Evolution, 4th Edition

548 CHAPTER 21

Where Did We Come From?

Humans are descended from the last universal com-

mon ancestor (LUCA) of all living organisms on

Earth. We share fundamental features with them:

inheritance based on nucleic acids (DNA and RNA),

the genetic code, proteins composed of L amino

acids, and much more. Among the grand events in

our earliest ancestry was the origin of the eukary-

otes, a symbiosis between an archaean and a bac-

terium (FIGURE 21.1). That momentous event led to

the evolution of diverse unicellular forms and sev-

eral multicellular lineages, which then evolved tis-

sues and organs. One lineage was the progenitor

of animals. A descendant then gave rise, some 550

Mya, to the sea stars and other echinoderms on the

one hand, and to the chordates on the other hand.

Much later, from among diverse vertebrate chor-

dates, arose the ancestor of tetrapods, with legs that

evolved from fins. About 150 My after the first land-

dwelling tetrapod, some of its descendants stood on

the brink of mammalhood. By about 70 Mya, some

familiar groups of mammals had appeared, includ-

ing the first primates.

Our closest living relatives

About 35 Mya, the Old World monkeys and apes

(the catarrhine primates) arose. Humans are in a

group called the apes, which are most visibly distin-

guished from other primates by the lack of an exter-

nal tail (FIGURE 21.2). Our closest relatives are the chimpanzee (Pan troglodytes)

and the bonobo (Pan paniscus). The lineage that includes our own species is called

FIGURE 21.1 The path of evolution leading from the origin of life to

Homo sapiens. Some of the key traits we gained along the way are indi-

cated. Several of the dates for the origins of the traits (shown at right) are

very uncertain because fossils showing those transitions are not available.

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Bipedalism

Tail lost

Binocular vision

Placenta

Amniotic egg

Legs

Bone

Dorsal nerve cord

Nerves and muscle

Multicellularity

Mitochondrial symbiosis

Origin of life

4

BacteriaArchaeaYeastPotatoSea starSea squirtCarpFrogLizardMouseMonkeyChimpanzee Mya

18

70

150

350

370

500

550

650

1000

2000

3500

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Sinauer Associates

Troutt Visual Services

Evolution4e_21.02.ai Date 01-09-2017

Gibbon Orangutan Gorrilla Human Bonobo Chimpanzee

7 Mya

FIGURE 21.2 Phylogeny of some of the living apes, illustrated by the white-handed

gibbon (Hylobates lar), orangutan (Pongo pygmaeus), western gorilla (Gorilla gorilla),

human (Homo sapiens), bonobo (Pan paniscus), and chimpanzee (Pan troglodytes).

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