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Four closely related genera of green algae, showing a
progression from unicellular to colonial and multicellular
organization. (Courtesy of David Kirk.)
Evolutionary relationships among some of the organisms
mentioned in this book. The branches of the evolutionary tree
show paths of descent but do not indicate by their length the
passage of time. (Note, similarly, that the vertical axis of the
diagram shows major categories of organisms and not time.)
Multicellular Life
The single-celled prokaryotic and eukaryotic cells lacked the specialization and
organization we see in many plants and animals today. Single-celled organisms need
to perform every life function within the confines of a single cell. Multicellular
organisms, like ourselves, however, can have cells specialized for a particular
function. Thus, a liver cell doesn't have to "worry" about all the chemical reactions
and interactions with other cells that a retinal cell in the back of your eye has to
worry about. The liver cell can specialize and be very good at its job. Likewise, the
cells in your eye and making up the rest of the organ systems in your body can
specialize and be very good at what they do. This specialization allows for more
efficiency.
There are three (not mutually exclusive) theories on the evolution of multicellular
life. It is likely that multicellularity evolved independently in several groups over
time; for example, among the Protists.
Symbiotic Theory: Two or more unrelated organisms "cooperated" in a symbiotic
relationship and eventually each became so specialized and dependent on one
another that they fused genetically into a single organism. Although we see such
dependencies today (lichens), the algal and fungal species have separate
reproductive cycles making this theory unlikely.
Syncytial Theory: This suggests that multinucleate protists or slime molds may
have evolved cellular membranes between their floating nuclei (organisms with
multinucleate cytoplasms are said to be syncytial). Among the protists, however the
multiple nucleii usually have separate functions. In paramecium, for example, the
macronucleus serves to support day-to-day cellular function while the micronuclei
are involved in reproduction (see conjugation here). Syncytial slime molds such as
Physarum are multinucleate over part of their life cycle and it appears that the
nuclear variability in slime molds is not random suggesting that there would be a
mechanisms for radiating cell lines (see also Hexactinellids). See slime mold videos
here, here and here. Syncytia are also common among higher animals (developmental
stages of many insects, giant axons of squid, skeletal muscles) though these form
through the fusion of single-nucleated cells.
Colonial Theory: This is again a symbiotic theory but involves symbiosis within a
species. The advantage of this mechanism is that such relationships can be seen in
extant species (Volvox, etc.). This is the most-accepted theory and is the one
presented here.
Figure 1 depicts a pair of single-celled protists (Chlamydomonas and Euglena;
protists are organisms that are usually single-celled or found in loose colonial
organizations. More optional information on protists can be found here). Each has
it's own mechanism for procuring food (chloroplasts), moving about (a pair of hairlike flagella for Chlamydomonas, a single flagellum for Euglena). They have lightsensitive eye spots, excretory systems (contractile vacuoles), and are capable of
reproduction on their own. A perfect example of a jack-of-all-trades cell. Although
there are advantages in being self-sufficient, a major disadvantage is that their
small size and they can't grow much larger without organ systems (which would
require multicellularity). Their small size makes them easy prey for predatory
forms.
The first move toward multicellularity can be seen in organisms like Pandonna
(Figure 2) and Eudoria (Figure 3). Pandonna is a fresh-water colonial protist that
typically is seen as a colony of 8 cells. Eudoria, on the other hand, can be
considerably larger, composed of 16, 32, 64, or 128 cells. In both species the paired
flagella of each cell point toward the outside of the ball and serve to propel the
colony through the water. As the colony moves, it rolls, providing each cell with
their share of the sunlight. The colonies in both these examples are covered with a
clear protective capsule that helps to hold the cells together. In Eudoria's case,
the capsule allows the cells to occupy only the outer skin of the colony. No cells are
shaded at the center and, with a hollow interior, the colony can be even larger
making it considerable more difficult to chomp down.
Volvox (Figure 4) takes coloniality to extremes. These colonies can have 500 or
more individual cells each with a pair of flagella pointing toward the outside of the
the sphere (Figure 5). Daughter colonies grow in the interior of the mother colony.
When mature, the mother colony splits and releases the smaller spheres. Volvox
colonies can grow very large and can be easily seen with the naked eye. Despite
their size, however, these are not truly multicellular since there is no specialization
of the cells into tissues. Tissues are groups of cells that have more-or-less the
same morphology and who work together for a specific purpose. Muscle tissue in
your arm, for example, is groups of muscle cells all working toward movement. Bone
tissue is a grouping of cells all working toward support. No such specialization is
seen in Volvox or the others. Instead, each individual cell is like the others. For this
reason, these groupings of cells are known as colonies to set them apart from the
truly multicellular organisms.
One theory on the development of the metazoans (multicellular animals) suggests
that they had their origins in a group of protists known as choanoflagelates (Figure
6). Choanoflagelates have a collar formed from fused cilia (short hair-like
extensions). A single flagellum protrudes from the center of the ciliary
Choanoflagelates spend most of their time attached to rocks and other support
surfaces as a colony. The collared end of the cell points out and movements of the
flagellum create a vortex at the collar to draw small prey to the cell.
Sponges, the simplest of the animals, use the same method to move water through
their bodies to capture prey. The right-hand image in Figure 7 shows an internal
pore in a sponge surrounded by flagellated collar cells (choanocytes). The figure on
the right shows the choanocytes with their flagella pointing toward the top of the
picture. Like the choanoflagelates, these collar cells serve to trap prey for the
sponge. Unlike most animals, sponges do not have an organ level of cellular
organization and have only managed the tissue level. Because of this, you can take a
sponge, push it through a wire screen to tear it into thousands of pieces, and it will
re-assemble itself (try THAT with a puppy or baby bird!). Because sponges have no
true organ systems, they are often referred to as the parazoa (beside the main
animal line; see this figure for the placement of the Porifera). See Figure 8 for a
picture of a sponge; more optional information on sponges can be found here.
The first animals belonged to a diverse group of organisms known as the ediacarans
(Figure 12; more can be seen by clicking on Figure 12). This group was an
"evolutionary experiment" and non survive today. Their bodies were arranged as a
series of bag-like structures that probably housed symbiotic algae. Many were
sessile and incapable of movement, while others probably were able to move along
the ocean floor. There is no evidence of a mouth, anus, or other digestive tract
features; they probably fed by absorbing nutrients from the ocean around them
(with the help of their symbiotic algae). Both radial and bilateral forms are known
from the fossil record). Most are radial in their symmetry (Dickinsonia.
Astylospongia, Cyclomedusa, Eoporpita, and Nemiana), while others seem to have
made the first moves toward bilateral symmetry (Spriggina and Charnia; though
that is almost certainly just an artifact of fossilization).
The world was a very different place 600 million years ago when these beasts ruled.
It was much calmer and less violent with no visible predation; the only predators
being the unseen single-celled protists. Then somebody developed a mouth,
eventually got tired of sucking muck off the ocean floor, and began to feed on one
another. The dim-witted ediacarans must have been easy prey. More on ediacarans
can be found here.
The first of the bilateral metazoans to survive to this day were a group of flat
worms (Platyhelminths) known as acoels (optional readings on the platyhelminths
can be found here). The acoels are a primitive flat worm, sometimes placed in their
own group. They are the first to show the focused bilateral symmetry and we see
the beginnings of the first really organized neurosensory systems. Read "From a
Flatworm: New Clues on Animal Origin.