HUMAN BIOLOGY

prinCiples of evolution 449

hoW do extinction and the rise of neW

species fit into the history of life on earth?

• Extinction and the appearance of new species are part of the
  natural course of evolution.


• Species are always going extinct. In a mass extinction, major
  groups of species go extinct in a short period of geological time.


• In adaptive radiation, new species rapidly (on a geological time
  scale) fill a range of habitats.

taKe-Home message

In adaptive radiation, new species arise

In adaptive radiation, new species of a lineage move into
a wide range of habitats during bursts of micro evolution.
Many adaptive radiations have occurred during the first
few million years after a mass extinction. Fossil evidence
suggests this happened after dinosaurs went extinct. Many
new species of mammals arose and radiated into habitats
where dino saurs had once lived.
Adaptive radiations also have occurred in the human
lineage. The ancestors of modern humans, including the
tool-using species Homo habilis (“handy man,” pictured in
Figure  23.3B), apparently remained in Africa until about
2 million years ago. Around that time, genetic divergence

Figure 23.12 Many mass extinctions have occurred during Earth’s
history. Each extinction has been followed by a period of slow
recovery, and the resulting mix of species is not the same as before.

Figure 23.13 Many marine species

died out during the mass extinction

at the end of the Devonian period.

One of them was Dunkleosteus, a

massive placoderm (“plate skin”).

Here you see a cast of its skull, which

was reconstructed from fossils.

(Man: Photo by Lisa Starr; Skull: Courtesy of

John McNamara, http://www.paleodirect.com)

PALEOZOIC

CAMBRIAN

PERMIAN

MESOZOIC

CENOZOIC

CARBONIFEROUS

TRIASSIC

JURASSIC

CRETACEOUS

TERTIARY

QUATERNARY

SILURIAN

DEVONIAN

Period

ORDOVICIAN

Major extinction event

Major extinction event

Major extinction event

Major extinction event

488

443

416

359

299

251

199.6

145.5

65.5

1.8 mya

Slow recovery after Permian extinction,

then adaptive radiations of some marine

groups and plants and animals on land.

Asteroid impact at K–T boundary, 85% of

all species disappear from land and seas.

542

Pangea forms; land area exceeds ocean

surface area for first time. Asteroid impact?

Major glaciation, colossal lava outpourings,

90%–95% of all species lost.

Massive glaciation; 79% of all species lost,

(Precambrian) including most marine microorganisms.

Second most devastating extinction in

seas; nearly 100 families of marine

invertebrates lost.

More than 70% of marine groups lost.

Reef builders, trilobites, jawless fishes, and

placoderms severely affected. Meteorite

impact, sea level decline, global cooling?

Major extinction under way

With high population growth rates and cultural

practices (e.g., agriculture, deforestation),

humans become major agents of extinction.

Era

Major extinction event

led to new species, including Homo erectus, a human spe-

cies that the fossil record places on the evolutionary road

to modern humans. H. erectus coexisted for a time with

H. habilis. But while members of its sister species also were

upright walkers, H. erectus really earned the name. Its popu-

lations walked out of Africa, going left into Europe or right

into Asia. Judging from Middle Eastern fossils, our species,

Homo sapiens, had evolved by 100,000 years ago.

What kinds of selection pressures triggered this adaptive

radiation? Although we don’t know for sure, this was a

time of physical changes. One group of early humans, the

Neanderthals, had large brains and were massively built

(Figure  23.14). Some Neanderthal populations were the

first humans to adapt to colder climates, such as in Europe.

Later genetic changes resulted in anatomically modern

humans. Scientists now consider Neanderthals our closest

extinct relatives.

Figure 23.14 Adaptive radiation of humans included the evolution

of several sister species, including Neanderthals and our own

species, Homo sapiens. These skeletons are both from males.

Scientists reconstructed the Neanderthal skeleton from fossils

found in several locations, designated by different colors.

Courtesy of Blaine Maley, Washington University, St. Louis

Homo neanderthalensis Homo sapiens

© Cengage Learning

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).