Many owls’ ears are asymmetrically positioned, to allow them to better pinpoint sound sources.
Of course, there’s a Wiki page for this.
I’ll just point out there are multiple geometric notions of “symmetry”.
Starfish and sand dollars are not bilaterally symmetrical. To a gross approximation they are radially symmetrical.
From an evolutionary standpoint, starfish are interesting. Radial symmetry antedates bilateral symmetry. Starfish fall among Bilateria, the larvae are not radially symmetric. The adults are a secondary return to radial symmetry.
We have Porifera (sponges) and Placozoa without any symmetry.
Then Cnidaria (jellyfish etc.) and Ctenophora (comb jellies) with radial symmetry.
Other animals fall under Bilateria, descended from a common ancestor that evolved bilateral symmetry. Within Bilateria there are some interesting cases like the adult starfish that returned to a radially symmetric body plan, and molluscs with spiral shells. As well as countless examples of more minor deviations from bilateral symmetry.
A prior related thread.
They get over it.
Strictly speaking, anything radially symmetrical is also bilaterally symmetrical. It’s just not the maximum degree of symmetry they have.
But in any case, adult sand dollars (unlike other urchins) are secondarily bilateral, not purely radial. Their anus has migrated off to one side, and they have preferential fronts and backs.
There are good accounts of symmetry in the evolution of development here:
“I went to a planet where the native lifeforms had no bilateral symmetry, and all I got was this lousy F-shirt.”
Not necessarily. The letter S, for instance, or a swastika shape, is radially symmetric, but not bilaterally symmetric.
No. Those have two- and four-fold rotational symmetry, they are not radially symmetrical.
Rotational symmetry has axes of symmetry, radial symmetry has planes of symmetry.
I seem to recall something Carl Sagan wrote in one of his earlier books, about humans populating a distant planet, and discovering copious amounts of plant life and animals that could be used for food. He mused that the humans could be disappointed when they find that on that planet the DNA strands were twisted in the opposite direction from ours, and were thus indigestible, to great disappointment. (Frankly I don’t know if that’s a real thing; is it?)
Yes, chirality in biochemistry is a real thing. And enantiomers can have very different biological effects. I don’t know the specifics of DNA chirality well enough to know if e.g only the helix could be opposite but the amino acids all still can have the same chirality as ours, in which case it would still be perfectly digestible.
But we don’t generally get most of our nutrient intake from DNA. It’s the chirality of the amino acids themselves in proteins, as well as that of sugars, that would have more of an effect. And yes, mirrored aminos and sugars would fuck us over, bigtime.
The Honey Badger subspecies signata is bilaterally symmetrical in other respects but has 2 molars on its left lower jaw, and only 1 on the right.
Too late to edit, but the wiki on Mirror Life is a good start to the concept:
As far as I’m aware, we don’t know the chirality of the amino acids we’ve detected in space, but we have detected at least one chiral organic molecule there, so chirality may be universal and Sagan’s scenario may not be likely. Or for every planet, it may be 50/50 -the Murchison meteorite contains a mix of amino chiralities .I think the jury’s still out on it.
Chirality in DNA wouldn’t render food indigestible, would it?
Not if it’s just the helix that’s chiral, no. But if the sugars that are in it are mirrored, there’s problems, as far as I’m aware. But like I said, the digestibility of DNA itself isn’t a major concern, it’s hardly a major nutrient for humans (prurient jokes aside)
But like I said, I’m not sure how mirrored DNA codes to peptide chains, if it requires mirrored aminos, or what the story is there.
I understand it’s proteins that are the problem.
Life on earth uses L-amino acids and D-sugars, with the chirality of DNA deriving from the sugars in the backbone of the spiral. If we tried to eat mirror life, as @MrDibble said the digestibility of proteins is certainly an issue, the direct digestibility of DNA probably is not, I’m not sure about sugars themselves.
But the underlying question is whether partially-mirrored life is possible, where DNA & sugars are mirrored but proteins are not. The relationship appears to be arbitrary in life. There is no obvious reason that different translation machinery (enzymes and transfer RNA) could not exist to translate mirrored DNA in non-mirrored protein or vice versa. There’s nothing obvious that forbids life with L-amino acids and L-sugars.
But there may be a deeper relationship that ties L-amino acids to D-sugars.
There are some wild theories about the origin of homochirality on earth. It may not have been a chance event, and it may have arisen before life evolved. Amino acid precursors with enantiomeric excess (ee) may have been delivered on asteroids and comets, with the ee subsequently amplified on Earth abiotically. The extraterrestrial ee may have derived from circularly polarized light.
If these theories about the origin of homochirality on Earth are correct, there may be deterministic prebiotic mechanisms that relate L-amino acid chirality to D-sugar chirality, as described here: