The origin of the anus
The extraordinary evolution of our most
embarrassing organ
Presented by
Matt Walker
Yes that’s a picture of a puppy, with a little
bit of its rear end showing. And no, we
couldn’t actually illustrate this story with an
anus, now could we?
And before we discuss the origin of the anus;
let’s back up a little. It’s a subject surrounded
by, how should we put it, a bit of cheek. A
topic right for puns, or a touch of verbal
diarrhoea as we can’t but help see the
innuendo.
See what I mean? So we try to get serious, to
focus, and ask why has no one gotten to the
bottom of this particular mystery before? Is it
a crappy research topic, or by not addressing
it, have other scientists fallen behind? Is even
reporting such a subject, well, a little anal?
Perhaps, if jokes and innuendo are all we care
about. But if we’re interested in some of the
most fundamental questions about how
animals evolved and function, then read on.
Because scientists have just published a
review into the origin of the anus, and in
doing so, they articulate how the subject is far
more important than it at first appears.
That’s because the anus is one of the most
important parts of many animals; an essential
structure that changes how an organism’s
digestive system works. Our puppy would
struggle to eat and grow without it. But
intriguingly, not all animals have one. Some
have simple versions, others have many, while
a few organisms uniquely appear to have
transient anuses, which come and go. Others
have anuses that are… how shall we put it…
multifunctional?
While several organ systems have been
investigated, such as the nervous system,
the anal opening has been largely
neglected
But those that do have an anus, an organ we
can’t help but smirk and joke about, have
vastly improved digestion. They can eat and
grow more effectively, and reach much larger
sizes. And the story of the origin of the anus
is actually a story about how animals evolved,
diverged from one another, and became
sophisticated creatures.
The review into the evolutionary origins and
development of the anus is published in the
journal Zoologischer Anzeiger - A Journal of
Comparative Zoology
.
Molecular biologist Dr Andreas Hejnol and Dr
Chema Martín-Durán, of the University of
Bergen, Norway undertook the research in
part, because no one else had. Over recent
years, scientists have been able to study how
genes affect the development of a range of
species.
There are a select few animals that
appear to have a ‘transient’ anus
“That revived the interest in the origin of our
organ systems. Where did brains come from?
How did blood evolve?” Dr Hejnol told BBC
Earth. “But while several organ systems have
been investigated, such as the nervous
system, the anal opening has been largely
neglected.”
But through evolution, the anus has appeared
in many forms, occasionally disappearing
again. “It is a fascinating subject to
investigate how changes on the molecular
level during evolution led to the shaping of
this part of the gut,” says Dr Hejnol.
Digesting the facts
The appearance of the anus is of course
inextricably linked to the evolution of the
whole digestive system.
The digestive tract is one of the most
important organs in any animal; it allows the
efficient digestion of food and uptake of
nutrients to maintain growth and sustain the
body.
Despite this, some types of animal cope even
though they lack a digestive tract, including
sea sponges (Porifera) and tapeworms, for
example. These species, perhaps obviously,
also lack an anus.
Others have a very simple gut, a single sac
with a single opening.
These have an anus of sorts; as their single
gut opening acts as both mouth and anus –
being used to both ingest food and expel
excrement.
Many gelatinous-type creatures that live or
float in the open ocean, including sea
anemones, corals and jellyfish (types of
animals known as Cnidaria) and comb jellies
(Ctenophora), process food this way. As do
many types of flatworm (Platyhelminthes).
Sessile animals often possess a U-shaped gut
with the gut and anus in close proximity.
"They manage," says Dr Hejnol.
Then there are a select few animals that
appear to have what Hejnol and Martín-Durán
describe as a ‘transient’ anus.
The tiny jaw worm called Haplognathia , which
is just a few millimetres long, has a temporary
opening to its gut, through which researchers
speculate it may occasionally defecate,
although the creature has never been caught
in the act.
A similar transient anus is found in one of the
smallest animals known, a microscopic
creature called Limnognathia, which was only
discovered in 1994, living in the hot springs of
Disko Island, Greenland.
Trying to understand when and why the anus
evolved is also complicated because it
appears and disappears again within many
groups of animals.
While most flatworms lack an anus, some
species have independently evolved one. Some,
such as the polyclad flatworm ( Thysanozoon
nigropapillosum) have multiple anuses,
situated on their back.
Most deuterostomes, a superphlum of animals
including sea stars, sea cucumbers, and all
vertebrates such as birds, fish and mammals
such as humans, have an anal opening (in
some groups, such as reptiles and birds, the
anal opening is fused with the reproductive
opening, forming a structure called a cloaca).
But one group of deuterostomes, the sea stars,
now lack the organ.
Just imagine if we have to wait for lunch,
because we still have not gotten rid of
the remnants of the breakfast
And the history and function of the anus gets
even more complicated. Some animals grow
up with an anus, but then lose it during their
own lifetime.
A rare group of scorpions, for example, will
occasionally break off their tails in order to
escape an attack by a potential predator, a
process called autotomy.
While the benefits of doing so are clear, as the
scorpions avoid being eaten, the act has
consequences for the animals’ own eating
habits. For when shedding their tails, they
also shed their own anuses.
Since the tail never grows back, and the
wound scars over, the scorpion (Ananteris
balzani) can never defecate again, their
abdomens swelling with the build up of poo
.
