This post began turning out longer than I intended, so I’m going to divide it in two, with the second part up tomorrow.
When we consider major organs or features of animals, they can be bilaterally symmetrical, with the traits the same on both sides, or bilaterally asymmetrical, with differences between left and right. And there are two major forms of bilateral asymmetry.
In antisymmetry (which can be considered a macroscopic form of “fluctuating asymmetry” see here), there is no directionality to the trait, so the asymmetry is random with respect to the side of the body. One example of this is the lobster claws, in which one becomes a “crusher” claw and the other a “cutter”, as in this individual.
Now this asymmetry is adaptive in the sense that it’s useful for a lobster to have one claw that can crush and another that can cut; it’s like a crustacean Swiss Army knife that can do multiple things. But if you look at lobsters, you’ll find that the crusher claw is on the right as often as it is on the left; in other words, the asymmetry is random in direction among individuals.
This is still an evolved trait, as it’s clearly adaptive to have the two functions, but it doesn’t really matter to the lobster which side does which.
We know how this asymmetry develops—at least proximally. What happens is that the claw that is used most often after the fourth molt develops into the crusher claw, and the other one into the cutter. (I recommend having a look at the link, which details some clever experiments.) This means that there is some developmental program in the lobster’s genes that turns on “crusher” genes in the most stimulated claw, and that, in turn, may activate genes (or repress genes) on the other side of the body leading to the development of the cutting claw. In this case the environment itself, or rather the behavior of the animal interacting with the environment, activates the genetic program, and since it’s apparently random which claw is most stimulated, we get half the lobsters with a crusher on the left, and half with the crusher on the right.
Here’s another example of antisymmetry, the big vs. small claws of the male fiddler crab, Uca deichmanni (the females’ claws are the same size). There are equal proportions of right-clawed and left-clawed males:
Another type of asymmetry is directional, that is, the left and right sides differ, but always in the same direction. We’re familiar with this in our own bodies, in which the heart and viscera are directionally asymmetrical. The bulk of the heart, for instances is on the left side of the body, which is why you feel your heartbeat on that side. Quirks of Human Anatomy gives more examples:
Our right lung has three lobes but our left lung only two. Our heart is shifted to left, our spleen is located on the left, and our stomach bulges to the left, whereas our liver is shifted to the right. Our colon curls into a question mark, although its exact path can vary from person to person.
There are rare individuals in which every directionality like this is reversed due to a condition called situs inversus; these individuals are usually normal, but their innards are mirror images of the much more frequent “normal” individuals.
These kinds of directional asymmetries are not infrequent. Male narwhals, for instance, have a grossly enlarged canine tooth (up to 3 meters long) that forms a tusk, and it’s always on the left side, as shown in this photograph below. (Females don’t usually have tusks, which might imply sexual selection via male-male competition, but it looks as if the tooth/tusk is a sensory organ that males, use to communicate with each other when they rub tusks.)
Another example of directionality are some nocturnal owls in which the ear openings are asymmetrical; this helps them localize prey. In the barn owl, the left ear opening is higher than the right. Here’s another owl, the boreal owl, showing directional asymmetry in the skull:
Flounder species show both forms of asymmetry. In some species of flounders, which begin swimming vertically, they subsequently flatten so that they lie on their left side, with the left eye migrating over the head to the right side, while other species lie on their right side with the eye migrating the other way. These species are directionally asymmetrical, but in opposite ways. Still other species of flounders also flatten, but in a random direction, so some individuals lie on their left sides, and others on their right; this, of course, is antisymmetry.
Like antisymmetry, directional symmetry is often adaptive in that it’s useful to have only one side enlarged, and if you have to enlarge a tooth to make a tusk, it’s got to lead to asymmetry. Here, however, we face a genetic problem: the induction of the tooth on a given side is not due to random environmental stimuli, but is somehow to the genes themselves. There must be a genetic program in narwhals, for example, that says “make left tooth grow large,” regardless of the environment. And that means this: those genes know whether they’re on the right or left side of the body! It’s easier to envision genes knowing whether they’re in the front or back half of the body, as an anterior-posterior gradient is set up in the egg or early zygote. But such gradients aren’t obvious for the right versus left sides of the body in animals that are, by and large, bilaterally symmetrical.
Let me add first that while it may be important to be asymmetrical, as with the lobster, there probably aren’t many cases in which directional asymmetry is more important than antisymmetry (can you think of examples?). In these cases which form of asymmetry evolves may just be a result of whether the genetic variation promoting asymmetry is of the antisymmetric or the directional sort.
