewis-

sen. So he wondered if hind limbs exist in whale

or dolphin embryos (dolphins are also mammals

and are closely related to whales). And if so, what

makes them subsequently disappear before the

animal is born.

Examining spotted dolphin embryos, Thewis-

sen saw that when the embryos are the size of a

pea, they do develop hind limb buds, but by the

time they grow into the size of a bean, the limb

buds are gone. In 2006, he and researchers at

several other universities studied the genes that

are active in whale and dolphin embryos and

concluded that whales’ hind limbs regressed

over millions of years through small changes in

species of whales, such as the crocodile-like

Pakicetus, that lived in rivers and lakes but

did not swim in the ocean are found near India

and Pakistan. “It makes sense,” says Thewissen.

“You don’t have crocodiles crossing the Atlan-

tic.” But fossils of fully aquatic protocetids,

which emerged about 40 million years ago, are

geographically much more widespread; they

have been found as far away from Pakistan

as Canada. “Protocetids are good swimmers,

so we find their fossils all around the world,”

says Thewissen.

Growing Together

Though Thewissen has built a career on find-

ing and describing whale fossils, he has recently

become enamored with another vein of evolu-

tionary evidence: embryology. A major predic-

tion of evolution is that organisms should carry

within themselves evidence of their evolution-

ary past, and they do. Evidence of evolution can

be observed in shared patterns of embryonic

development.

Once again, these common patterns are

caused by descent from a common ancestor.

Rather than evolving new organs “from scratch,”

new species inherit structures that may have

been modified in form and sometimes even in

function.

Upon fusion of sperm and egg, an animal

embryo begins to grow and develop. The manner

in which an embryo develops, especially at the

early stages, may mirror early developmen-

tal stages of ancestral forms. For example,

anteaters and some baleen whales do not have

teeth as adults, but as fetuses they do. And the

embryos of fishes, amphibians, reptiles, birds,

and mammals (including humans) all develop

pharyngeal pouches or gill slits (Figure 11.15).

In fish, the pouches develop into gills that adult

fish use to absorb oxygen underwater. In human

embryos, these same features become parts of

the ear and throat.

“I was interested to get embryos to look at

some of these processes that we see happen in

evolution, to see if they happen in development,”

says Thewissen. The first trait he examined was

Fish Reptile Bird Human

Gill slits Tail

Embryos all share gill slits and tails because

fishes, reptiles, birds, and humans all evolved

from a common ancestor that had these features.

Figure 11.15

Evolutionary history can be extrapolated from similarities in

embryo development

Complex structures in descendant species are generally elaborations of

structures that existed in their common ancestor.

Q1: Why are the similarities among organisms during early

development evidence for evolution? Give an example.

Q2: Are the similar structures among vertebrate species during

embryogenesis homologous structures? Explain.

Q3: Why do embryonic structures still exist at points during

embryogenesis if they are not used after birth?