Benton M.J. Introduction to paleobiology and the fossil record

Подождите немного. Документ загружается.

468 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

probably fed on lush plants in the ponds and

rivers of Africa. Later, many lines of probos-

cideans diverged, distinguished by an aston-

ishing array of tusks, which are modiﬁ ed

incisor teeth. Some had tusks in the upper jaw

(as in modern elephants), others had tusks in

the lower jaw, and others had tusks in both

as in Deinotherium (Fig. 17.15b). The Pleis-

tocene mammoths were abundant in cold

northern ice age climates, but died out as the

ice retreated 10,000 years ago.

Among the other afrotheres, the hyraxes

and sirenians are close relatives of probosci-

deans. The aardvark (Tubulidentata), as well

as golden moles and tenrecs, seem to form a

second afrothere group.

The Xenarthra, or edentates (“no teeth”),

are a peculiar South American group. They

include modern armadillos, tree sloths and

anteaters, as well as their remarkable ances-

tors, known especially from the Pliocene and

Pleistocene. There were giant armadillos such

as Glyptodon (Fig. 17.15c), and giant ground

sloths such as Mylodon (Fig. 17.15d) that

reached a length of 6 m and fed on coarse

leaves from the treetops. The ground sloths

survived in South America until 11,000 years

ago, and their subfossil remains include

clumps of reddish hair and caves full of

unrotted dung that occasionally ignites

spontaneously.

The “northern” placentals

The Laurasiatheria include about half of

modern placental mammals, and they are as

diverse as shrews, bats, cattle, whales and

lions. The Lipotyphla (sometimes Insectivora)

is composed of dozens of species of hedge-

hogs, moles and shrews, all small animals

with long snouts that feed on insects. The

oldest Lipotyphla are Paleocene in age, and

for the most part the fossil forms probably

looked like the modern ones. One exception

is the giant spiny hedgehog Deinogalerix,

which was 0.5 m long.

Next in the cladogram (see Fig. 17.14) are

the Chiroptera, or bats, a diverse group today

of some 1000 species. The earliest bats such

as Icaronycteris from the Eocene (Fig. 17.16a)

show the typical wing structure in which the

ﬂ ight membrane is supported on four ﬁ ngers

of the hand that spread out. The feet turn

backwards, and Icaronycteris could have

hung upside down. It also had large eyes and

the ear region was modiﬁ ed for echolocation.

Icaronycteris, like most modern bats, hunted

insects at night, using its large eyes to pick up

movements, and sending out high-pitched

(a)

(b)

100 mm

(c)

(d)

0.5 m

ilium

ischium

1 m

Figure 17.15 Afrotheres and xenarthrans: (a, b)

skulls of the Eocene proboscidean Moeritherium

(a) and the Miocene proboscidean Deinotherium

(b); and (c, d) Pleistocene edentates from

Argentina, Glyptodon (c) and Mylodon (d).

(Based on Gregory 1951.)

(b)

(c)

(f)

0.5 m

(d)

(e)

1 m

reduced hindlimb

0.5 m

(a)

10 mm

elongate

humerus

fused radius

and ulna

100 mm

(g)

(h)

(i)

20 mm

200 mm

paddle-like hand

Figure 17.16 Diverse laurasiatherians: (a) the Eocene bat Icaronycteris; (b) the small four-toed

artiodactyl Messelobunodon, showing the complete skeleton and a mass of chopped plant material in

the stomach area, from the oil shale deposit of Messel, Germany; (c) the Pleistocene giant Irish deer

Megaloceros; (d) the middle Eocene whale Ambulocetus; (e) the late Eocene whale Basilosaurus; (f) the

Miocene horse Neohipparion; (g) the Pleistocene sabre-toothed cat Smilodon; (h) the Eocene dog

Hesperocyon; and (i) the Miocene “seal” Allodesmus. (a, based on Jepsen 1970; b, courtesy of Jens

Franzen; c, e–i, based on Gregory 1951; d, based on Thewissen et al. 1994.)

470 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

squeaks to detect its prey by the echoes they

made.

The next clade is perhaps a little unex-

pected. Much evidence links whales and artio-

dactyls, the even-toed ungulates (larger plant

eaters), as a clade called Cetartiodactyla.