When I was younger I pondered this question at length. Yes, you can determine front and back in the egg, and then top vs. bottom (dorsal versus ventral), but, unless there’s some directional left-right gradient set up in the egg (and I wasn’t sure how that would work), I couldn’t see how a gene would know, from its internal environment, which side of the body it was on. (Draw a box with a front-back and a top-bottom chemical gradient; you’ll see that the concentrations of the “morphogen” chemicals are the same on the left and the right.) How, then, I wondered, could directional asymmetry, which must involve genes taking cues from their local environments, ever evolve?
Well, if we start with a single trait being directionally asymmetrical, that would be all that is required for subsequent traits to cue on that, or on each other, to themselves evolve directional asymmetries. And even organisms that look pretty bilaterally symmetrical, like Drosophila, can have subtle directional asymmetry (flies have asymmetrical guts and the male genitalia rotate in a given direction during development.)
But that still leaves a problem: Assuming that organisms evolved from a common ancestor that was completely bilaterally symmetrical (right vs left), how did the very first directional asymmetry evolve? With gradients the same on both sides of the organisms, how could gene variants accumulate that would be activated (or silenced) on a consistent side of the body?
I’ll leave this for readers to ponder. If you’re a biologist, you may already know some of the answers. I’ll discuss some solutions (and some selection experiments) in the next installment.
For more on directional versus antisymmetry, go to Rich Palmer’s website at the University of Alberta.
I’m not a biologist (nor do I play one on TV) but I’ve often wondered about how asymmetry develops. This may go back to my experience being a left-handed child in a right-handed-dominated world. Is the development of handedness in humans at all relevant to the question?
Yes, it is, though handedness is sort of fluctuating, in that individuals vary. However, right-handedness is more frequent There is some genetic basis for right versus left-handedness, but it’s complex.
I was wondering if lobsters have a left/right hand and if that would shed any light on the crusher/cutter asymmetry. Since it indicated the split was 50/50, I figure it isn’t related to a human’s handedness.
If you’re still curious about it after Prof. Ceiling Cat’s posts, there’s a wonderful book, ‘Right Hand, Left Hand’ (2003) by Chris McManus. It goes deep into the origins of asymmetry in evolution and development.
Great post. The crusher experiments are very interesting.
Thanks Jerry. I don’t think about these issue very often and this post was very enlightening. One of my favorite examples of handedness (which is a bit different from what is covered here) is the right versus left handed scale-eating cichlids. I think crossbills (conifer seed eating birds) may also have righties and lefties.
Yes, I was going to look up the crossbills; I can’t remember what form of asymmetry they have.
I forgot about those cichlids! I learned about them in George Barlows very interesting book The Cichlid Fishes: Nature’s Grand Experiment In Evolution .
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The cichlids are an interesting example, and is one I will be describe in my evolution class later this week, as it happens.
Interesting post, I look forward to part 2.
My cousin has Situs inversus and as far as I’m aware hasn’t had any issues.
btw, I think you meant symmetrical here:
“But such gradients aren’t obvious for the right versus left sides of the body in animals that are, by and large, bilaterally asymmetrical.”
Yes, you’re right. I’ll fix it, thanks!
I’m sure her heart’s in the right place.
But seriously, in the old days when they used to take chest x-rays and develop them on plates, the tech would have to mark the left and right side of the x-ray so the reading radiologist when seeing a patient with S.I., would know that the film was not placed backwards(which always amuses me when they hang the x-ray up on tv and invariably the film is backwards and no one notices)
I carry a size scale (CM and inches – not because the inches are useful except to make it clear that the other measure is cm (and hence mm for the finer scale). That scale has writing on it, which means that handedness of the image is also defined within the image.
I thought that radioiogists did the same with something X-opaque.
I’m curious to know if a cardiovascular surgeon would find it difficult to operate on someone with Situs inversus.
Not sure, maybe just stand on the other side of the table 🙂
What would concern me more is if the layout of – for example – the aorta, the recurrent vagus nerve and the clavicular arteries was entangled in an unusual way.
Is it to do with slightly different amounts of matter and anti-matter in the early universe?
OK, I admit I’m guessing.
Fascinating. The gradient problem is intriguing.
Asymmetries are everywhere in nature. Light-dark, water-air, hot-cold, acidic-basic. These would lead naturally driven, to long term morphological changes to cells and to multicellular organism.
