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THIS
IS NOT HOW EVOLUTION OCCURS!! What this
animation
implies is that one type of animal became
another
type of animal. It shows a chimp becoming a
human.
If so, how come there are still chimps? This
suggests
direct descent with adaptation and this is a
fallacy.
What is correct is that the animals shown did all
share
a common ancestor at some point.
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Every living thing you see around you, and all the
living things you can’t see around you, are descended from Luca. But here is
the part that’s a little harder to understand – it hasn’t been a straight line
from Luca through all other life forms to you.
It’s the difference between common descent and direct
descent. Just because humans and pine trees have a common ancestor, it
doesn’t mean that we are direct descendents of the same organism.
At some point, about 3.5 billion years ago, archaeal prokaryotes were recognizable.
Luca gave rise to archaea and also to bacteria, but we can’t say that bacteria
descended directly from archaea. There are as many differences between archaea
and bacteria as there are between you and a pine tree, maybe more. All we can
say is that they diverged from a common ancestor, which is why they have some
common characteristics (from a common ancestor), and many
different characteristics (because they diverged).
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This
is a much better representation of evolution
through
common descent and adaptation. We may not
know
what exact organism was the last common ancestor
between
any two branches since only the fossil record only
represents
about 1% of species that lived.
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Archaea, as opposed to bacteria, diverged again and again,
giving rise to the rest of the kingdoms we know today – protists, plants, fungi,
and animals. The question is, why did they diverge so much, while bacteria
stayed so much the same?
I don’t know the answer, but a good theory is the
development of organelles. We have talked before about how an archaea swallowed
a bacteria, but didn’t destroy it, and how that the eukaroytic nucleus. Another
instance in this endosymbiosis theory
formed the mitochondrion. The more complex something is, the more chance for it
to respond to changes in the environment – more adaptation could have led to
more divergence.
Through billions of years of struggle and adaptation, the
eukaryotic kingdoms emerged – but again, you can’t say that one descended directly from
another. Contrast evolution to the alphabet. C follows B, which follows A.
Humans like to think linearly. But consider that there may be millions of A’s.
Must they all become B’s and then C’s?
No, millions of A’s may breed and remain A’s over the years,
while what became a B breeds and stays a B. Maybe later on a couple of A’s
breed and have a little baby that looks like neither an A nor a B. Now we have
C. B and C or both diverged from A, but B and C are different from one another.
They are linked by common, but not direct, descent.
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This
is a phylogenetic map of the life on Earth. Notice that
we
don’t know just what the last common
ancestor was, but
you
see that eukaryotes diverged from archaea and that
animals,
plants, and fungi diverged from protists
(microsporidia,
flagellates, ciliates, slime molds) rather
recently.
Trees may look a bit different based on what DNA
target
gene is being compared.
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A long time ago, a few archaeal descendents had diverged enough
to be called protists. They were eukaryotic now, with organelles and special
functions, and were starting to think about working together as multicellular
organisms. The protists of that time
were direct descendents of archaea, but the different protists we know today
(see our last few posts here, here, and here) aren’t necessarily directly
descended from one another or even from the same archaea.
Some protists diverged from one another. Many stayed as
protists, even though they might have evolved to different orders or families over
the years. However, others became animals, fungi, or the plants. Our last
common ancestor with plants was a protist of some sort, so we have a common
ancestor with a pine tree, we just can’t draw a direct line through humans and a pine tree to get to that ancestor.
Let’s use undulipodia
(eukaryotic flagella and cilia) to illustrate, or confuse, the situation. We
saw that protists – most all of the different protists, use flagella or cilia
in one or several ways. Flagella and cilia are characteristics that have been
retained in the various descendents of archaea…. But didn’t we also say that
prokaryotic flagella and eukaryotic undulopdia are completely different in
structure and genes? Yes we did - so what gives.
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This
is a typical fungus that someone might think of. This is the
basidiomycete
mushroom, Phallus indusiatus. Fungi
can range
from
single cell yeasts (yeah beer!) to hyphal forms that make
your
tongue fuzzy, to mushrooms that can take up thousands of
acres.