Whales and artiodactyls, for example, share

a similar pulley-like ankle joint (do whales

have ankles?) – not now, but the early forms

did (see below). The artiodactyls arose in the

Eocene (Fig. 17.16b), and the group includes

pigs, hippos, camels, cattle, deer, giraffes and

antelopes, all with an even number of toes

(two or four). Pigs and hippos share ancestors

in the Oligocene, at the same time as vast

herds of oreodonts fed on the spreading grass-

lands of North America. Oreodonts are related

to the camels and the ruminants. The ﬁ rst

camels were long-limbed and lightly built

North American animals: it was only later

that camels moved to Africa and the Middle

East and evolved adaptations for living in

conditions of drought.

Most artiodactyls today are ruminants,

animals that pass their food into a forestom-

ach, regurgitate it (chew the cud) and swallow

it again. The multiple digestive process allows

ruminants to extract all the nourishment from

their plant food, usually grass, and to pass

limited waste material (compare the homoge-

nous excrement of cattle with the ﬁ brous

undigested droppings of horses, which do not

ruminate). Ruminants became successful after

the mid-Miocene, when a great variety of

deer, cattle and antelopes appeared. These

animals usually have horns or antlers, seen in

spectacular style in the Irish deer Megaloceros

(Fig. 17.16c). The headgear is used in all cases

for displays and ﬁ ghts between males seeking

to establish territories and win mates.

The whales, Cetacea, evolved from a

raccoon-sized artiodactyls that fed on aquatic

plants along the edges of streams and ponds

(Thewissen et al. 2007) – the oldest whale

Ambulocetus (Fig. 17.16d) still has fully

developed limbs, and these show the pulley-

like ankle bone of artiodactyls. By the late

Eocene, whales such as Basilosaurus (Fig.

17.16e) had become very large, at lengths of

20 m or more. Basilosaurus had a long thin

body, like a mythical sea serpent, and a rela-

tively small skull armed with sharp teeth. It

was probably a ﬁ sh eater, like the toothed

whales today. The baleen whales, the biggest

of all modern whales, arose later, and they

owe their success to their ability to ﬁ lter vast

quantities of small crustaceans, krill, from

polar seawaters.

The second major ungulate group, the

Perissodactyla, consisting of horses, rhinos

and tapirs, all have an odd number of toes –

one or three. The horses provide a classic

example of evolution (see p. 543). The ﬁ rst

horse, Hyracotherium, was a small wood-

land-living animal that had four ﬁ ngers and

three toes, and low teeth used for browsing

on leaves. During the Oligocene and Miocene,

horses became adapted to the new grasslands

that were replacing the forests, and they

became larger, lost toes and evolved deep-

rooted cheek teeth for grinding tough grass

(Fig. 17.16f).

Tapirs and rhinoceroses are probably

related. Eocene and Oligocene rhinos were

modest-sized, running animals, not much dif-

ferent from some of the early horses. Tapirs

later became a rare group, restricted to Central

and South America and Southeast Asia. The

rhinoceroses ﬂ ourished for a while, producing

monsters such as the Oligocene Indricothe-

rium, the largest land mammal of all time:

5.5 m tall at the shoulder and weighing 15

tonnes.

The Carnivora – cats, dogs, hyenas, weasels

and seals – are characterized by sharp cheek

teeth (carnassials) used for tearing ﬂ esh. The

cats have a long history during which dagger-

toothed and saber-toothed forms evolved

many times. The sabertooths such as Smilo-

don (Fig. 17.16g) preyed on large, thick-

skinned herbivores by cutting chunks of ﬂ esh

from their bodies. The saber-toothed adapta-

tions evolved independently in some South

American marsupials (cf. Fig. 17.13a). Early

dogs such as Hesperocyon (Fig. 17.16h)

were light, fast-moving animals, close to the

ancestry of modern dogs and bears. Some car-

nivores related to raccoons and weasels entered

the sea during the Oligocene, and gave rise to

the seals, sealions and walruses. Early forms

such as Allodesmus (Fig. 17.16i) had broad,

paddle-like limbs and fed on ﬁ sh.

The monkey-rabbits

The second boreoeutherian clade (see Fig.

17.14) is the Euarchontoglires, a mouthful that

is made from a combination of the group

DINOSAURS AND MAMMALS 471

names Archonta and Glires, and it means some-

thing like “true primate–treeshrew rodent–

rabbits”. The name tells the whole story!

