Evolution, 4th Edition

THE HiSToRy oF LiFE 443

other classes of arthropods, molluscs, and echinoderms, as well as animals that are

hard to classify into later phyla (FIGURE 17.9). This diversification, surely the most

dramatic adaptive radiation in the history of life [92], is called the Cambrian explo-

sion because it transpired over such a short time (“only” 20 My). The Cambrian

explosion is a conundrum: how can the long prior history of the phyla, revealed by

molecular divergence, be reconciled with their absence, and then sudden appear-

ance, in the fossil record?

The bilaterian lineages that existed before the Cambrian probably fed on detritus

and plankton (by filter-feeding). Only at the start of the Cambrian did they evolve

hard parts and acquire novel ways of living, such as predation and burrowing into

sediments. A combination of genetic and ecological causes may account for this diver-

sification [29, 49, 60]. Environmental changes, such as an increase in atmospheric

oxygen, may have played a role [48]. Mechanisms of gene regulation may have

undergone major evolutionary changes at this time, leading to new morphologies.

For example, microRNAs, which affect the precision of translation of mRNAs into

proteins, are more diverse in morphologically more complex animals [71a]. Some

of the resulting morphological changes may have led to novel interactions among

different organisms, such as predation, that further enhanced

diversity by selecting for protective skeletons and new ways of

overcoming such defenses. Other changes, such as those that

enabled animals to burrow, provided access to new environ-

ments. Some of these activities modified physical and chemical

aspects of the environment, providing ecological opportunities

for yet other novel ways of life.

Paleozoic Life

Between the start of the Paleozoic era 541 Mya and its end

252 Mya (FIGURE 17.10), life on Earth became wonderfully

diverse. The era begins with the first evidence of the mod-

ern phyla, and ends with seas populated with great predators

and with dense forests on the continents, inhabited by some

familiar insects and the early ancestors of mammals.

Many animal lineages diversified during the Cambrian,

including early chordates such as Haikouichthys, which had

eyes, gill pouches, a notochord, and segmented musculature,

but no jaws or limbs (FIGURE 17.11A) [84]. Conodonts, first

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(A) (B) FIGURE 17.9 Two animal groups

that first appeared during the

Cambrian explosion. Both of these

fossils were uncovered in the

sandy shales of southern Utah,

an area that once was covered

by shallow seas. (A) A Cambrian

trilobite (Paraceraurus), phylum

Arthropoda. More than 17,000

species of trilobites have been

described from the Paleozoic. The

group became extinct at the end

of the Permian. (B) An echino-

derm (Gogia spiralis) from the

early Cambrian. Several groups

of echinoderms—which, along

with chordates, are deuterostome

animals—flourish in the modern

fauna.

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Evolution4e_17.10.ai Date 01-13-2017

Cenozoic

Mya

252

299

359

419

443

485

541

Permian

299–252 Mya

Carboniferous

359–299 Mya

Devonian

419–359 Mya

Silurian

443–419 Mya

Ordovician

485–443 Mya

Cambrian

541–485 Mya

Mesozoic

Paleozoic

Proterozoic

Archean

Precambrian

Phanerozoic

FIGURE 17.10 Time line of the Paleozoic

era, illustrating a noteworthy organism from

each period except the Silurian.

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