Biology concepts – bilateral symmetry, radial symmetry,
planulozoa hypothesis, cephalization, last animal common ancestor, porifera,
platyhelminth, cnidarian, echinodermata
 |
|
Halloween was a classic slasher film.
Jamie Lee
Curtis
looks so young, decades before Freaky
Friday or yogurt commercials. Michael
Myers
could
cut a man in half with his machete, but
could
he produce two mirror image halves?
|
But how many ways can you be cut in half? Top to bottom
is one way, leaving you with your head attached to one half and your feet
attached to the other. Or you could be cleaved through your ears and down
through your body. Then you would have your nose attached to one half and your
bum attached to the other.
However, there’s only one way to slice you that will give two
mirror images, each with the same components. If Chucky happens to catch you
through the top of your head, down through your nose and straight down to where
your legs split, each half will have one eye, one arm, one leg, one ear. This
can only occur because you are bilaterally
symmetric. Most animals (about 99%) have bilaterally symmetric bodies, so we have to at least consider the possibility that this provides some
sort of advantage.
 |
|
Cnidarians
like jellyfish have radial symmetry,
but
not spherical. You still have to cut them in
half
from top to bottom through the center. When
some
move on their own, instead of floating, they
move
like bilaterally symmetric animals – could
this
have been the start of bilateral symmetry?
|
If bilateral symmetry is advantageous, why are jellyfish
still radial? Because it works for them; no pressure/ random mutation
combination sent them on that path. Remember, evolution doesn’t have a
plan, it is neither reactive nor proactive. Random mutations are always
occurring, and sometimes a change in environment makes renders a random mutation advantageous. It’s simply hit or miss. If the mutation or the pressure occurred at some other time, they would miss each other.
Radially symmetric animals tend to be sessile (non-moving), free-floating, or very slow movers. They
don’t chase prey down, so they don’t need to be fast. This is the advantage of
bilateral symmetry; it coordinates movements so that an animal can move in a
particular direction faster. In fact, one 2102 paper puts forth the idea that
maneuverability is the main reason for the maintenance of bilateral
symmetry in animals.
However, fast movement wouldn’t be much use if you didn’t
know where you were going. This is why bilateral animals also have a head. A
head is a place to store your sensory apparatus and your neural tissue to
process those sensory inputs. You think its an accident that our brain is
located the same place as our eyes, ears, nose, and mouth?
Slow or sessile animals (like cnidarians) that filter feed
or catch what runs into them have no head. They have few sensory neurons, and
only loosely associated ganglia of neural tissues spread throughout their
bodies.
 |
|
Humans
decided to become bipedal (two footed)
and
this made it hard for us to lead
the
way with our head. Our foramen magnum
(hole
where spine emerges) moved down and
below
our skull to support the weight of our head
and
keep our sensory organs pointed in
the
right direction.
|
Bilateral animals have a head, and radial animals don’t have
a head. This sounds like a fairly plain story – as animals diverged and
evolved, some developed a head and became bilateral. Or..... did they become
bilateral and then develop a head? Maybe the animals can tell us which way it
was.
The flatworms (platyhelminthes)
were the first divergence of animals to have their neural ganglia clustered in
their anterior end. Going along with this, they have sensory systems located at
that end too. They have eyespots, although they are really just patches that
detect light or dark.
 |
|
Platyhelminthes
have a define head ganglion of
nerves
and have started develop more senses at
the
anterior end; see the sensors that stick up.
And
they move faster and in a straight line. They
are
headed, and headed in a particular direction.
|
So we have gone from animals with no head and radial
symmetry to animals with a head and bilateral symmetry. This doesn’t help
answer the question of which came first. Aren't there any animals in between?
Yes, there are, and they give us a little bit of a clue as
to which came first. The ctenophora (pronounced "ten", cteno = comb and phora = bearing) is a phylum of animals that lie between the
cnidarians and the platyhelminthes. Ctenophoran animals are the comb jellies. Both cnidarians and comb jellies have been around for over 500 million years, so they’ve had time to
settle in to a niche.
The comb jellies look round at first glance, but their
architecture is a bit more complex than the cnidarian jellyfish. They have
internal and external features that allow only for two planes of symmetry that
give mirror images (see picture). These especially include the combs, rows of
fused cilia that line their sides, and the fact that they don’t have stinging
cells (cnidocytes). Remember that ONLY cnidarians have cnidocytes.
 |
|
Here
is a cladogram that shows the divergence of
each
phylum of animal from their last common
ancestor.