Rats and rabbits, representing the Rodentia

and Lagomorpha, belong to one group

(Glires), and they shared an ancestor with

buck teeth and a propensity for breeding.

Rodents are the largest group, consisting of

over 1700 species of mice, rats, squirrels, por-

cupines and beavers. They owe their success

to their powerful gnawing teeth: the front

incisors are deep-rooted and grow continu-

ously, so that they can be used to grind wood,

nuts and husks of fruit. The oldest rodents,

such as Paramys (Fig. 17.17a) already had the

front grinders, and this ability to chew materi-

als ignored by other animals triggered several

phases of rapid radiation. Beavers, porcupines

and cavies radiated in the Miocene. The cavies

include a giant Pliocene guinea pig that

weighed 1 tonne, and was the size of a small

car (Rinderknecht & Blanco 2008); why the

South American rodents became so large is

hard to understand. Rabbits and their rela-

tives (Lagomorpha) have never been as diverse

as the rodents. Fossil forms in the Oligocene

have elongate hindlimbs used in jumping.

Primates, consisting of monkeys, apes and

humans, are part of a larger clade, Archonta,

which also includes the rare treeshrews and

ﬂ ying lemurs (dermopterans) – but it is the

primates that have attracted most attention.

THE LINE TO HUMANS

Early primates

The Primates is one of the oldest of the modern

placental mammal groups. The name

“primate” (from primus, “ﬁ rst”) does not

refer to this, but to the fact that humans are

primates, and so “ﬁ rst” among animals: the

namer, Homo sapiens (“wise person”), has

the privilege of choosing the name! For most

of their history, the primates were a rare and

rather obscure group. All primates share a

number of features that give them agility in

the trees (mobile shoulder joint, grasping

hands and feet, sensitive ﬁ nger pads), a larger

than average brain, good binocular vision

and enhanced parental care (one baby at a

time, long time in the womb, long period of

parental care, delayed sexual maturity, long

lifespan).

(a)

powerful

incisors

50 mm

grasping

hand

no tail

long arm

(b)

(c)

50 mm

Figure 17.17 Diverse euarchontoglirans: (a) the

Eocene rodent Paramys; (b) the Paleocene

primate Plesiadapis; and (c) the Miocene ape

Proconsul. (a, based on Wood 1962; b, based on

Lewin 1999.)

472 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Early relatives of the primates such as Plesi-

adapis (Fig. 17.17b) were squirrel-like animals

that may have climbed trees and fed on fruit,

seeds and leaves. Various basal primates radi-

ated in the Paleocene, Eocene and Oligocene,

and gave rise to the modern lemurs, lorises

and tarsiers.

True monkeys arose in the Eocene and they

diverged into two groups, the New World

monkeys of South America, and the Old

World monkeys of Africa, Asia and Europe.

The New World monkeys, such as marmosets

and spider monkeys, have ﬂ at noses and pre-

hensile tails that may be used as extra limbs

in swinging through the trees. The Old World

monkeys, such as macaques and baboons,

have narrower projecting noses and non-pre-

hensile tails, or no tails at all.

The apes arose from the Old World monkeys

before the end of the Oligocene and the group

radiated in Africa in the Miocene. Even early

forms, such as Proconsul (Fig. 17.17c), have

no tail, and a relatively large braincase, indi-

cating high intelligence. These apes ran about

on the ground and along low branches on all

fours, and fed on fruit. The apes spread out

from Africa into the Middle East, Asia and

southern Europe by the mid-Miocene and

gave rise to some of the modern ape groups

at that time. Fossil and molecular evidence on

phylogeny (see pp. 133–4) suggests that the

gibbons of Southeast Asia are the most primi-

tive living apes, having branched off 25–

20 Ma, followed by the orangutan 20–15 Ma

(Fig. 17.18). The focus of ape (and human)

evolution remained in Africa.

