
Why are there no animals with three legs? - lelf
https://phys.org/news/2019-10-animals-legs.html
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perl4ever
"Almost all animals are bilateral," he said. The code for having two sides to
everything seems to have got embedded in our DNA very early in the evolution
of life—perhaps before appendages like legs, fins or flippers even evolved.
Once that trait for bilateral symmetry was baked in, it was hard to change."

What about starfish and other echinoderms? They may not have exactly three
limbs, but they aren't bilaterally symmetrical, and they are animals.

~~~
avar
You've misunderstood what bilateral means, it doesn't mean "two of each", it
roughly means that there exists a point at which you could bisect the animal
with a mirror and "see" the hidden side.

So e.g. humans are bilateral even though we only have one nose, mouth, anus,
penis etc, because if you mirror half of a human bisected down the middle you
"see" the other side.

The same can apply for animals that have an odd number of appendages such as
starfish and other echinoderms.

~~~
lucideer
If that's the definition of bilateral, the article title makes no sense as
this property wouldn't preclude animals having three legs. So the definition
intended in the article must be narrower.

~~~
avar
Neither the article nor the paper makes that claim about bilateral symmetry.
From the abstract of the paper:

"As is reviewed here, the strongest constraint on the evolution of a triped is
phylogenetic: namely, the early genetic adoption of a bilaterally symmetrical
body plan occurring before the advent of limbs. Presumably, this would greatly
constrain any three‐legged animal from ever evolving."

In other words, an animal can have an odd number of legs and be bilateral, but
due to how animals grow and develop it's less likely to evolve than an even
number of legs.

~~~
lucideer
From the article:

> why are there no animals with three legs? That might go back a long, long
> way, Thomson said.

> "Almost all animals are bilateral," he said. _The code for having two sides_
> to everything seems to have got embedded in our DNA very early in the
> evolution of life—perhaps before appendages like legs, fins or flippers even
> evolved. Once _that trait for bilateral symmetry_ was baked in, it was hard
> to change.

(emphases mine)

Thomson suggests bilateral symmetry as the reason for not having three legs.
It's strongly implied that they're mutually exclusive. Not sure how else you
could read this excerpt.

~~~
avar
By "that trait" I believe he's referring to that particular way of getting to
bilateral symmetry, not that an organism that doesn't do it like that can't be
bilaterally symmetric.

As an example, think of what it would take to add a third leg to a human
being. Now we have two and one leg is a mirror image of the other. Those would
presumably be the outer legs, how's the central leg going to look like? Is it
going to be a copy of either the right or left leg, or some combination with 9
toes (4 outer, big toe in the middle) etc.?

Carrying that sort of information in the DNA is going to be harder than
following the default template of "make this here and mirror it", which is how
most structures are encoded in DNA.

Furthermore I think the reason the paper's abstract mentions phylogenetics
(but I don't know, I can't access the full thing) is to say something to the
effect that if an organism doesn't have any hands or legs, adding them in
pairs of two is inherently simpler than adding three, since the rest of the
body structure is already mostly following that template.

Once you start going down that road it's hard to "go back", which is why
modern animals carry a lot of structures that are only the way they are today
because they evolved from some initial past state.

~~~
perl4ever
"As an example, think of what it would take to add a third leg to a human
being"

Apparently the way radial symmetry actually develops is that one side of the
body grows and the other withers away, and the remaining side produces the
radial parts.

------
java-man
There were attempts to evolve, during Ediacaran period.

[0]
[https://en.wikipedia.org/wiki/Tribrachidium](https://en.wikipedia.org/wiki/Tribrachidium)

[1]
[https://en.wikipedia.org/wiki/Trilobozoa](https://en.wikipedia.org/wiki/Trilobozoa)

~~~
locusofself
fascinating .. so there we go. there was an attempt!

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jedberg
I wonder if anyone has tried an evolutionary locomotion algorithm that has bi-
pedal and tri-pedal animals compete. See if it can even come up with a
reasonable try-pedal model.

~~~
NohatCoder
Most quadroped mammals can run by moving their legs in pairs, in this mode one
of the pairs could be replaced by a single leg. Would make normal walking
awkward though.

~~~
Sharlin
And indeed some almost exclusively move like that, namely leporids (rabbits
and hares). They do not have a regular walking gait because the back legs are
so long and specialized for jumping.

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aurbano
I actually dislike this style of article writing, or title writing.

The title leads me to believe that I will find an answer to that question in
the article, but it just talks about the subject for a while until they decide
to end it.

The article was good though, so perhaps just a different title?

~~~
_nalply
I think that this is a trick to make people stay with the website. This works
similar to a cliffhanger.

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PeterStuer
Some do consider the Kangaroe to be a 3 legged species

[https://www.discoverwildlife.com/animal-facts/mammals/how-
ma...](https://www.discoverwildlife.com/animal-facts/mammals/how-many-legs-
does-a-kangaroo-have/)

~~~
anotheryou
Well I'd say they walk 5-legged
[https://youtu.be/HWiLyIqcK24?t=12](https://youtu.be/HWiLyIqcK24?t=12)

and "run" 2 legged
[https://youtu.be/ftgY63SlmKY?t=67](https://youtu.be/ftgY63SlmKY?t=67)

[https://youtu.be/S-V2MqNLgdg?t=14](https://youtu.be/S-V2MqNLgdg?t=14)

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3leggedcatman
My cat has three legs.

~~~
saagarjha
But cats, in general, don’t.

~~~
kqr
Oh, but they do. Indeed, it is a prerequisite for having four legs!

~~~
kijin
Three legs shalt thou not count, excepting that thou then proceed to four.
Five is right out.

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RenRav
If "aquatic locomotion" counts... seals, walruses, dugongs, manatees. Look at
their skeletons if you have time, it's pretty interesting.

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joe_the_user
The Kangaroo moves with two legs and a tail. How close is that to three legs?
Kangaroos hop with two feet but use their tail for slower movement.

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snissn
Whales and dolphins have fused legs and if you count their two flippers they
have 3 legs.

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jjtheblunt
Tarantulas have 3 legs, with 5 spares, i thought, having just seen one this
morning out with our dog, though i realize that's a technicality wrt the posed
question.

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m463
I thought the octopus has 3 brains, but it might not. looking it up, it might
have 9 brains and 3 hearts.

In any case the octopus is a pretty different kind of creature.

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cellular
"The code for having two sides to everything seems to have got embedded in our
DNA "... Heart, liver, gallbladder

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cellular
I've always wondered how birds evolved to fly. It seems like wings are useless
until they can fly.

~~~
ken
[https://en.wikipedia.org/wiki/Origin_of_avian_flight](https://en.wikipedia.org/wiki/Origin_of_avian_flight)

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Kaibeezy
Plenty of fictional tripeds. Watch this Idiran go...

[https://m.youtube.com/watch?v=iC6sJ-
Ysb2Y](https://m.youtube.com/watch?v=iC6sJ-Ysb2Y)

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meggar
Technically a centipede does have 3 legs.

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ganitarashid
Because you can’t walk on three legs

~~~
no1youknowz
Jake the Peg, did just that [0].

[0]:
[https://www.youtube.com/watch?v=KJleJbn9G6Y](https://www.youtube.com/watch?v=KJleJbn9G6Y)

