Arriving Late, Traveling Far: The Evolution of Human Beings

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

BIOLOGYA GUIDE TO THE NATURAL WORLD

FOURTH EDITION

DAVID KROGH

Arriving Late, Traveling Far:The Evolution of Human Beings

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.

20.1 The Human Family Tree


The Human Family Tree

• Human evolution is the study of the taxonomic grouping called the Hominini or human-like primates.

• Every member of this group is referred to as a hominin, including human beings.



• A common primate ancestor is believed to have given rise to both the chimpanzee and the human family evolutionary lines between 6 and 7 Mya.



• The structure of the hominin tree is a matter of considerable debate among researchers.

• We do not have enough fossil evidence to say with certainty which species were ancestral to which others.


20.2 Human Evolution in Overview


Human Evolution in Overview

• All the members of the hominin grouping are extinct except for Homo sapiens, the human species.

• Human beings were late in arriving among the hominins, but all hominins are late arrivers when the entire sweep of evolution is considered.



• Until the past 10 years, evidence indicated that most early- and mid-period hominin evolution took place in east Africa.



Figure 20.2

Africa

Chad

South Africa Indian Ocean

Ethiopia

Kenya

Tanzania

Sahelanthropustchadensis

Orrorintugenensis

Earliest modernhuman(Homo sapiens)

Australopithecusfossil discoveries

Paranthropusfossil discoveriesAustralopithecusand Ardipithecusfossil discoveries



• But in 2002, the range of likely hominin fossils was expanded 2,500 kilometers to the west, to Chad, with the discovery of the 6- to 7-Mya remains of a primitive hominin named Sahelanthropus tchadensis or Toumaï.



Figure 20.1

A Hominin Family Tree

Present

H. habilis

K. rudolfensis

Kenyanthropusplatyops

H. mauritanicus

H. neanderthalensis

H. erectus

P. boiseiP. robustus

H. floresiensis

H. heidelbergensis

H. cepranensis

H. sapiens

Au. africanus Paranthropusaethiopicus

ancestral primate

Homoergaster


Ardipithecuskadabba

Ar. ramidus

Orrorintugenensis

Australopithecusanamensis

Au. afarensis

Au. garhi

Au. bahrelghazali

1

2

3

4

5

6

7

recent

Plei

stoc

ene

Plio

cene

Mill

ions

of y

ears

ago

mon

keys

oran

guta

ns

goril

las

chim

ps

hom

inin

s


20.3 Interpreting the Fossil Evidence


Interpreting the Fossil Evidence

• In human evolution studies, molecular evidence—usually the sequencing of DNA—has increased in importance in recent years.

• But the primary evidence in the field remains fossil evidence.



Figure 20.2

Africa

Chad

South Africa Indian Ocean

Ethiopia

Kenya

Tanzania


Orrorintugenensis

Earliest modernhuman(Homo sapiens)

Australopithecusfossil discoveriesParanthropusfossil discoveriesAustralopithecusand Ardipithecusfossil discoveries



• The researchers who find and analyze hominin fossils are known as paleoanthropologists.



• Paleoanthropologists interpret features of fossils in order to make judgments about where a given fossil form lies in the hominin family tree.

• Interpretations can differ, however.



• The two most important defining characteristics of a hominin are tooth structure and upright or “bipedal” walking.


20.4 Snapshots from the Past: Three Hominins


Australopithecus afarensis

• The bipedalism seen in our own species clearly existed in the hominin species Australopithecus afarensis, whose most famous individual, Lucy, lived 3.18 Mya in what is now Ethiopia.



• Lucy had a much smaller brain than modern humans do, however, and she probably was partly arboreal, or tree-dwelling.



Figure 20.6

Smallerbrain

Longer arms(shorter legs)

Graspingfeet



• Lucy’s set of features demonstrate that hominin features developed in a mosaic pattern—different features evolved at different points in time and in different species.



• Lucy’s species is generally regarded as being ancestral to the Homo genus that human beings are a part of.

• Thus, Lucy probably is a human ancestor.


Three Hominims

Figure 20.5


Homo ergaster

• A change to a physical form and mental capacity much closer to ours comes with the evolution of Homo ergaster, exemplified by Turkana Boy, who lived 1.6 Mya.


Homo ergaster

Figure 20.7


Homo neanderthalensis

• In their modern form, the Neanderthals (Homo neanderthalensis) populated Europe as well as parts of Asia for about 130,000 years.



• The last of them died 28,000 years ago in Europe.

• The modern caricature of the caveman comes from the Neanderthals, but this image is difficult to reconcile with facets of Neanderthal life such as the burying of the dead.



• Even given these facets, however, it is clear that the Neanderthals were a primitive species in comparison with H. sapiens.


20.5 The Appearance of Modern Human Beings


The Appearance of Modern Human Beings• Human beings appear to have evolved into their

modern anatomical form in Africa prior to the time they began to migrate into the wider world.


The Appearance of Modern Human Beings• This “out-of-Africa” hypothesis stands in

contrast to the less-accepted “multiregional” hypothesis, which holds that several species of hominins migrated from Africa at different times, interbred, and evolved into H. neanderthalensis and H. sapiens.


The Appearance of Modern Human Beings• The initial wave of human migration out of

Africa was initiated sometime between 85,000 and 55,000 years ago.


The Appearance of Modern Human Beings

Figure 20.8

Africa

Indian Ocean

Malay PeninsulaAndaman Islands

Niah Cave,Borneo

Asia

Australia

Lake Mungo

ArabianPeninsula

Easterly migration

Humanoriginsin Africa


The Appearance of Modern Human Beings• The earliest fossils we have of modern human

beings outside Africa and its immediate environs date from 46,000 years ago and were found in Australia.


The Appearance of Modern Human Beings• Molecular evidence indicates that modern

human beings evolved no earlier than 200,000 years ago.

• The earliest human fossils we have date from 195,000 years ago and were found in Ethiopia.


The Appearance of Modern Human Beings• The arrival of modern human beings in Europe

40,000 years ago was followed by the extinction of the Neanderthals 12,000 years later.

• The arrival of human beings in the Far East, at least 46,000 years ago, was followed by the extinction of the Homo erectus species 6,000 years later.


The Appearance of Modern Human Beings• If there is a consensus about why we human

beings are the only living species of hominin, it is that we “replaced” such species as H. erectus and H. neanderthalensis by out-competing them after having migrated to Asia and Europe.


20.6 Next-to-Last Standing? The Hobbit People


Homo floresiensis

• In 2004, researchers reported finding, on the Indonesian island of Flores, fossils of a previously unknown hominin, Homo floresiensis, who stood only 3 feet tall and seemed to have survived until 18,000 years ago.


Homo floresiensis

• This interpretation of the Flores fossils has been challenged, however, by two teams of researchers who have concluded that H. floresiensis actually was a small, modern human being suffering from the medical condition of microcephaly.


Homo floresiensis

Figure 20.9


Homo floresiensis

• If H. floresiensis is a new species of hominin, one possibility is that it evolved from H. erectus.

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Arriving Late, Traveling Far: The Evolution of Human Beings