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BIO 156

Chapter 23

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HumanEvolution

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How Evolution Works

Genetic variation is the raw material of

evolution.

Changes in species occur because of

changes in genetic material. Genetic variation in a species arises from

several processes including:

Mutations

new combinations resulting from sexual

reproduction.

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Natural selection is a process by which

organisms become better adapted to

their environment. Natural selection is a process in which slight

variations, if useful, are preserved.

Abiotic and biotic factors contribute tonatural selection.

Natural selection ensures that the fittestorganisms survive and reproduce.

Fitness is a measure of reproductive

success.

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The Evidence Supporting Evolution

The fossil record yields some of the best

supporting evidence for the existence of

evolution.

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Common anatomical features in different

species support the theory of evolution.

Comparing homologous structures permits

scientists to determine evolutionary

relationships.

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The common biological makeup oforganisms also supports the theory of

evolution.

Studies of common developmentalpatterns supports the theory of

evolution.

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Experimental evidence

supports the theory of

evolution by naturalselection.

The study ofbiogeography helps

explain evolution.

Biogeography is the

study of thedistribution of plants

and animals

worldwide.

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Early Primate Evolution

Humans belong to the ordercalled primates, which

includes two suborders:

Prosimians (premonkeys

Anthropoids (monkeys,apes, and humans.

The prosimians probably

evolved from tree-dwellingmammals that lived during

the age of the dinosaurs.

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Primates are characterized by graspinghands, forward-looking eyes, and large

brains.

The fossil evidence of primate evolution

is limited, making it difficult to determine

the exact progression of early human

ancestors.

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Evolution of the Australopithecines

The earliest hominids were members of the genus

Australopithecus.

The oldest known australopithecine skeleton, named Lucy, is

believed to be about 3.5 million years old.

The skeleton belongs to a group calledAustralopithecus

afarensis and stood about

3 feet high.

About 3 million years ago,A. afarensis was replaced byA.

africanus.

A. robustus appeared approximately 2.3 million years ago, andA. boiseiappeared approximately 2.2 millions ago.

Australopithecines disappeared about

1 million years ago; no one is sure why.

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Evolution of the Genus Homo

The first truly humanlike creatures were

Homo habilis.

Homo habilis gave rise to Homo

erectus.

H. erectus moved from Africa to Europe

and Asia.

Homo erectus is believed to be the direct

ancestor ofHomo sapiens.

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Modern humans belong to Homosapiens.

Homo sapiens emerged about 300,000years ago

Homo sapiens consists of two subspecies:

Homo sapiens neanderthalensis (the

Neanderthals Homo sapiens sapiens

Cro-Magnons are the earliest knownmembers ofH. sapiens sapiens.

The Neanderthals disappeared about40,000 years ago.

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Human races result from variations

caused by geographic separation.

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End of Chapter 23