By the way, in New Guinea they worship P.
indusiatus as
sacred
and that lacy web is called an indusium.
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Yet, if you look for undulipodia in the fungal species on
Earth today, you’ll find them in only one of the five phyla. Again, what gives?
Didn’t we just say that fungi descended from undulipodia containing ancestors?
Four of the phylums of fungus are grouped by
characteristics, both genetic and life cycle – the Chytridiomycetes, the
Zygomycetes, the Ascomycetes and the Basidiomycetes. The fifth phylum is called
the imperfect fungi because we
haven’t found a life cycle in them that we can place into one of the other four
phyla. But when we do, as we have for some individuals imperfect fungi, they
usually fall into the ascomycete or basidiomycete phylum.
However, only chytrid fungi have flagella. Every species in
every other phylum of fungi has lost their undulipodia. If you are thinking in terms of
phylogenetics – what does this suggest to you?
Yes, very good – the chytrids are probably the common
ancestor for all the other fungi. Those that didn’t diverge enough and stayed chytrids
retained their undulipodia, but at some point, others lost their flagella
and kept on changing into all the other phyla of fungi over time.
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Here
is a phylogenetic tree of the fungi, showing that flagella
were
lost after the other phyla diverged from the
Chytridiomycetes.
On the bottom right is a microscopic image
of
the mature sporangium of a chytrid fungus. The
zoospores
are inside the sporangium. The dark filaments are
rhizobia,
not flagella.
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What is so different about the chytrids that they kept their
flagella, while the others were suited to live without them? There must be
something about the chytrids life cycle and/or environment that required the
retention of their eukaryotic flagella. Well, they all live in water, maybe
they need flagella to swim around. But look at the picture of the chytrid on
the right, I don’t see a flagellum anywhere there.
The chytridiomycete name comes from the Greek chytridion, which means little pot. The pot holds all their zoospores, their reproductive form.
It is the zoospores that have the flagella, so they can swim away and establish
themselves somewhere with less competition. But that isn’t a good enough
explanation. Many water borne fungi come from the basidiomycete or ascomycete
phyla, and they don’t have flagella in any of their life cycle stages. Hmmm.
We have tens of thousands of extant (living today) fungal
species, and only about 750 or so have any undulipodia. This makes the chytrids
exceptional, and is directly related to their being the common ancestor for all
the other fungi. If undulipodia are so important that animals, protists, some
fungi, and even some plants have them, then why is it that so many fungi seem
to get along fine without them? Maybe they’re the exception.
Let’s tell one story of how the chytrid flagella are
important. A species of chytrid, called Batrachochytrium
dendrobatidis,
is responsible for perhaps the largest vertebrate mass extinction in the
history of the Earth - and it's going on right now. Most chytrids are sabrophyitc, meaning they eat dead
tissue, but a few are parasitic, mostly on plants.
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Although
the chytrid infection is a large reason for amphibian
collapse,
there are others. Mutagens in pollution are responsible
frogs
with extra or missing limbs. You can imagine how that
might
interfere with long life and mating. It turns out that
amphibians,
being in and out of water, are especially vulnerable
to
environmental changes.
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You may have heard of the frightening loss of amphibian
diversity on the past decades. The reasons for this are known to be several,
including toxic pollutants, climate change, and habitat destruction. But B. dendrobatidis has played a
significant role as well. Fortunately, a 2011 study showed that a water flea
has a voracious appetite for our frog foe and might be used as a control
measure. Unfortunately, a 2013 study showed that chytrid-susceptible amphibians
may be succumbing to at least two different, and probably more, species of Batrachohytria. So the problem is
probably more complex than we thought.
And yes, you read correctly a couple of paragraphs above. Some
plants have cells that have flagella. There are moving plants cells! Those will
be our exceptions for next week.
For
more information or classroom activities, see:
Common
descent –
http://www.ibtimes.co.uk/four-billion-year-old-mystery-last-universal-common-ancestor-solved-1460866
Fungi
–
Amphibian
collapse –
Phylogenetics
–