Asymmetries can draw out the symmetry breaking, but what helps developing symmetry, i.e., right and left hands, after the original symmetry is broken can also come from physical laws. Fluid dynamics is a great example. Anyone who has ever swum or paddled a boat knows it is hard, if not impossible, and certainly energetically unfavorable to have jut one paddle on the side of a torso.
Swimmers partitions the strokes into short axis (like fly) fully symmetric along the belt line. In these cases, propulsion needs only one fin (like a dolphin). The long axis strokes, require handedness along the length of the body.
Since you mentioned fluid dynamics – assymmetry seems to develop naturally as a consequence of fluid flow in some instances. I’m thinking of a kayak, where the flow past the back of it tends to sweep across from one side to the other – either way is possible, but once the flow is set up it’s hard to switch. I found this when paddling a kayak – the flow at the rear tends to make it turn one way, and once it starts to turn that way it takes quite a lot of effort to ‘break out’ of it. Paddling a kayak in a straight line takes quite a lot of skill and concentration, you can’t just give alternate strokes of the paddle each side and expect it to go straight.
I believe at least one model of airliner used to experience this – it would fly through the air ‘crabbing’ slightly.
And I *think* the little finlets along the back of tuna, which are alternately angled in opposite directions, are ‘spoilers’ to prevent this.
Plus in humans there is also the issue of the ‘dominant eye’. I don’t know if this is genetic or some quirk of the developmental environment but it can affect some skills. As I found out learning to shoot (clay pigeons with a shotgun firing steel shot) – being mostly right handed with a dominant left eye meant I wasn’t shooting quite where I was looking.
Yes, in the rare case of being right-handed and left-eyed, the solution is a specially-made “crossover” stock, which lines up the barrels with the left eye.
In my case (it was only a ‘team building exercise’) the solution was an eyepatch over the left eye. Pirate clay pigeon shooting.
Always dangerous to get nit-picky about things outside my expertise (if any…), but I’m pretty sure that fluctuating asymmetry is used to refer to small random deviations from normal symmetry. Measurements of FA are commonly used as a proxy for stress and/or ability to resist stress during development.
The type seen in the left-right crabs is usually referred to as antisymmetry. The Palmer web page backs me up on these.
Yes, that’s true. But it’s a continuum more or less, and cases of “fluctuating asymmetry” have also been referred elsewhere as cases of “antisymmetry”. When a small deviation becomes large is a subjective matter.
UPDATE: I’ve changed my terminology in the post to conform to Palmer’s; after all, he is the expert in this area.
What I think makes the distinction worth keeping is that most studies of FA (usually implicitly) assume that the asymmetries are not adaptive, while the large-scale antisymmetries are at least potentially adaptive.
There are rare individuals in which every directionality like this is reversed due to a condition called situs inversus; these individuals are usually normal, but their innards are mirror images of the much more frequent “normal” individuals.
This would seem a more appropriate use of prefixes like ‘cis’ and ‘trans’ than the use to which they are currently put.
…and we are left in suspense; tune in next week for the conclusion of the cliff hanger. Now a word from our sponsor…
Another interesting asymmetry is in echo locating cetaceans. The baleen whales have two dorsal blowholes, but the toothed, echolocating whales have only one, usually the left. The right side develops into their echolocating apparatus. The old whalers could differentiate the sperm whales by the left leaning spray when they blew at the surface.
Interesting…thanks for the added tidbit.
Very interesting. I think I know a bit about how directional asymmetry emerges during early embryonic development in vertebrates, but I won’t say here. Looking forward to the rest.
In cases such as narwahl tusks, if the tusk is broken off or damaged, would the other side then grow? Would that depend on when during it’s growth the damage occurs? Is there are a point when the adjustment “door” closes?
I really hope someone answers that – it’s a fascinating question.
I guess I stumped ’em? 😉
This sounds great – I love this stuff
No intelligent comments to make; and, needless to say, no answers to your questions; but please keep this sort of stuff coming. Looking forward, after the break, to Part 2!
I’m not up to intelligent comments on this, but I loved the post too, and wanted to say so.
And I love it when I actually understand a whole biology post! (There are usually bits that leave me baffled.)
Thanks Jerry.
As a chemist I always wondered at the asymmetry of organic molecules. Amino acids universally have L-configuration at the 2-carbon. It seems the orientation was fixed before the origin of life.
