Robert Banas, Katelyn Holland, Kyle McMullen, Brittany Notor
Honors / Period 4
13 June 2007
the field of biology there are only a few truly great mysteries left. One of these mysteries is the evolutionary path that
has lead to contemporary angiosperms. Angiosperms, also known as flowering plants, have troubled evolutionary biologists for
decades; as common features of the world today, the fact that their evolution cannot be explained with absolute conviction
troubles scientists. Despite these difficulties scientists have been able to make great strides in explaining as well as they
can the evolution of angiosperms, and at this point any uncertainty about their origins may come more so from the conflicting
views from scientists than from lack of data. This paper will summarize the findings that scientists have made over the years
in analyzing the evolution of flowering plants; though it is still important to note that only one view will be focused on,
and that other proposed paths do exist.
In order to save some measure of time such things as the creation of the universe
and the formation of the galaxies and solar systems (including our own) will be skipped over to get to the more exciting aspects
of angiosperm evolution. The first of the steps of angiosperm evolution is something that is for the most part an accepted
fact among biologists at this point in time. That step, or rather organism is cyanobacteria. Cyanobacteria are aquatic, photosynthetic
bacteria that are sometimes called by the more deceptive name of blue-green algae. As they are bacteria, they are also prokaryotic
and usually unicellular organisms. Unlike most bacteria, however, cyanobacteria often grow in large colonies that can actually
be large enough to be visible to the unaided human eye.
There is something very interesting about cyanobacteria that
give them so much credibility as the first organism in the process of angiosperm evolution, and that is the process of endosymbiosis.
The theory behind endosymbiosis is that cyanobacteria evolved into chloroplasts after other eukaryotic cells absorbed them
as endosymbionts (the technical name for any organism that lives with the body or cells of another organism). Now that the
cyanobacteria are no longer individual cells and are instead chloroplasts within another organism, it is time to introduce
the next organism that is part of this proposed sequence for angiosperm evolution.
The next organism in this sequence
is chlorophyta. Chlorophyta are very similar to cyanobacteria in that both of them are photosynthetic aquatic organisms, however
chlorophyta’s alternate name green algae is in no way deceptive in this instance, as it is indeed a eukaryotic organism.
There are relatively few differences between chlorophyta and charophyta, the next organism in the evolutionary sequence Charophyta,
like chlorophyta is a member of the group classified as green algae. Charophyta is also, however, the closest known relative
to the embryophytes; which includes anthocerotophyta, hepatophyta, and bryophyta all of which as a group are informally called
bryophytes. The bryophytes represent the nonvascular evolutionary sequence from charophyta; they are nonvascular because they
lack vascular tissue, or xylem to transport water and thus require a moist environment to survive. The bryophytes as a group,
however, do not lead to angiosperms.
Since the bryophytes do not evolve into angiosperms, a divergence clearly occurred
in the evolutionary sequence at charophyta. This divergence was into two groups; one was the above group that is made up of
nonvascular plants and the other which was a group of vascular plants. The progenitor of the vascular plants was psilophyta.
Psilophyta is the group of vascular plants that produce with spores as opposed to seeds, and unlike some later plants do not
have a true root system and instead had a rhizome, which sprouted small rhizoids that helped the plant maintain structure.
to psilophyta, and thus next in the sequence of evolution is lycophyta. Lycophyta is regarded to be one of the first groups
of true vascular plants because of its extreme age. Lycophyta is akin to psilophyta because they both use rhizomes to stay
erect and maintain structure as opposed to a true root system. The next group of plant in this sequence, sphenophyta, is very
similar to lycophyta and both were contemporaries of one another that evolved around the same period in time.
the plants that evolved from lycophyta, was a group of plants that was very similar to lycophyta and as such the divergence
from lycophyta to sphenophyta is one that even the most pessimistic can accept. After lycophyta diverged from sphenophyta,
it was only a few more million years before sphenophyta would diverge and form pterophyta. Pterophyta and sphenophyta, of
course, have numerous similarities; including the fact that both of them produce lignin and use spores instead of seeds. The
key difference between these two groups was that pterophyta was the first group that exhibited true leaves.
later from pterophyta would be coniferophyta that, like charophyta, is a key divergence point in the evolution of flowering
plants. It is at this stage that the plants cease to reproduce by means of spores only and move onto more advanced means;
the members of coniferophyta reproduce by way of cones. This key change in divergence leads into the split of evolution that
would lead to seed bearing plants and the continuation of seed-less plants. The seed-less plant group that immediately followed
coniferophyta was cycadophyta; cycadophyta is very similar in appearance to other gymnosperms, but is more like ginkgos or
angiosperms in structure, their being seedless being one of the few but major differences.
The seed bearing descendent
of coniferophyta is ginkophyta. Ginkophyta is regarded as the link between pterophyta and the other gymnosperms and the angiosperms.
Ginkophyta is one of the first groups of plants that exhibits a means of reproduction similar to that of seeds; having cones
that more closely resemble small berries or fruit than previous plants. This presence of seeds, along with the logical pattern
that leads to it, is what has lead scientists to believe that ginkophyta is the group of plants that leads to gnetophyta;
the plant group that scientists feel develops into the contemporary angiosperms. The most important similarity between ginkophyta
and gnetophyta is therefore the fact that both are seed bearing plants.
This leads into just how gnetophyta would evolve
and develop into the angiosperms of today. This question, which has plagued scientists for years without end has numerous
answers, and due to the number of answers that do exist, only one will be explored herein. The idea behind the evolution of
gnetophyta to angiosperms is that the two distinct leaves that are present within gnetophyta are comparable to the two cotyledons
that are present within the dicotylodonea group of angiosperms.
This explanation of the evolution of angiosperms does
not yet totally answer the question of the way in which angiosperms have evolved into the way that they appear today. To take
this into account, the theory that this explanation is based off of states that through various mutations that dicotylodonea
group of angiosperms could eventually diverge to form the lilliopsida group of angiosperms which has only a single cotyledon.
The converse of this theory is also held to be plausible; that lilliopsida evolved first and then mutated to become dicotylodonea.
It is this evolutionary sequence that some biologists feel angiosperms came to be what we see today. Despite the difficulties
faced in determining this sequence, biologists are growing more confident each day that this is indeed the way in which angiosperms
evolved. While it may be that some new discovery is made in recent years which proves for certain that one step or the whole
process is different, or that certainty in whether lilliopsida or dicotylodonea came first it is most likely that this is
the order that scientists will consider to be the evolutionary path of angiosperms.