THE HiSToRy oF LiFE 447
The colonization of land
Terrestrial plants, including mosses, liverworts, and vascular
plants, are a monophyletic group that evolved from green algae
(Chlorophyta) (FIGURE 17.14) [43]. Living on land required the
evolution of an external surface and spores that are resistant to
loss of water, structural support, vascular tissue to transport
water within the plant body, and internalized sexual organs,
protected from desiccation. The first known terrestrial organ-
isms are mid-Ordovician spores and spore-bearing structures
(sporangia) of very small plants, which were apparently related
to today’s liverworts [96]. By the mid-Silurian there were small
vascular plants, less than 10 cm tall, that lacked true roots and
had sporangia at the ends of short, leafless, dichotomously
branching stalks (FIGURE 17.15A). A staggering amount of
adaptive evolution ensued: by the end of the Devonian, about
75 My later, terrestrial plants had evolved deep root systems,
wood, leaves, and complex, diverse reproductive structures.
These plants included ferns, club mosses, horsetails, and seed
plants: all the major groups of plants except the flowering
plants. Many were large trees (FIGURE 17.15B). Over the course
of the Devonian, the amount of terrestrial biomass increased enor-
mously, and it had huge effects: it increased atmospheric oxygen,
created organic soil, increased the weathering and erosion of
rocks, and consumed carbon dioxide, resulting in lowered tem-
perature [3]. Life continued to alter the planet.
The earliest terrestrial arthropods are known from the Silurian.
They fall into two major groups, which both arose in the ocean.
The chelicerates included spiders, mites, scorpions, and several
other groups that still exist. The earliest mandibulates included
detritus-feeding millipedes from the late Silurian, followed
in the Devonian by predatory centipedes and primitive wingless insects, which
evolved from crustaceans. Later, the importance of insects in terrestrial ecosystems
became immense: as herbivores, insects have profoundly affected plant evolution;
as predators, they affect the evolution of other insects; and as prey, they support
the majority of the terrestrial animals that do not feed on plants.
The first terrestrial vertebrates evolved from lobe-finned fishes late in the Devonian.
The Sarcopterygii, or lobe-finned fishes, appeared in the early Devonian, about
408 Mya. They include coelacanths and lungfishes, a few of which are still alive,
and the osteolepiforms, which had distinctive tooth structure and skull bones
(FIGURE 17.16A). Osteolepiforms, such as Eusthenopteron, had a tail fin and fleshy
paired fins, with a central axis of several large bones to which lateral bones and
slender, jointed rays (radials) articulated. They could not flex their head relative
to the body, and their braincase had a joint between the anterior and posterior
sections, as it does in living sarcopterygians. The first definitive tetrapods (four-
legged vertebrates), such as Ichthyostega from the very late Devonian, had the same
tail fin and distinctive teeth and skull, but the gill cover bones at the rear of the
skull had been lost, and the head could now be moved on a more flexible neck.
Most importantly, they had larger pectoral and pelvic girdles and fully developed
tetrapod limbs that bore more than five digits (unlike almost all later tetrapod
vertebrates) [20].
Clearly, ichthyostegids show a mosaic of sarcopterygian and tetrapod features, and
are intermediates in the evolution of a major new clade of vertebrates. Until recently,
only a few fossils provided evidence of intermediate steps in the transition from Futuyma Kirkpatrick Sinauer Associates Evolution, 4e
Troutt Visual Services
Evolution4e_1715..ai Date 11-02-2016Aglaophyton, a
very early land
plant, had no
roots or leaves.Huge lycophyte
"trees" dominated
Carboniferous
forests.Dichotomous
branches(A) (B)FIGURE 17.15 Paleozoic vascular plants, portrayed at different
scales. (A) Aglaophyton, from the Devonian, was probably less
than 15 cm tall. (B) Lepidodendron, a Carboniferous lycophyte
tree, was as tall as 30 m. (A from [47]; B, from [89].)17_EVOL4E_CH17.indd 447 3/22/17 1:37 PM