Biology Now, 2e

Neanderthal Sex ■ 303

some 700 million years ago, descended from flag-

ellated protists, single-celled organisms with

whiplike tails that thrived in wet habitats and

began to use oxygen to break down food into

energy. From those humble origins, count-

less animal species evolved—beginning with

sponges and including mollusks such as snails

and clams, the annelids (segmented worms),

and arthropods like crustaceans, spiders, and

insects (Figure 17.2).

Chordates make up a large phylum that

includes all animals with backbones, such as

fishes, birds, and mammals (see Figure 17.3 and

quickly driven to extinction, and therefore the

two did not live side by side. Others claim the

opposite: that Neanderthals, Homo neandertha-

lensis, were slowly incorporated into the popula-

tion of newly incoming humans, Homo sapiens.

But to know how humans evolved, it is import-

ant first to realize where we came from.

All species in the Homo genus, of which H.

sapiens is the only one alive today, are in the

Animalia kingdom. Animals are multicellular

ingestive heterotrophs; that is, we obtain energy

and carbon by ingesting food into our bodies

and digesting it internally. Animals first evolved

No

symmetry

Radial

symmetry

Bilateral

symmetry

Notochord

Domain

Bacteria

Domain

Eukarya

Domain

Archaea

Kingdom

Protista

Kingdom

Fungi

Kingdom

Animalia

Common
ancestral
cell or
universal
ancestor

Kingdom

Plantae

Sponges

Mollusks

Flatworms

Annelids

Nematodes

Arthropods

Echinoderms

Chordates

Cnidarians

Growth front to back

(mouth first)

Growth back to front

(mouth second)

Growth by

molting

Figure 17.2

Animals are heterotrophic eukaryotes

Animals ingest their food, which can be organisms from any of the other domains and kingdoms, and then digest it internally.

Q1: Are mollusks more closely related to flatworms or to annelids? Explain your reasoning.

Q2: If you found an animal with no symmetry, to what group would it belong? Give an example of an animal that is radially

symmetrical, and one that is bilaterally symmetrical.

Q3: How do we know that a sponge is an animal, and not a plant or an alga?