Two Kidneys

Representative Trantis said:

Why did animals develop two Kidneys and two testicles/ovaries?

You asked this in sci.anthropology.paleo and I answered you there. There is seldom any reason to
post to two news groups. And if you do, you should learn to cross-post rather than post separate
messages in each. That way, the answers to your questions will also get cross posted.

[moderator's ah, but: Ah, but I frown on crossposting; I'm likely to allow it within the sci.*
hierarchy, so long as I don't step on the toes of some other moderated newsgroup, but this question
is patently NOT specific to humans, and hence, is not appropriate for
s.a.p. I agree with other answers, however, in thinking that the question should be, since we're
bilaterally symmetrical, why are some organs NOT duplicated (when, of course, developmentally
speaking, the heart is symmetrical to begin with, but torsion messes up all that lovely
symmetry, for example). - JAH]

Trantis asks:

Quoted message said:

Why did animals develop two Kidneys and two testicles/ovaries?

Once evolution has settled on a bilaterally symmetric design plan during embryogenesis, duplicating
any structure simultaneously on each side of the midline is probably the simplest design possible.
The same question could be asked of why do we have two eyes, two ears, two arms, two lungs, two
brain lobes or two nares?

The more difficult question to answer is why do we have only one heart, one liver, one pancreas, one
stomach, one gut, each asymmetrically placed against the midline?

Wirt Atmar

"Representative Trantis" <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Why did animals develop two Kidneys and two testicles/ovaries?

For the same reason they have 2 lungs! If one organ craps out, or is injured, or is diseased, or is
... etc., the other one is able to take over the duties of its fallen comrade. Redundancy is crucial
for higher organisms.

"Lane Lewis" <[email hidden]> wrote in
message "]news:[email hidden]...

Quoted message said:

"Wirt Atmar" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Trantis asks:

Quoted message said:

Why did animals develop two Kidneys and two

testicles/ovaries?

Quoted message said:

Quoted message said:

Once evolution has settled on a bilaterally symmetric

design plan during

Quoted message said:

Quoted message said:

embryogenesis, duplicating any structure simultaneously

on each side of

Quoted message said:

the

Quoted message said:

midline is probably the simplest design possible. The

same question could

Quoted message said:

be

Quoted message said:

asked of why do we have two eyes, two ears, two arms,

two lungs, two brain

Quoted message said:

Quoted message said:

lobes or two nares?

The more difficult question to answer is why do we have

only one heart,

Quoted message said:

one

Quoted message said:

liver, one pancreas, one stomach, one gut, each

asymmetrically placed

Quoted message said:

against

Quoted message said:

the midline?

Wirt Atmar

Perhaps those organs that were close to the center did not

need to have a

Quoted message said:

double but those out side the center needed two to be

symmetrical. The

Quoted message said:

doubling of organs might not be needed for survival just

symmetry. It may be

Quoted message said:

just easier to produce. The one organ that might bring

this down is the

Quoted message said:

brain which even though is along the center has two

complete sides.

Quoted message said:

Lane

Perhaps an even deeper question that might be asked is, why are all larger animals bilaterally
symetric, rather than symetric from front to back, or even "tetra-symetric?" The latter should have
even greater survival value (other things being equal. 🙂) ...tonyC

"Wirt Atmar" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Trantis asks:

Quoted message said:

Why did animals develop two Kidneys and two testicles/ovaries?

Once evolution has settled on a bilaterally symmetric design plan during embryogenesis,
duplicating any structure simultaneously on each side of

the

Quoted message said:

midline is probably the simplest design possible. The same question could

be

Quoted message said:

asked of why do we have two eyes, two ears, two arms, two lungs, two brain lobes or two nares?

The more difficult question to answer is why do we have only one heart,

one

Quoted message said:

liver, one pancreas, one stomach, one gut, each asymmetrically placed

against

Quoted message said:

the midline?

