Evolution, 4th Edition

THE HiSToRy oF LiFE 441

share many genes that underlie cell and developmental processes [76], suggesting

that the genetic toolkit of animals evolved during the Ediacaran period (635–541

Mya) [28, 49]. Among living animals (Metazoa), sponges (phylum Porifera) are

thought by many researchers to be the sister group of the other animals, although

the relationships among the sponges, the radially symmetrical Cnidaria (jelly-

fishes, corals), the Ctenophora (comb jellies), and the Bilateria are still uncertain

[25]. The Bilateria—bilaterally symmetrical animals with a head, often equipped

with mouth appendages, sensory organs, and a brain—include all the other ani-

mal phyla. The origin of all these phyla is one of the biggest unsolved problems in

the study of evolution.

The final two periods of the Proterozoic are the Cryogenian (720–635 Mya),

when Earth experienced lowered temperatures, and the Ediacaran. From about 575

to 541 Mya, there are fossils of a variety of enigmatic animals known as the Edia-

caran fauna. Most of them were soft-bodied and appear to have been flat creatures

that crept or stood on the sea floor (FIGURE 17.7). They are thought to have become

extinct without leaving any post-Cambrian descendants. Ediacaran animals seem to

have lacked features, such as mouthparts or locomotory appendages, that might be

used in interacting with other animals, and they left no burrows in sediments. Nor

did the bilaterians that are thought to have existed at this time. Based on calibrated

DNA sequence divergence, the phyla of living animals stem from a common ancestor

FIGURE 17.7 Members of the

Ediacaran fauna. (A) Tribrachidium

heraldicum. The triradial form of

this animal differs from that of

any Phanerozoic animals. (B) The

relationship of the wormlike Dick-

insonia costata to later animals is

unknown.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_1707.ai Date 11-02-2016

(A) (B)

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.06.ai Date 12-31-2016

(A)

Choanocyte

(B)

FIGURE 17.6 (A) Choanoflagellates are

unicellular eukaryotes that can form simple

colonies. They are the closest known rela-

tives of animals, and structurally resemble

(B) choanocyte cells of sponges. (B, photo

from [23b].)

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