Pleistocene

Pliocene

Miocene

Oligocene

Tertiary

Eocene

Paleocene

5 Ma

Latest

Cretaceous

(a) (b)

HOMINIDAE (APES)

Paromomyidae

Plesiadapidae

Lemuriformes

Tarsiidae

New World monkeys

Old World monkeys

Gibbons

Orangutan

Gorilla

Humans

Ardipithecus ramidus

A. afarensis

A. africanus

A. boisei

H. erectus

H. habilis

Neandertal

H. sapiens

PRIMATES

ANTHROPOIDEA

Chimpanzee

Figure 17.18 Phylogeny of the primates, showing some of the main fossil and living groups (a), and the

detail of one view of human evolution (b). A., Australopithecus; H., Homo; M, Mesozoic; Q,

Quaternary.

DINOSAURS AND MAMMALS 473

Gorillas, chimpanzees and humans appear

to be very closely related: they share many

anatomic characters and more than 94% of

their DNA is identical. Gorillas seem to have

diverged ﬁ rst, about 10 Ma, and the ancestors

of humans and chimps separated about

8–6 Ma.

Human evolution

Humans are set apart from other primates by

their large brain and their bipedalism, walking

upright. We might like to think the large brain

and human intelligence evolved ﬁ rst, but all

the evidence shows that the move from four

to two legs came ﬁ rst, and this may have been

because of a climatic accident in Africa in

the late Miocene. Much of Africa had been

covered with lush forests in which the ances-

tral apes ﬂ ourished, but climates then became

arid and the East African rift valley began to

open up, separating the forests in the west

from the arid grasslands in the east. Tree-

living apes (chimps and gorillas) retreated

west and the remaining apes (our ancestors)

remained in the eastern grasslands. They had

to stand upright to look for enemies, to permit

them to run long distances in search of food

and to free their arms for carrying food.

Humans evolved from bipedal ape-like ances-

tors that had no special high brainpower.

Until recently, the oldest human remains

were about 4 Ma, which seemed to match the

molecular estimate of 5 Ma for the split

between humans and chimps. Then, in 2001

and 2002, there was a ﬂ urry of excitement

when two French teams announced human

fossils that were 6 Ma. There was immediate

dispute about whether these fossils truly were

human, and which was oldest, and the debate

rumbles on (Box 17.7).

A number of incomplete human fossils

were reported during the 1990s from Africa

from rocks dated between 6 and 4 Ma. It may

be that Sahelanthropus and Orrorin were

already bipedal by 6 Ma, but the oldest clear

evidence for bipedalism was the ﬁ nd of some

human tracks in volcanic ash from Tanzania,

dated at 3.75 Ma. The oldest substantial skel-

etons, of Praeanthropus afarensis, come from

rocks dated at about 3.2 Ma and also show

clear anatomic evidence for advanced biped-

alism, but still an ape-sized brain. The famous

skeleton of a female P. afarensis from Ethio-

pia, called Lucy by its discoverer Don Johan-

son in the 1970s (Fig. 17.20a), has a rather

modern humanoid pelvis and hindlimb (Fig.

17.20b). The pelvis is short and horizontal,

rather than long and vertical as in apes, the

thighbone slopes in towards the knee, and the

toes can no longer be used for grasping. Lucy’s

brain, however, is small, only 415 cm

for a

height of 1–1.2 m – not much different from

a chimpanzee.

The human genus Australopithecus contin-

ued to evolve in Africa from about 3 to

1.4 Ma, giving rise to further small species,

and some large robust ones (Fig. 17.21a, b).

The larger australopithecines reached heights

of 1.75 m, but their brain capacities did not

exceed 550 cm

, a rather ape-like measure.

The leap forward to modern human brain

sizes only came with the origin of a new

human genus, Homo. The ﬁ rst species, H.

habilis (Fig. 17.21c), lived in Africa from 2.4

to 1.5 Ma, and had a brain capacity of 630–

700 cm

in a body only 1.3 m tall. H. habilis

may have used tools. It is a remarkable fact

that, for over 1 myr, three or four different

human species lived side by side in Africa.

So far, the focus of human evolution had

been entirely in Africa, but a new species, H.

erectus, which arose 1.9 Ma in Africa, spread

to China, Java and central Europe. H. erectus

had a brain size of 830–1100 cm

(Fig. 17.21d)

in a body up to 1.6 m tall, and there is clear

evidence that this early human species had

semipermanent settlements, a basic tribal

structure, knew about the use of ﬁ re for

cooking, and made tools and weapons from

stone and bone.