I don’t know about that. How do you explain the side of the road different countries have configured driving norms on their roads? LOL
Pretty obvious really. We drive on the right side of the road and the Brits drive on the wrong side.
Its’s an interesting distraction. Allegedly we started riding on the left to leave our sword arm free, but then we drive boats on the right.
In the UK most people are right handed and so by driving on the left we can continue to steer with our dominant hand while the left hand is free to change gear (stick shift I believe you call it) or apply the hand brake. Which perhaps is another reason why most American cars are automatic or had the gear change on the steering wheel,and (I’ve read) Americans tend not to set the parking brake.
Most standard transmission cars in the US have been a stick on the floor. There were indeed some that had the shifter on the steering column (“3 on the tree” configuration, a 3 speed standard transmission and “the tree” is the steering column, is one I’ve driven), but that was definitely not the norm.
Umm, no, after all European countries all drive on the right and their cars are mostly manual. And I can say, having driven all my life on the left side sitting on the right with a left-hand gearchange, driving on the right (in France) sitting on the left with a right-hand gearchange comes quite naturally.
What causes more trouble is the indicator (flasher) lever. Traditionally cars had it on the opposite side from the gear lever. That is, older British cars had the ‘indicator’ stalk on the right (as do Japanese cars still, half the cars in NZ are Jappers). European cars had the stalk on the left, and British cars swapped over about 1970 probably for conformity of manufacturing. Then when the windscreen wipers acquired a stalk it was on the opposite side from the indicator lever. So now you have a stalk each side.
And if you own (as we do) one Japper, one German and one British car, (all of course right-hand drive), confusion reigns mightily when hopping from one into another.
cr
As another chemist (at least way back when), I don’t have any explanation for the initial predominance of L-amino acids; but I think that the continuing predominance may be explained by “seeding” – once you have a handedness that helps a reaction/crystallization/whatever, that handedness will gradually take over in a sort of self-catalysis.
I wondered whether chemical-level chirality was related to the problem discussed on this page.
From what little I know one always has to have something else asymmetrical to favour another asymmetry: so ultimately some electron spun up rather than down (by chance, or the bohemian equivalent) and that got it all started.
—
However: I have a biological question too – what in these animals and so on “remembers” the influence from the environment? Is it like metal memory, or is there a nervous system or immune system like matter involved?
That’s *Bohmian*, not bohemian. Sheesh. 😉
One pathway that seems obvious to me could be demonstrated with a simple handshake.
Whether one shakes hands with the left or the right is fundamentally arbitrary. But once you’ve got some sort of protocol established, the handedness becomes self-sustaining.
For a more obvious but less biological example, consider traffic flows. Left- or right-hand drive is arbitrary, but it’s for damned sure that you’ve got to be on the same side as everybody else. Indeed, simply being a passenger in a taxi in either London or Tokyo scares the shit out of me!
Jerry, you mentioned that narwhals interact with their horns. That right there is more than enough to establish a side preference. I’ll go out on a limb and suggest that there’s no comparable interaction between lobsters and crabs.
As for the flounders, I’d expect the location of the genitalia to have something to do with which ones don’t care about which side is up and those that do. At least, I’d expect it enough to bet a cup of coffee on it, but maybe not an entire meal….
Cheers,
b&
As I recall, the right-hand handshake is said to have developed because, most people being right-handed, it meant that you were shaking hands with your weapon-holding hand and therefore displaying trust by not being able to hold a weapon. The Boy Scouts used to, maybe still do – it’s a long time since I was a Boy Scout, shake hands left-handed, showing greater trust (I think this was a Baden-Powell invention).
As for traffic, think of the fun it must have been in Sweden when they converted from driving on the left to driving on the right in 1967 (http://rarehistoricalphotos.com/dagen-h-sweden-1967/).
Or in Okinawa, Japan in 1972 when they did it overnight. Only this was right to left.
I’ve enjoyed watching fiddler crabs for years and never noticed that they exhibit fluctuating asymmetry.
Interesting post. I look forward to part II.
There is also a recent article on asymmetrical biology in Quanta Magazine.
https://www.quantamagazine.org/20170131-how-life-turns-asymmetric/
Yes, that’s the impetus for my piece, though I won’t talk much about the Quanta article, which is very good.
As a general problem to solve, if both up & down and front & back are already known then left & right can be “calculated” directly from that. Could a mechanism evolve that allows a particular genetic program to “figure out” what left and right is based on both the up-down gradient and the front-back gradient together? It is trivial to do that sort of thing with, for example, a simple digital circuit. It seems like biological systems could do it too.
