Transcript Homo sapiens - McGraw

The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 27
How Humans Evolved
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
27.1 The Evolutionary Path to Apes
• the story of human evolution begins around 65 M.Y.A.
• this time marks the explosive radiation of a group of
small, arboreal mammals called the Archonta
 they were likely nocturnal and were arboreal and insectivorous
 their radiation gave rise to the different types of mammals,
including
• bats, tree shrews, and primates
• primates are the order of mammals that includes humans
27.1 The Evolutionary Path to Apes
• primates are mammals with two distinctive
features that allowed them to succeed in
the arboreal, insect-eating environment
 grasping fingers and toes
• the first digit in many primates is opposable and at
least some of the digits have nails
 binocular vision
• this permits the brain to judge distance precisely
27.1 The Evolutionary Path to Apes
• about 40 M.Y.A. the earliest primates split into
two groups
 prosimians
• surviving representatives today include the tarsiers, lemurs,
and lorises
• most are nocturnal
 anthropoids
• these higher primates included monkeys, apes, and humans
• most are diurnal
Figure 27.1 Prosimians.
Figure 27.2 A primate evolutionary
tree.
27.1 The Evolutionary Path to Apes
• the monkeys are a very successful group of
primates
 New World monkeys
• South American descendants of African ancestors
• all are arboreal, have flat spreading noses, and prehensile
tails
 Old World monkeys
• descendants of the ancestral anthropoids that remained in
Africa
• none have prehensile tails
• include both ground-dwelling and arboreal species
Figure 27.3 New and Old World
monkeys.
27.2 How the Apes Evolved
• hominoids evolved from anthropoid
ancestors from the split that formed Old
World Monkeys about 25 M.Y.A.
 hominoids are comprised by the apes and the
hominids (i.e., humans and their direct
ancestors)
Figure 27.4 The living apes.
27.2 How the Apes Evolved
• considerable controversy surrounds the
identity of the first hominoid
 very few fossils have been recovered from the
period 5 to 10 M.Y.A.
 past candidate ancestors include
Ramapithecus and Proconsul
27.2 How the Apes Evolved
• studies of ape DNA have revealed much about
how living apes evolved
 Asian apes evolved first
• gibbons diverged from other apes about 15 M.Y.A.
• orangutans split off about 10 M.Y.A.
• neither are closely related to humans
 African apes evolved more recently (between 6 to 10
M.Y.A.)
• these apes are the closest living relatives to humans
27.2 How the Apes Evolved
• chimpanzees are more closely related to
humans than gorillas are
 chimpanzees diverged from the ape line less than 6
M.Y.A.
 the genes of humans and chimpanzees have not had
time to evolve many differences
• humans and chimpanzees share 98.6% of their nuclear DNA
 gorilla DNA differs from human DNA by about 2.3%
• gorillas split off from the ape line around 8 M.Y.A.
27.2 How the Apes Evolved
• the common ancestor of apes and hominids is
thought to have been an arboreal climber
• much of the subsequent evolution of the
hominoids differs with respect to locomotion
 hominids evolved bipedal walking
 apes evolved knuckle-walking
Figure 27.5 A comparison of ape
and hominid skeletons.
27.3 An Evolutionary Tree with
Many Branches
• bipedal locomotion is the hallmark of
hominid evolution
• Australopithecus is an old and smallbrained lineage of hominid
 a skull specimen of Australopithecus
africanus was discovered in 1924
• this specimen was dated to be 2.8 million years old
27.3 An Evolutionary Tree with
Many Branches
• other australopithecine specimens were
discovered after A. africanus
 in 1938, Australopithecus robustus
 in 1959, Australopithecus boisei
 in 1974, Australopithecus afarensis
• this was the famous “Lucy” skeleton
• in the past 10 years, three additional kinds of
australopithecines have been reported
• an initial radiation of australopithecine seems to
have initiated the evolution of hominids
Figure 27.6 Nearly human.
27.3 An Evolutionary Tree with
Many Branches
• hundreds of fossils from australopithecine have
been examined
 all have been found in Africa only
• the structure of these fossils indicates that they
walked upright
• their dentition was hominid but their cranial
capacity was not any larger than that of apes
27.4 The Origins of Bipedalism
• the evolution of bipedalism marks the beginning of
hominids
• which came first, bipedalism or bigger brains?
 evidence of bipedalism extended back 4 million years ago
 Indications of brain expansion appeared roughly 2 million years
ago
• the real reason why bipedalism evolved is controversial
Figure 27.7 A reconstruction of an
early hominid walking upright.
Figure 27.8 The Laetoli footprints.