Modern peoples

Truly modern humans, H. sapiens, may have

arisen as much as 400,000 years ago, and

certainly by 150,000 years ago, in Africa,

having evolved from H. erectus. It seems that

all modern humans arose from a single African

ancestor, and that the H. erectus stocks in

Asia and Europe died out. H. sapiens spread

to the Middle East and Europe by 90,000

years ago. The European story is particularly

well known, and it includes a phase, from

90,000 to 30,000 years ago, when Neandertal

man occupied much of Europe from Russia

to Spain and from Turkey to southern England.

Neandertals had large brains (on average,

474 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 17.7 Vive la différence!

The world of paleoanthropology was rocked in 2001 when a team based in Paris, Senut et al. (2001),

announced a new hominid, Orrorin tugenensis from Kenya, dated as 6 Ma. The remains were rather

scrappy, as hominid remains often are: teeth, jaw fragments and limb bones. Brigitte Senut and her

colleagues argued that the teeth were rather ape-like, and that the arm bones suggested Orrorin

could brachiate – that is, swing through the trees arm-over-arm, like an ape. However, the femur

(thigh bone) showed that Orrorin stood upright, and so this was a true early human. Other paleo-

anthropologists questioned these claims: they accept the great age of the fossils, but some have

doubted whether the remains all belong to the same species, and others question whether Orrorin

really could have walked upright.

Soon after, another French team, this time from Poitiers, announced a second ancient hominid,

from Chad in North Africa: Sahelanthropus (Brunet et al. 2002). Sahelanthropus is based on a

fairly complete skull (Fig. 17.19), some fragmentary lower jaws and teeth. The sediments in

Chad are hard to date, but are about 7–6 Ma, perhaps the same as Orrorin, or maybe older.

The Sahelanthropus skull indicates a brain volume of 320–380 cm

, similar to a modern

chimpanzee, but the teeth are more human-like, with small canines. The position of the

foramen magnum is disputed: Michel Brunet and his colleagues claim the foramen magnum, the

great opening in the skull through which the spinal cord passes, is located beneath the skull, so

implying that Sahelanthropus stood upright. Critics have said the specimen is too shattered to be

sure, and that the foramen magnum might lie more towards the back of the skull, so suggesting

that the spinal cord exited horizontally and that Sahelanthropus stood on all fours. These critics

have cheekily suggested Sahelanthropus (“Sahel man”) should be renamed Sahelpithecus (“Sahel

ape”)!

Read more in the original papers by Senut et al. (2001) and Brunet et al. (2002), and at many

web sites, linked from http://www.blackwellpublishing.com/paleobiology/.

Figure 17.19 Our oldest ancestor? The spectacular skull of Sahelanthropus from the upper

Miocene of Chad, over 6 Ma. (Courtesy of Michel Brunet.)

DINOSAURS AND MAMMALS 475

1400 cm

), heavy brow ridges (Fig. 17.21e)

and stocky powerful bodies. They were a race

of H. sapiens adapted to living in the continu-

ous icy cold of the last ice ages, and had an

advanced culture that included communal

hunting, the preparation and wearing of

sewn animal-skin clothes, and religious

beliefs. Some paleoanthropologists see the

femur

indicates

upright

posture

(a)

(b)

100 mm

Figure 17.20 The origin of bipedalism in

humans: (a) the Pliocene hominid Praeanthropus

afarensis, known as “Lucy”; and (b) comparison

of the hindlimb of an ape (left), Lucy (middle)

and a modern human (right). (Based on Lewin

1999.)

(a)

(b)

(c)

(d)

(e)

(f)

50 mm

Figure 17.21 Skulls of fossil humans in front

and side views: (a) Australopithecus africanus;

(b) A. boisei; (c) Homo habilis; (d) H. erectus;

(e) H. sapiens, Neandertal type; and (f) modern

H. sapiens. (Based on Lewin 1999.)

476 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 17.8 The quest for the Hobbit

An extraordinary human fossil was unearthed in 2003 on the island of Flores, Indonesia, in Southeast

Asia. The bones came from a woman who was fully grown, and yet barely 1 m tall, and she was

named as the exemplar of a new human species, Homo ﬂ oresiensis. This is an astonishing claim,

that a distinct human species existed in Indonesia at the same time as H. sapiens was striding across

northern Asia towards the Bering Strait, and Middle Stone Age peoples in Europe were painting

wonderful hunting scenes in the caves of France.

