Encyclopedia of Evolution.pdf - Online Reading Center

and others form more complex structures, but none of them
rival the gigantic kelp in size or complexity. However, they
have important similarities to land plants on the cellular
level:
• Green algae include many freshwater species. Almost all
brown and red algae, for example, are marine, as are many
of the green algae. Land plants evolved from freshwater,
not marine, ancestors.
• The cells of green algae are chemically and structurally
similar to those of land plants. They have the same kinds
of chlorophyll, store the same kinds of carbohydrates, and
have the same kinds of cell walls.
The green algal origin of land plants, long known from the
fossil record, has been confirmed by DNA studies (see DNA
[evidence for evolution]) in which green algae of the family
Charophyceae have been found to be more similar to land
plants than are any other algae.
Freshwater algae often live in shallow water, and shallow
water frequently dries up. Algae that had adaptations that
allowed them to continue to function when water became
mud would be favored by evolution. The first land plants, as
seen in the fossil record, were small plants in wetlands, consistent
with this explanation.
Nearly all organisms have a sexual life cycle in which
a haploid phase alternates with a diploid phase. Cells with
unpaired chromosomes are haploid, cells with paired chromosomes
are diploid. A type of cell division known as meiosis
separates chromosome pairs and produces haploid cells;
in turn, some haploid cells come back together, in a process
called fertilization, to restore the original diploid number (see
sex, evolution of). In animals, the haploid phase consists
only of eggs and sperm, but in plants, the haploid phase is
multicellular. As green algae evolved into land plants, they
diverged:
• Some of them evolved into bryophytes (see figure), in
which the haploid multicellular phase is relatively large
and independent, while the diploid multicellular phase is
relatively small and depends on the haploid cells to supply
their needs. They reproduce when sperm swim through a
film of water to eggs. None of these plants grow taller than
a few centimeters. These plants were already established on
the Earth by at least 350 million years ago.
• Some of them evolved into plants in which the diploid
multicellular phase was relatively large and independent,
while the haploid multicellular phase was relatively small
and sometimes dependent on the diploid cells to supply
their needs. These plants were the earliest known land
plants. They lived in marshlike habitats during the Silurian
period 435 million years ago, although a few fossil
spores have been found from the previous Ordovician
period. Some of their fossils have been so well preserved
that the vascular pipes can actually be seen in rock cross
sections. They were the ancestors of pteridophytes and
seed plants.
The earliest land plants, in the swamps of the Silurian
period, were only about three feet (a meter) tall or less, and
seedless plants, evolution of
Mosses do not produce seeds. They produce spores, which are singlecelled
reproductive structures, in the capsules shown in this photo. Each
spore grows into a new moss plant. (Photograph by Stanley A. Rice)
were hardly more than branching stems with reproductive
structures at the tips. They had no roots or leaves. They
included:
• the rhyniophytes such as Cooksonia. The rhyniophytes
apparently became extinct without descendants. Even
though they looked very much like modern whiskferns,
they are unrelated to them; the whiskferns are more closely
related to the ferns and apparently evolved from a more
complex into a simpler form in more recent times.
• the zosterophylls. Some zosterophylls evolved into the
ancestors of the club mosses.
• the trimerophytes. Some trimerophytes evolved into the
ancestors of horsetails, whiskferns, and true ferns. These
plants were common during the Silurian and Devonian
periods. Their evolutionary descendants included large
forest trees of the Carboniferous period (see below) as
well as smaller plants, which are today’s ferns and fernlike
plants.
By the Carboniferous period, many of these plants had
grown very tall. Some of them, such as Lepidodendron,
were relatives of modern club mosses, while others such as
Calamitales were relatives of modern horsetails. Then as now
some ferns reached the size of small trees. Their leaves were
long, narrow, and very simple. Their water pipes allowed
them to grow tall, but their upward growth was limited by
their inability to continue producing new wood. They dominated
the Carboniferous swamps. They could only live in
moist environments, such as swamps, because their sexual
reproduction required sperm to swim through water, just as
seedless plants do today. Therefore, the forests of the Carboniferous
period were found only in swamps; the hillsides
and dry plains were most likely barren. Fortunately for them,
swamp conditions were extremely widespread on the Earth