27.5 The Beginning of Hominid
Evolution
• recent fossil specimens of early hominids
have been found that date as far back as 6
to 7 million years
 these fossils are confusing because they
show a mixture of primitive and modern traits
 there are too few of these very old fossils to
make certain their connections to
australopithecines and humans
27.5 The Beginning of Hominid
Evolution
• in 1995, hominid fossils dating 4.2 million
years old were found in the Rift Valley of
Kenya
 they were assigned to the species
Australopithecus anamensis
 they represent an intermediate form between
apes and A. afarensis
 this species represents a base of the family
tree for human evolution
Figure 27.9 A hominid evolutionary
tree.
27.6 African Origin: Early Homo
• the first humans evolved from australopithecine
ancestors about 2 million years ago
• Homo habilis
 this “handy man” was described because fragments
of the first skeleton of this species was found in the
1960s interspersed with stone tools
 it had a larger brain volume than Australopithecus but
was similarly short in stature
27.6 African Origin: Early Homo
• Homo rudolfensis
 described from a specimen discovered in
1972 with a larger brain capacity than H.
habilis
• Homo ergaster
 a species used to describe specimens that
have a larger brain capacity than H.
rudolfensis and a skeleton more like modern
humans and less like australopithecines
Figure 27.10 Early Homo.
27.6 African Origin: Early Homo
• too few fossils have been found of early
Homo to explain with certainty the
evolution of Homo
 if three species are accepted, then it would
appear Homo underwent an adaptive
radiation
27.7 Out of Africa: Homo erectus
• Homo erectus is definitely a true human and has
been supported by many specimen finds
• Homo erectus was taller and had a larger brain
than H. habilis
 the shape of the skull interior suggests that it was
able to talk
 it was a social species
• although younger specimens have been found in
Asia, the origin of the oldest specimen of H.
erectus is Africa
Figure 27.11 Brain size increased
as hominids evolved.
27.8 The Last Stage of Hominid
Evolution
• modern humans first appeared in Africa
about 600K years ago
• there have been three species of modern
humans
 Homo heidelbergensis
 Homo neanderthalensis
 Homo sapiens
27.8 The Last Stage of Hominid
Evolution
• Homo heidelbergensis is the oldest known
modern human
 an Ethiopian fossil dates back to 600K years old
 it co-existed with H. erectus in Africa but had more
advanced features
 it had a bony keel running along the midline of the
skull and a thick ridge over the eye sockets
 its range included parts of Africa, Europe, and
western Asia
27.8 The Last Stage of Hominid
Evolution
• about 130K years ago, Homo
neanderthalensis appeared in Europe
 compared to modern humans, Neanderthals
were short, stocky, and powerfully built
 their skulls were massive
Figure 27.12 Our own genus.
27.8 The Last Stage of Hominid
Evolution
• the oldest known fossil of Homo sapiens is
130K years old and occurred in Africa
• outside of Africa and the Middle East, the
earliest known fossils of H. sapiens are no
older than 40K years old
• did Homo sapiens evolve first in Africa and
then migrate to the rest of the world?
27.8 The Last Stage of Hominid
Evolution
• Recently-Out-of-Africa model
 this view of Homo evolution states that Homo
sapiens evolved in Africa and then migrated to
Europe and Asia
• Multiregional hypothesis
 this view of Homo evolution states that the
human races evolved independently from
Homo erectus in different parts of the world
27.8 The Last Stage of Hominid
Evolution
• studies of human mitochondrial DNA (mtDNA) have
helped to clarify the controversy over the origin of Homo
sapiens
 because DNA accumulates mutations over time, the oldest
populations should show the greatest genetic diversity
 all modern humans of different ethnic backgrounds share
common ancestor dating back 170K years ago
 only 52K years ago do Africans separate from non-Africans
 essentially the migration of Homo sapiens out of Africa followed
the same paths taken by Homo erectus taken half a million years
before
Figure 27.13 Out of Africa—
many times.
27.9 Our Own Species: Homo
sapiens.
• Neanderthals
 this species of modern human were common in
Europe in Asia around 70K years ago
 they did not interbreed with members of Homo
sapiens
• Cro-Magnons
 fossils of these early members of Homo sapiens date
back as late as 100K years ago in Europe
 they appear to have completely replaced the
Neanderthals around 34K years ago
Inquiry & Analysis
• Is the angle of the
premolar tooth in Lucy’s
jaw set at a sharp angle
to the tooth row, or
square to it?
• Based on the
comparisons, does Lucy’s
upper jaw more closely
resemble a chimpanzee
or a human jaw?
Figure of Jaws of Chimpanzee,
Human, and Lucy