The Flores remains consist of a more or less complete skull and skeleton, as well as the remains

of seven other individuals. The skull (Fig. 17.22) shows this hominid had a brain size of only 380 cm

It was named as a clearcut new species, H. ﬂ oresiensis, by its discoverers, Peter Brown of the Uni-

versity of New England, Australia, and colleagues (2004). They argued that the remains show evi-

dence for a unique population of little people – soon dubbed the hobbits by the press – that,

remarkably, hunted pygmy elephants and giant lizards on the island. The discoverers argued that

this unique species had lived on the island for some time, unaware of its larger human relatives

elsewhere, and that Flores man had become small as a result of island dwarﬁ ng. It is a relatively

common observation that mammals isolated on islands may become smaller (or sometimes larger)

because there are usually fewer species than on the neighboring mainland, and each species ﬁ nds a

new place in the ecosystem. The elephants on Flores doubtless became small too so they could survive

in a smaller area.

As soon as the paper appeared, many paleoanthropologists were immediately critical. Surely

this could not be a distinct species, merely a local variant of H. sapiens, either a pygmy race, or

perhaps a microcephalic individual (Jacob et al. 2006), that is an individual with an unusually

small head and brain? There have been accusations that researchers on both sides of the

dispute have been less than willing to share the specimens, that others have borrowed specimens

and then refused to hand them back, that specimens have been damaged, and other multifarious

examples of skullduggery. The majority view still seems to be that H. ﬂ oresiensis is truly a distinct

species, but the inaccessibility of the location and legal problems may make further collecting

difﬁ cult.

Read more about the hobbit in Morwood and van Oosterzee (2007) and Culotta (2007), as well

as web presentations available through http://blackwellpublishing.com/paleobiology/.

Figure 17.22 Skulls of Flores man, Homo ﬂ oresiensis (left), and of a typical modern human, H.

sapiens, to show the great difference in size. (Courtesy of Paul Morwood.)

DINOSAURS AND MAMMALS 477

Neandertals as distinct enough to be given

their own species, H. neanderthalensis.

The Neandertals disappeared as the ice

withdrew to the north, and more modern

humans advanced across Europe from the

Middle East. This new wave of colonization

coincided with the spread of H. sapiens (Fig.

17.21f) over the rest of the world, crossing

Asia to Australasia before 40,000 years ago.

There is much debate about the dating of

these migrations, and how the various human

populations are related. New discoveries from

Indonesia have stirred up real controversy

over whether there was a unique small-sized

human species, H. ﬂ oresiensis, living there

only 18,000 years ago (Box 17.8).

Equally controversial is the question of

when modern humans reached the Americas.

All agree that native Americans walked across

from Siberia to Alaska, and colonized south-

wards over some hundreds or thousands of

years. It is commonly said that humans

reached North America 11,500 years ago,

and yet apparently older remains are reported

from time to time. These fully modern humans,

found worldwide, with brain sizes averaging

1360 cm

(see Fig. 17.21f), brought more

reﬁ ned tools than those of the Neandertals,

art in the form of cave paintings and carvings,

and religion. The nomadic way of life began

to give way to settlements and agriculture

about 10,000 years ago.

Review questions

1 Why were dinosaurs so huge? Establish

the typical size range of dinosaurs in com-

parison with modern mammals, and read

about ideas past and present about why

dinosaurs were an order of magnitude

larger than mammals.

2 Were dinosaurs warm-blooded? Read

around the topic, back to the debates in

the 1970s and 1980s, and through to the

present day. List the different lines of evi-

dence used to suggest endothermy and

evaluate the arguments for and against.

3 Did mammals radiate explosively after the

Cretaceous-Tertiary event? Investigate the

“classic” story of a massive diversiﬁ ca-

tion/adaptive radiation of mammals

65 Ma, and consider why molecular phy-

logenies and dates seem to indicate a much

earlier diversiﬁ cation.

4 What happened to mammalian faunas

11,000 years ago, at the end of the ice

ages? Read about the climate change and

overkill hypotheses for Pleistocene extinc-

tions, and decide which side of the debate

has the best evidence.

5 Track the discoveries of new human fossils

over the past 20 years, and focus on the

question of dating human origins. What

were the oldest human fossils in 1980,

1990 and 2000, and what is the view

today?