Wirt Atmar

Perhaps those organs that were close to the center did not need to have a double but those out side
the center needed two to be symmetrical. The doubling of organs might not be needed for survival
just symmetry. It may be just easier to produce. The one organ that might bring this down is the
brain which even though is along the center has two complete sides.

Lane

Anthony Cerrato said:

"Lane Lewis" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

"Wirt Atmar" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Trantis asks:

>Why did animals develop two Kidneys and two

testicles/ovaries?

Quoted message said:

Quoted message said:

Once evolution has settled on a bilaterally symmetric

design plan during

Quoted message said:

Quoted message said:

embryogenesis, duplicating any structure simultaneously

on each side of

Quoted message said:

the

Quoted message said:

midline is probably the simplest design possible. The

same question could

Quoted message said:

be

Quoted message said:

asked of why do we have two eyes, two ears, two arms,

two lungs, two brain

Quoted message said:

Quoted message said:

lobes or two nares?

The more difficult question to answer is why do we have

only one heart,

Quoted message said:

one

Quoted message said:

liver, one pancreas, one stomach, one gut, each

asymmetrically placed

Quoted message said:

against

Quoted message said:

the midline?

Wirt Atmar

Perhaps those organs that were close to the center did not

need to have a

Quoted message said:

double but those out side the center needed two to be

symmetrical. The

Quoted message said:

doubling of organs might not be needed for survival just

symmetry. It may be

Quoted message said:

just easier to produce. The one organ that might bring

this down is the

Quoted message said:

brain which even though is along the center has two

complete sides.

Quoted message said:

Lane

Perhaps an even deeper question that might be asked is, why are all larger animals bilaterally
symetric, rather than symetric from front to back, or even "tetra-symetric?" The latter should have
even greater survival value (other things being equal. 🙂) ...tonyC

It is easy to see why animals with an anterior/posterior axis have a clear "advantage" over radially
symmetric ones. Locomotion specialized for movement in one direction can be much more efficient. The
anterior part then encounters new environments first and specialized sensory structures are
concentrated there as well as central nervous structures to process the sensory information and
prepare appropriate behavior. Given a one-way complete digestive tract, it is also useful to put the
mouth at the front and anus at the rear where the anterior brain and sensory structures can be
associated with feeding. A dorsal/ventral axis (contrasted with a cylindrical body) also allows for
postural and locomotory specalization. Given anterior/posterior and dorsal/ventral, then left/right
follows logically. The duplication of internal organs provides some redundancy and the duplication
of external organs is important for balanced sensation and locomotion. Other systems of symmetry, or
total lack of symmetry, do not allow for such structural and functional specialization.

Of course, you do have to take into account many of the specifics. The flatworms are bilateral
without the complete digestive system. The gastropod molluscs have modified the strict
anterior/posterior axis and the scaphopod and cephalopod molluscs have modified the strict
bilaterality. The echinoderms have become secondarily radial. Still, the bilateral planula larva of
the Cndaria, with an anterior/posterior axis, does go way back evolutionarily and may form the basic
pattern for the development of the bilateral animals. And the bliateral plan (anterior/posterial,
dorsal/ventral, left/right) has clearly been enormously successful for the arthropods and the
vertebrates, the "major advanced" animals we now see.

Incidentally, the heart is no exception to the general plan of bilateral symmetry. This leaves the
digestive system is the sole unpaired system. According to the Nature Encyclopedia of Life Sciences:

"Heart development starts with progenitor cells that are allocated from mesoderm at the anterior
domain of the embryo and become committed to a cardiogenic fate (cardioblast, heart specification).
These clusters of cells (cardiac precursors) form a bilaterally symmetric cardiogenic field that
develops further into bilateral cardiac primordia. The primordia then come closer together and
eventually fuse at the midline to form a single linear heart ."

([censored], Qing and Chen, Qiuyun (December 1999 ) Cardiovascular Disease and Congenital Heart Defects.
In: Nature Encyclopedia of Life Sciences. London: Nature Publishing Group. els.netels.netOpen ↗
[doi:10.1038/npg.els.0001907] )