(Okay, I should not start thinking about interesting subjects on a lazy Sunday morning. This post went waaay long so I'll just put my conclusions at the start as a summary. You can read the rest if you like. I'll have to proof read after letting it set for a time so sorry for the typos.)
Kinetic projectiles are not favored evolutionarily because:
- They require a lot of intermediate forms that in turn require a specific sequence of environmental conditions. Existing forms place developmental constraints on exact forms the projectile system can take.
- When the individual runs out of projectiles, it's out of defense. It's like the military axiom that logistics trumps tactics because a gun is only a weapon if it has ammunition. An animal could invest a great deal of energy into a projectile weapon just to find itself defenseless or unable to hunt. (Note that carrying around a bunch of ammo also requires energy and imposes tradeoffs of mobility and increased likelihood of accidental injury. Just ask a modern soldier humping half an ammo dump around everyday.)
- Muscle powered projectiles are not very effective. Human hunters using muscle power and natural materials like hardened wood, bone and even flint, wound animals many more times than they kill them outright. Most hunter-gathers who use kinetic projectiles wound an animal and then run the bleeding animal into the ground (humans can run farther than any other animal on earth.) The instant kill, the goal of all hunters, requires relatively high technology, usually metal, a great deal of skill and a dose of luck. It is unlikely an evolved system can duplicate that.
- As species (A) evolves a projectile offense or defense, that creates selection pressures on species (B-Z) to evolve a counter. It's the "Red Queens" arm's race. Even if we image a plausible scenario for the evolution of a kinetic projectile, we would also have to image the countermeasure. At every intermediate form of the evolution of a projectile, a moderately effective counter-measure could kill the advantage of the proto-projectile system and eliminate the selection pressure that would have produced the projectile.
- Kinetic projectiles would be very expensive to create and use. The projectiles are literally thrown away so all the energy that goes into their creation is lost. Every miss is a huge waste of resources for the animal. The payoff for projectiles would have to be enormous.
I think (5) is the real deal breaker. A lot of organisms throw a lot of things at prey or predators but it's always something lightweight and innately disposable, e.g saliva, venom, musk (skunks), vomit (vultures), feces (monkeys, vultures and way to many others.) In none of these cases is the primary modality of the "projectile" its kinetic energy. Instead these are all chemical payloads that irritate, poison, entangle, or chemically burn (vulture vomit) and which were already something the organism benefits from getting rid of eventually.
Let's walk through several scenarios in the evolution of a plausible projectile systems for organisms. For brevity, let's just concentrate on issue (1): intermediate forms.
Understanding intermediate forms is absolutely critical to understanding evolution, especially the evolution of specific attributes.
We must always remember that all species have specific forms right here, right now because the environment right here, right now shapes them so. Evolution has no goals, no progress, no direction, no conception of the future.
When looking at the evolution of any phenotype, we have to evaluate each step without hindsight. Any phenotype we see in hindsight as leading to the phenotype was itself a fully functional (or side effect of a fully functional) attribute of the species at the time it first appeared.
Remember, this not engineering. Species don't start with an end goal in mind and then slowly evolve solutions to that goal. Instead, environmental forces shape organism from moment to moment constantly. Species don't develop attributes because they will need them later.
Darwin noted, "nature does not make leaps" meaning that the development of a new attribute requires an accumulation of smaller change termed "intermediate forms" e.g. birds just didn't one day suddenly spout wings. Feathers started as near microscopic fuzz used for insulation, and only slowly grew large enough for for other functions. Wings started out as grasping limbs that combined with feathers, broke falls. Controlling falls led to gliding. Evolving muscular and skeletal structure to add power to gliding led to flying. Each of those changes which in hindsight led to feathered flying birds, was an intermediate form.
So, to understand why something did or did not evolve, we need to understand what intermediate forms would be required, which in return would require understanding environmental pressures that would create that intermediate form.
(There are also developmental constraints. When the environment shapes a particular form that precludes others e.g. if the environment creates a heavy, armored creature like a turtle, that precludes the development of flight anytime soon because the advantages of mass and density are diametrically opposed the needs of flight; low-mass and low density.)
