Biology Now, 2e

Neanderthal Sex ■ 307

Making up the third category, monotremes are

egg-laying mammals that lack a placenta alto-

gether. The only living species of monotremes

are just one platypus species and several echidna

species, all confined to Australia and New Guinea.

Moving now from class to order, humans and

Neanderthals are in the order of the primates

(Figure 17.7). Like all other primates, we have

flexible shoulder and elbow joints, five functional

fingers and toes, thumbs that are opposable (that

is, they can be placed opposite each of the other

four fingers), flat nails (instead of claws), and

brains that are large in relation to our body size.

Within the primates we are members of the

ape family, the hominids. We are not just closely

related to apes; we are apes. As such, we share

many characteristics with other apes, especially

chimpanzees, including the use of tools, a capac-

ity for symbolic language, and the performance

of deliberate acts of deception. But we are part of

a distinct branch of apes called hominins—the

“human” branch of the ape family that includes

the extinction of dinosaurs; if dinosaurs still

roamed Earth, it is likely that mammals would

be nothing but dinner. Lucky for us, mammals

have replaced dinosaurs as the top predators in

most terrestrial habitats, and they thrive in both

marine and freshwater environments. Only one

type of mammal can fly—bats—although a few

others can glide through the air.

Mammals can be divided into three broad cate-

gories, all of which feed their offspring with milk

(Figure 17.6). More than 95 percent of mammals

alive today are eutherians, including humans.

A unifying characteristic of eutherians is that

the offspring are nourished inside the mother’s

body through a special organ called the placenta

and are therefore born in a relatively well-devel-

oped state. Marsupials, the second category of

mammals, have a simple placenta, resulting in

offspring born early, which then complete devel-

opment in an external pocket or pouch. Marsu-

pials are found mainly in Australia and New

Zealand, with a few species in the Americas.

Eutherian Marsupial Monotreme

Figure 17.6

Three kinds of mammals

All mammals have milk-producing mammary glands for suckling their young. Eutherians like the polar bear

give birth to well-developed young. Kangaroos are marsupials, and give birth to immature young that

finish developing in a pouch. Monotremes like the platypus lay eggs instead of giving birth to live young.

Q1: The Virginia opossum, or possum, is the only North American marsupial. Describe how its

young are born and develop.

Q2: If you were a monotreme, would you still produce milk and nurse your young?

Q3: What kind of mammal is a cow? How about a human?