You’ll have to explain that further because I don’t see how it can be true. Mathematically, left-right is orthogonal to both up-down and front-back.
If you only know one then yes, but if you know both at the same time you have what you need to know to distinguish one side from the other as well.
No you don’t unless you assume some sort of symmetry which as the article suggests isn’t always present.
I am surprised that I missed that in the article. Was I that sloppy reading it or did Jerry add that parenthetical in an edit?
Regardless, my mistake was a wrong model from the get go. I was mistakenly considering the problem to be a cell being able to distinguish it’s own left & right rather than left & right sides of the body it’s in.
Assuming that organisms evolved from a common ancestor that was completely bilaterally symmetrical (right vs left), how did the very first directional asymmetry evolve?
Is that a safe assumption?
Because, from what little I know about human anatomy, it seems that (perfect) bilateral symmetry is not the default, even when there is a strong adaptive advantage for it–as in the muscular-skeletal system. Many people have one leg that’s slightly longer than the other, or one foot that’s slightly larger. And whereas it might not matter, in terms of selection, on which side of your body an organ resides (as long as it works), having both sides of your body move the same way directly impacts how effective your locomotion is. And effective locomotion is HUGE for selection, right?
Now, body asymmetry may be more epigenetic than genetic (imagine a child playing baseball regularly from the age of five, throwing a ball with one arm, swinging a bat on one side of their body, running in the same counter-clockwise direction around the bases, etc.). But is it possible that body asymmetry is actually the default, and that symmetry is the evolved trait?
Yes, you’re right: we don’t know for sure that there were any genetically based directional asymmetry in the bilaterian ancestor. I’m assuming, as a Gedankenexperiment, that there weren’t, and even if that was the case one could think of ways that directional asymmetry could evolve. We’ll talk about that tomorrow.
Yes, that came immediately to my mind too.
How bilateral are the oldest eukaryotes? And the oldest ‘multicellulars’?
Or would bilaterality evolve so quickly once one starts moving that it would be undetectable in the fossil record?
And if the fossil record can’t answer, maybe genetics (and epigenetics 😶) can in future?
Forgive me if I’ve missed something but if a developing embryo has a distinct top and bottom and front and back then it seems straightforward for right and left to be established, if one uses nautical terms for them instead; port and starboard. Bear with me for a minute.
Those terms arose because the right or left side of ship depends on which way one is facing – so by having unambiguous terms for one side or the other, commands to the sailors won’t get so easily confused. The key is that the port and starboard terms only work with a priori knowledge of fore and aft; poor sailors don’t know the front of the boat from the back.
So if genetic programs are capable of distinguishing port from starboard (rather than right from left) it then does become possible (in my simple understanding) for a set of genes to “know” which side it is on. We already know that gene expression patterns “know” the front and back (and top and bottom) of a growing embryo. As mentioned this is often accomplished by biochemical gradients that control gene expression.
In the developing male Narwhal embryo the “make right tooth grow large” genetic program happens in the region of the developing jaw that is in the superior, anterior (topside, near the bow) part of the jaw. That much can be explained by mechanisms we already know about.
So perhaps once the fore/aft and topside/bottomside orientations of the embryo are established, then it seems to me that port and starboard are defined by default such that whatever gradient there is for the genetic programs – promoting the tooth growth on the starboard side and or suppressing it on the port side can act.
That’s my guess.
???
No, I don’t think so. Think about it. Top and bottom and fore and aft are established. And that may produce chemical/physiological gradients. But at every point on the right side of the embryo, there is the same concentration of “morphogens” on that same point on the left side, no matter where it is.
I see. I guess I was thinking that the same processes that established the fore/aft topside/bottom orientation of the embryo could work in the port/starboard scenario once the fore/aft orientation is established.
I look forward to tomorrow’s discussion!
I’d had the same idea as you, but I see what I missed. Individual cells could conceivably distinguish one side from the other with respect to themselves if they could already distinguish both up-down and front-back. But that alone wouldn’t enable them to distinguish which side of the body they are on. There would be matching pairs of values for the two gradients mirrored on either side of the body.
I would guess that mutations in the Hox genes have something to do with the evolution of asymmetry.