Ctenophores and cnidarians diverged from
each
other recently (or did they, see article). Starfish
diverged
after everyone else on that end was
bilateral,
yet they are radial as adults. What gives?
|
A 2004 study investigated the relationships between biradial
and bilateral animals in evolution. If biradial is the link between radial and
bilateral, then would seem to suggest that bilateralism occurred before
cephalization. Called the Planulozoa Hypothesis, the authors
suggests that ctenophora are the sister clade of bilateralians, and that all three of the groups –
cnidarians, ctenophora and bilaterals – are the descendents of a single
bilateral ancestor.
Ctenophora larvae have bilateral features, so this supports
the planulozoa hypothesis (the free swimming larvae of all three phyla are called
planulae). This would then suggest that cnidarians were once bilateral and then
returned to radial symmetry.
Additionally, the if the planulozoa hypothesis holds, then
bilateralism would seem to predate cephalization (development of a head). The larvae of ctenophores and
some ctenophore features show that a move to true bilateral symmetry came before
platyhelminthes and the emergence of a head. The conclusion – the streamlined
body came before the head. But that confuses me, one isn’t much good without
the other.
 |
|
Ctenophores
– the comb jellies, often show
bioluminescence.
They only have two perpendicular
planes
of mirror image symmetry. You can see the
fused
cilia that form the combs on each ridge.
|
Wait a minute, there’s a fly in the bouillabaisse. Ctenophores
have a nervous system that is more complex than many other animals – it’s just
not centralized to a head. Centralizing the nervous system, with the sensory processing
and muscular control, is a crucial part of cephalization. They seem to
developed a strong neural system without adding the head itself.
A 2014 study of the genome of several ctenophores showed
that they do not have the same neuron-building gene regulation pathways as any
other phylum of animals, and they only use one of the most common
neurotransmitters; all their other neuron signalling molecules are unique to
ctenophores alone. This suggests that they evolved radically differently than
the phylums around them, cnidarians and flatworms. This does not support the planulozoa hypothesis at all. Ctenophores may have developed all on their own and therefore can't help us answer the question of which cam first the bilateral body or the head.
Other things about symmetry development make you say, “Huh?”
as well. Look at that same cladogram of animals above – see the right side
where the sea star is located? What’s a radially symmetric animal doing way
over there after everyone else switched to bilateral symmetry?
The echinodermata
(sea stars, brittle stars, sea cucumbers; echino
= spiny, and derm = skin) also
support the planulozoa hypothesis, since they seem to have undergone the same
regression as the cnidarians. Echinoderms include the brittle stars, sea stars,
sea cucumbers, barnacles and sea urchins. They have bilateral symmetry as
larvae, but many of them become radial (pentaradial or such, depending on the
number of arms) when they become adults.
Secondary radial symmetry
is term for when a bilateral larva becomes a radial adult; but it is more
interesting than that. The easy way for that transformation to occur would be for the arms to grow out of the
larva, with the top (aboral) and
mouth (oral) sides remaining the
same. But that’s not how it happens.
 |
|
The
brittle star, an echinoderm, walks like bilateral
animal,
even though it assumes pentaradial symmetry
as an
adult. One arm acts as the head, and two arms
on
each side work as mirror images. When it wants to
turn,
it just assigns another arm to be the head.
|
But like I say, it works for them. The adult sea stars and other echinoderms are
fairly slow. Their lifestyle doesn’t require a head or a bilateral body, so
they went biologically simpler and energetically cheaper and returned to radial symmetry. All the mechanics were still in their genomes - it was really pretty smart.
However much they have tried to regress as adults, the
brittle stars seemed to have retained at least a little bilateral activity. The
way they move is a lot like a bilateral animal, according to a 2012 study. One
arm points forward, the direction they are traveling. The arms on either side
then push the animal along, like a crawling bilateral animal. I guess you can’t
completely go home again.
Next week – Bilateral animals are simple - just two mirror
images, right? Well no. You won’t believe the number of complex animals that
break symmetry in order to give them a unique shape or function.
For
more information or classroom activities, see:
Biologic
symmetry –
Ctenophora
vs .cnidarians –
Echinoderms
-