We want to understand why no animal has evolved a primarily kinetic projectile weapon that shoots a dense, possibly sharp, projectile with enough momentum and accuracy to use practically for defense and hunting. Let's consider solely kinetic energy transfer weapons. Technological examples would be slung stones, arrows and bullets (but not poison darts, fire arrows or shells with explosive, chemical or biological payloads.)
Such a system would require several subsystems, each a fully functional intermediate form, each of which would have to exist before the projectile system could be useful:
- Target acquisition and aiming: The system must be able to identify targets, assess its motions and likely future positions, then translate all that to system specific information that will let it launch the projectile so that it will strike in a beneficial fashion.
- Energizing the projectile: The system has to add energy to the projectile to make it move. It could do so with muscles, air, fluid, chemical reaction or even (lamely) gravity.
- A shaped projectile: The point of the projectile in this system is to transmit kinetic energy destructively to the target. To do so, it must be some combination of sharp (wedged shaped), dense, and rigid.
- Predictable or guided flight: The projectile has to strike the target in order to transmit its energy but it is by definition severed from the creature that launches it. Therefore the projectile must either have predictable flight characteristics, like an arrow or a bullet, or it must be guided, like a seeking missile. We'll stick with predictable flight.
Each of these sub-systems must evolve as an intermediate form.
Target acquisition and aiming: Target acquisition would seem to have an obvious intermediate form of any animal's senses and neurological systems, which identify either predators or prey and then calculate how to move the body in order to evade or intercept the target. But shooting a projectile is an entirely different calculation problem than moving the entire body. There would have to be an environmental pressure that would shape the system to predict were the target will be in the future but that the organism itself won't be.
The easiest intermediate form that meets that criteria would be long limb, lets say a tail. Tails naturally evolve as clubs. The club starts out accidentally as just a tail striking a predator or prey in course of a contact fight. Environmental pressures to make all energy expenditures pay the highest dividend would lead to the evolution of a tail club that could strike intentionally and accurately. Some extant lizards use their tails quite accurately as whips, so this is plausible.
The targeting system begins as a means of directing the end of the tail to strike the target.
Energizing the projectile: If we start with a whip like tail, then the rapid motion of the tail can accelerate the projectile. Combined with the existing targeting for the tip of the tail, anything that flew off the end of the tail would automatically fly in the general direction of the target.
A shaped projectile: Living systems are governed by their energy budgets. As dynamic systems, they exist in the flow of energy through them. Everything they do, even just laying there, requires energy flow. Growing a disposable body part is a non-trivial expense even though animals do it all the time e.g. shed skin, hair, feathers, claws etc. You can see how important this is by the way that herbivore mammals eat the after-birth or many predatory species eat children that die or that they can't feed.
Forming a projectile will be a high energy endeavor because the projectile needs a specific combination of mass and form. Creating biological mass obviously takes energy but complex form also takes a lot of energy to create to the form's design. All that invested energy will then be thrown away.
It's easy to see how animals evolve whips and clubs on tails because the energy invested in making the weapon remains in the body - but a projectile requires a considerable energy investment that will be literally thrown away.
Fortunately we have previously existing systems to build on. Animal bodies are defined by the membranes that separate the inside from the outside. The outside facing layers of most animals (and all land and air animals) are dead tissues and sacrificial; e.g. skin cells, hair, scales, feathers, teeth, claws, shells etc. Our projectile can start out as dead sacrificial tissue; e.g. skin cells and teeth or extruded material; hair, claws, scales feathers, etc.
The easiest would be some kind of hard spike. If we start with the whip tail lizard, we could imaging scales on the tail tip evolving sharp edges to abrade the target. There would be pressure to make them cut more, so the scales would grow larger and larger eventually looking like sharp edged segments. The scale at the tip would be the most energized and have the most striking surface so it be the largest and the need to transmit energy destructively in the most efficient means possible would shape into a wedge or blade like form.
The lizard would have a series of large scales growing at the tip, when one was ripped off or destroyed, another would grow out in its place, Even being smaller and not at the tip, they would still form a cutting edge. Eventually, it would be shaped into something all dead scale like a rattlesnake's rattle.
Up to this point we have a barbed whip with a disposable and replaceable barb. It works by accelerating a wedge shaped cutting scale to a high velocity so that the wedge shape (the sharp edge) concentrates all the barb's kinetic energy into a tiny area on the target so they it destroys the tissue at that point. This barbed tail works as a final form itself to accurately strike a target and injure it.