This post triggered a deep memory. When I was a graduate student, a visiting professor led a seminar that look at famous “dead ends” in embryology, things that looked exciting in the late 19th and early 20th century but that had been dropped for lack of progress. The idea was to ask whether they might then (about 1966) be ripe for a new look. For my presentation, I ended up plowing through a very long paper in German (or maybe several papers) on situs inversus. Frankly, I don’t remember our conclusion. For Jerry’s question, I’m going to guess the answer has to do with the fundamental “handedness” of many molecules important in biology.
I remember when the cause for situs inversus in Kartagener syndrom was discovered. It was exciting!
From my scientifically ignorant perspective, this is one of the most fascinating science pieces you’ve done on WEIT while I’ve been reading. I read through the whole thing without skipping over parts. And, I stopped to think about the lobsters, crabs, narwhals, flounders, et al. Makes me wish that I’d been born with a much greater aptitude for science. I’m looking forward to tomorrow.
Thanks Jerry. Looking forward to post #2
Hmmm….maybe in cases of sexual selection? If female fiddler crabs preferred males with large right claws over males with large left claws, for instance, then directional asymmetry would be much more important than antisymmetry. Obviously not the case for fiddler crabs, so my example is just hypothetical.
I have no idea. But a curling or spiral feature on the centerline would be ‘seen’ by molecules on one side of the body as curling in a clockwise direction, and by molecules on the other side of the body as curling in a counter-clockwise direction (likewise, a simple right-angle forward “L” shaped feature would be seen as a chiral left-L or right-L, depending on which side of the body you’re on). So I would say that it’s at least theoretically possible that directional asymmetry is triggered by some reference to an internal chiral structural feature near the centerline of the organism. Is that the way it actually occurs? I have no idea. But that’s my completely ignorant try.
Snail shell chirality may affect their ability to mate.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1182695/
I wonder if at the very bottom of the asymmetry we rely on the asymmetry of molecules.
A helix defines a chirality that can then be used to determine a definite left/right direction once given a front/back direction. It may not be the shape of DNA but of some other chiral molecules that “seeds” the whole directionality of the body.
This is a bit tangential to the topic, but I notice a common tendency here for people to use “symmetry” as a shorthand for bilateral symmetry. However, there is another kind of symmetry: radial (e.g. starfish). This symmetry seems less common – is there something inherently advantageous in bilateral vs radial symmetry? Also, (returning to the main topic), is there radial asymmetry?
I think that it is easier to move fast if you are bilaterally symmetrical. Radially symmetrical animals tend to be slow.
Streamlining breaks the front-to-back symmetry, but it’s gravity and buoyancy that break the top-to-bottom symmetry. With two symmetries broken, bilateral symmetry is all that’s left.
There is also the spherical symmetry (sort of) of Volvox and such, no?
Gene could know that he’s on the right side of the bus when the windows are right of him. He could have instruction to locate where somehing else is (detecting type of neighbour cells). It could involve 90 degree rotation, where the front and back are transposed to left and right (i.e. unequal directions if rotated are preserved in unequal sides).
Is this at all related to spiraling growth of plants, such as trees, whether they spiral in one direction rather than the other?
Here’s my guess as a non-biologist:
Once the plane of bilateral symmetry is established during development, some cell near that plane divides and epigenetically imprints the two daughter cells differently. The daughter cells move away from each other along contours of constant concentration in the front-back and up-down chemical gradients; this takes them in opposite directions along the left-right axis. They then go on to found lineages of cells with different patterns of gene activation on the left and right sides, without any need for a global left-right gradient.
I admit I’m not clear on how you get consistent directional asymmetry from this mechanism. Perhaps the process of migrating perpendicular to the two existing gradients has something to do with it. If we picture the two gradients as forming a kind of color wheel, a cell migrating to the left sees the wheel in (let’s say) a clockwise orientation with respect to its direction of motion, while a cell migrating to the right sees it in a counterclockwise orientation.
Fascinating post, speaking as a non-biologist. As an aside, as a former lobster creel fisherman (Homarus vulgaris and Nephrops norvegicus) I can confirm that the cutting claw on Homarus vulgaris can indeed move very quickly. Once closed, the only way to save your finger from serious injury is to break the claw off from the lobster’s body. This action is not popular with the skipper, as the resultant lobster (known as a “cripple” in the trade) is worth substantially less at market.
Perhaps something to do with timing? A limb bud on one side begins developing before the corresponding bud on the other side, chemical signals from that process are sensed by the other limb bud altering its development? Of course that just pushes the question back to what could cause a regular, non-random start time difference.