Evolution, 4th Edition

T HE TREE of LifE 31

evolved from a symbiotic association (B) between an archaean and a bacterium

that evolved into the mitochondrion. So the mitochondria in each of our cells are

more closely related to E. coli bacteria than they are to the nuclei in the same cells!

Early eukaryotes evolved into diverse lineages. One of these became the green

algae, which acquired symbiotic photosynthetic cyanobacteria that evolved into

chloroplasts (C). Green algae gave rise to the true plants (D).

Remarkably, complex multicellular life forms evolved many times. Among

these groups are plants, brown algae (E), some fungi (F), and animals (G). Fungi

and animals stem from a single ancestor (H), so we are more closely related

to mushrooms than to plants. The closest relatives of animals are single-celled

choanoflagellates (I), which closely resemble some of the cells in sponges (see

Figure 17.6).

Several groups of animals have no heads. Some, such as jellyfishes, never had

one in their evolutionary past (J). Others had a head but then lost it. The echi-

noderms, such as starfishes, evolved radial symmetry from ancestors that were

bilaterally symmetric, and in doing so became headless (K).

The vertebrates (L) are most closely related to echinoderms (K) in this tree.

Among the tetrapod (four-limbed) vertebrates, research using fossils, compara-

tive anatomy, and DNA has revealed some unexpected relations. Birds are more

closely related to crocodiles than to any other living animals (M). Primates—

including people—are more closely related to rodents than to most other orders

of mammals. Whales are clearly mammals rather than fishes, and it turns out

that they are related to hippopotamuses (N).

The diversity of species within different groups is wildly uneven. With about

33,000 species, bony fishes (O) are the most diverse group of vertebrates. All the

other vertebrates combined sum to about 30,000 known species. But the diversity

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_02.05.ai Date 11-02-2016

Q: OK to place all red dot labels on lines?

Other protists

Brown algae

Plants

Green algae

Archaea

Chloroplasts

Cyanobacteria

Mitochondria

Proteobacteria

LUCA

Hippopotamus

Crocodiles

Birds

Bony shes

Echinoderms (sea urchins)

Coelenterates (jellyshes)

Sponges

Choanoagellates

Fungi

Arthropods

Molluscs

Primates

Rodents

Whales

O

K

J

I

F

E

D

P

C

N

ML

G

H

B

A

FIGURE 2.5 A phylogeny of some

major groups of organisms. “LUCA”

means Last Universal Common An-

cestor. The letters are keyed to de-

scriptions of some interesting events

in the history of life. The dashed

arrows show the origin of symbiotic

mitochondria and chloroplasts from

two lineages of bacteria. See the text

for explanations.

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