The tale of the scorpion’s tail, so to speak,
serves to reinforce how important anuses are
in animals that have a gut that opens at two
ends.
This basic digestive body plan is used by
most major animal groups. Insects, birds,
mammals, fish and amphibians, for example,
all have an alimentary canal, or continuous
through-gut, with an opening through which
they eat, the mouth, and another opening for
defecation.
Having a through-gut with two openings,
instead a simple sac, with just one opening,
has two main advantages, explains Dr Martín-
Durán.
“First, an animal can take up new food items
while it is still digesting a former meal,” he
says, because the food flows in one direction
through the body. Animals with a sac-like gut,
however, have to wait to finish their first meal,
and defecate out its remains, before eating
again.
“Just imagine if we have to wait for lunch - up
to nine hours - because we still have not
gotten rid of the remnants of the breakfast,”
he says.
Some animals are also the wrong shape to
cope with a sac-like gut. Ribbon worms,
reputedly the longest animals in the world,
can reach 60 metres long. “A sac-like gut
would make the sorting and digestion of food
difficult,” says Dr Hejnol. Or put another way,
50 metres or more is a long way for the waste
to come back up. Much easier to have another
opening, i.e. an anus, at the far end, to excrete
it from.
Second, a through gut, with mouth and anus,
can be subdivided into different, specialised
sections, each performing a different role in
digestion. Animals have evolved weird and
wonderful mouths; think how different the
mouths of a worm, bee, bird or fish are, for
example.
Many have evolved intricate guts, including
fore, mid and hind-guts, each with different
types of cells and functions. Cows are a
classic example of a ruminating mammal,
which have many gut chambers to
progressively break down hard plants and
grasses.
We, for example, chew with our mouths, digest
proteins in our stomachs, secrete bile to help
digest fats, and finally absorb most nutrients
via the small intestine.
“It is hard to imagine processing food as
efficiently with just a sack-like gut that lacks
an anal opening,” says Dr Martín-Durán.
So the advantages of a through-gut,
facilitated by the appearance of the anus,
seem clear.
Less clear is when and where the anus
originated in evolutionary history.
“The pattern of presence and absence of the
anus in animals is quite fascinating,” says Dr
Hejnol.
The shape of any organ, and the specialised
cells it is made from, depend on a network of
genes that contain the information used to
build it. These genes are inherited down the
generations, and the same genes are present
within many very different types of animal.
That means the early common ancestors of
these various species also had these genes,
and likely the same organ.
Hejnol and Martín-Durán’s review confirms
that two sets of genes in particular, known as
brachyury and ParaHox genes, which are
present in nearly all animals, play a key-role
in the formation of the anal orifice.
Animals that have an anus, almost uniformly
express these genes in the tissues surrounding
the organ. Those animals that do not have an
anus, do not.
Intriguingly, and perhaps confusingly, despite
the clear advantages of having an anus, some
species have undergone a second wave of
evolution, losing the anus that appeared in
their ancestors. They often lose too the genes
that code for the organ when it disappears,
raising questions as to why it was beneficial
to lose something so vital.
Dr Hejnol and Martín-Durán are conducting
further research in a bid to resolve these
mysteries.
They are sure that the anus has evolved
independently more than once among
animals. “But its deep origin remains unclear,”
says Dr Hejnol.
Our own hypotheses is that the anal
opening has some evolutionary
connection to the male gonopore
Their studies do provide tantalising evidence,
however, that the evolution of the anus is
linked to another structure that animals use
for sex.
They are researching a group of animals
called Acoela, which, in evolutionary terms,
are extremely primitive. Resembling tiny
flattened worms, no more than two millimetres
long, these creatures live free in the sea. They
have no gut at all, nor anus and even lack
circulatory or respiratory systems. In essence
they do not have any body cavities; their
bodies are solid.
But they do create sperm, which is released
through an opening in the body called a
gonopore. Many animals that lack back-bones
use gonopores to release sperm and eggs, and
genetic and molecular studies by Hejnol and
Martín-Durán and colleagues suggest the
development of the gonopore might be linked
to the origin of another opening – the anus.
It is very difficult to have a serious
conversation about this subject
“Our own hypotheses is that the anal opening
has some evolutionary connection to the male
gonopore,” Dr Hejnol told BBC Earth.
“This of course makes the whole subject even
more delicate. But this is how nature is -
nature does not care about taboos in human
society.”
“It is very difficult to have a serious
conversation about this subject,” he admits.
“It is much easier to talk about the evolution
of sex and sexuality than about the evolution
of the anus.”
“This indicates that the daily business we
have to do on the toilet is clearly a taboo in
industrialised societies.”
“Maybe our research can contribute to change
the discourse about the subject and
communicate about it more openly.”
And with that, perhaps we should all find a
new, less humorous, respect for what many
still see as our most embarrassing organ.
The sea cucumber (Parastichopus tremulus),
for example, has a wonderfully multifunctional
anus. Not only does it defecate through its
anal opening, it breathes through it too.
And if that fails to impress, and innuendo and
puns is what you’re still looking for, then the
phrases ‘stick it’ and ‘where the sun don’t
shine’ come to mind. Sorry, I couldn’t help
myself.
Or if you’re still reeling with the shock of it all,
here’s another cute picture for you, this time
of a little kitten.