Guidance: It's easy to see how a barb-scale could accidentally become a projectile. It's already evolved to be detachable and disposable so at the point of when the whip tail accelerates it to a high velocity and then snaps back, the barb-scale would experience significant stress at its attachment point. If the tail cracked back before actually hitting the target, the barb-scale could just pop off and go flying in the general direction of the target.
However, it would unlikely to be at this point an effective projectile.
- The barb-scale would have little mass. Being abradable and disposable, it would be fairly lightweight to conserve energy investment. Being attached to the tail, the mass of the tail would store the kinetic energy to be transmitted - not the barb-scale. When it detached it would rapidly loose energy by air resistance.
- An anchored cutting edge is not a stabilized flight form. Consider how difficult it is to throw a knife. A knife is optimized to transmit energy while held in a hand, not flying through the air. A thrown knife takes the the form of dart or javelin but a flying form makes for poor cutting or stabbing form accelerated along the curving path of an arch.
To evolve as an effective low-mass projectile, the barb-scale would need to evolve a dart or javelin shape while still being used as an attached weapon. The only reason to take that form would be if the barb-scale worked best as a puncturing weapon instead of a slashing one. But, that is a problem because the best form for an attacking puncturing weapon is a curve that follows the arch of travel of the limb that drives it; e.g. claws, fangs or pick-type metal weapons (for puncturing armor.) Any puncturing barb-scale would end up looking like a scorpion's stinger - which would have terrible flight characteristics.
There just isn't an intermediate form that works well as an attached cutting or stabbing form that also flies in a predictable manner. Once a creature had evolved an efficient cutting/stabbing whip tail, it would be precluded from easily evolving a projectile.
Surprise! Dead end.
So, we need to find another projectile. Kinetic energy is about momentum. Momentum is velocity times mass. High velocity with low mass gives the same momentum as low-velocity and high mass. In addition, a sharp projectile uses the wedge shape to concentrate the kinetic energy of its momentum in a small area, making it more destructive. That in turns requires controlled flight to ensure that the edge of the wedge strikes first.
But, if you go with high mass and low velocity, it becomes simpler. Enough mass hitting will transmit the energy regardless how it strikes. The obvious solution, with evolutionary precedent, is a massive ball on the end of the tail that works like a mace, a crude bludgeon. It could evolve from a scale built of very dense and close packed proteins (most scales have air pockets for insulation and weight saving, that's how feathers got started.) There is evolution precedence for such a system in Ankylosaurus and Doedicurus.
If the mace tip is detachable, it could fly off target. All the systems elaborated for the barb-scale would still apply. It would be like fighting by throwing bowling balls.
But, there are intermediate form problem.
Even if evolved from a disposable scale, the dense heavy bludgeon would require a strong attachment point lest it fall off while being accelerated. If it does, it's probably not even directed at the target but flies off on a tangent like an axehead that pops off during the swing. It might even strike the animal itself.
The strong attachment point precludes the easy evolution of the mass from bludgeon to projectile.
Hmmmm, but lets not give up.
The most effective bludgeon is a dead blow hammer. A dead blow uses impulse to transmit a great deal of force in a short time. It does so by having a movable element inside the hammer and space for it to move. Usually, this is lead weights in oil and an air pocket. In a normal hammer, the actual transmission of force is not instant but follows a somewhat bell shaped curve. This occurs because a real hammer is not an ideal rigid body and it flexes and compresses. Plus, the momentum of the back of the hammer takes time to transmit against the opposition force of object struck.
In a dead blow hammer, the mass of the free-weight inside flies forward to hit the front of the hammer in a much shorter time. More importantly, the equal and opposite force from the struck object is transmitted along the exterior container more than to the free-weight. The result is a peak shaped force transmission curve. Since it is usually the peak force that does the work/damage, a dead blow hammer is more effective even with the same overall momentum. (Impact wrenches, impact hammers and impact drills work on the same principle.)
So, lets imagine the evolution of a dead blow tail bludgeon. It would be easy to form if the bludgeon was made of layers of scales instead a single giant one. At first it would be like a clacker-rattle with only a slightly higher peak force but if it paid off, it would eventually form fairly large free-weights inside a relatively thin "hammer" shell. The real world precedent is a rattlesnake's rattle which is a thin shell of merged scales with a ball shaped detached scale inside. In a weapon, there would likely be several detached elements inside.
Now we have a dead blow mace weapon. It would more effective in transmitting impact than just a solid mass. One problem it would have, however, is that that the detached elements would be abraded to dust inside the airspace. The solution would be to evolve an opening in the outer shell to allow the particles to escape. The most effective point for shaking out the dust would at the tip in line with the maximal point of acceleration when the tail was whipped at a target and then yanked back. Waste not, want not - so the free-weight scales would evolve to disintegrate into jagged small units that could serve as an irritating dust.
Likely, the hole would evolve to be just under the size of the free-weight scales. Free-weights work best when symmetrical so if one broke or got worn small, best to get rid of it. Now the tail is not just a dead-blow bludgeon but also scatters irritating dust and shards. In some circumstances, like fending off small pack predators; throwing irritants over an area might be more effective that trying to smack one of the fast moving little buggers.
Now the environment might favor tossing the free-weights out the hole, instead of using the bludgeon. When throwing something lightweight, launching velocity becomes more important so the squat heavy bludgeon-tail begins to evolve into a thin, long fast moving whip with a hollow end full of dust and shards. As the tail began to accelerate more and more, the heavier shards would begin to store useful amounts of kinetic energy and become projectiles. No longer facing abrasion by smacking inside the bludgeon shell, they could become strong and dense.
As extrusions, scales naturally form an ellipsoid shape. Rattlesnake rattles start off as an irregular ellipsoid that gets worn into a sphere. But once its primary function becomes flying as a projectile, the projectile would work more effectively as an ellipsoid. Eventually the projectile scale would elongate into a more of a cylinder form. Anything on the back of the projectile-scale that produced drag, e.g. bumps, breakage, air pockets or strings of scale or other tissue, would cause it to orient back to front. This would create the intermediate form necessary to evolve a pointed tip.
So, we end up with a critter with long muscular tail, with a hollow rattle like structure at the end. Inside, its "barrel" might resemble a hi-li racket (jai alai cesta). It would flick dense ellipsoid or even dart shaped projectiles at high speed fairly accurately. The projectiles would be relatively small and grown at a lower energy cost.
But still, the core problems remain. Run out of projectiles, and the weapon is useless.
It's nearly impossible to digest scale, hair or other disposable extrusions so it's unlikely a projectile throwing beast could ever recover the energy invested in making the projectile by eating it, assuming it could find it. All the energy that goes into the the projectile is always lost whether the projectile produces a desirable effect or not.
To sum this up. Imagine if humans made arrow tips solely from a giant tooth we grew. How fast could we produce them? It would take weeks to grow an arrowhead sized tooth. Every shot would have a significant risk of breakage. In the time it took us to evolve the capacity to grow the arrowhead tooth, our prey would be evolving counter measures e.g. they can grow teeth or teeth like structures too. Just as metal can make swords or armor, teeth could be weapons or armor. What else could we do with all the energy we put into arrowhead teeth?
If you look at the arms race in human technology, muscle powered projectile weapons, it's clear that they were not a dominating weapon. It was not until paired with chemical energy that they became dominate and even that might be temporary. The return of body armor is forcing a redesign of projectile weapons back to heavier forms. The ability to intercept even small projectiles in flight will lead to other changes.
I think it's safe to say that biological kinetic projectiles would be hard to evolve and have little net payoff.
I do wonder if the lizard tail might evolve to scavenge rocks to use
for ammunition as the next step though?
For a rock thrower, you have figure out intermediate forms that don't involve actually throwing the rocks as weapons. You have to figure out a reason to move the rocks just bit and then another to move them some more and so on to reach a high velocity movement.
It might start by moving rocks accidentally with a whip tail which leads to some intentional movement to build a nest or uncover food. Optimization of moving rocks leads to a horizontal, blade like tail perhaps with a scoop at the end. It would still be used as a whip. At some point the use as a whip would accidentally overlap with moving rocks and a weapon would be born, assuming the coincidence happened often enough a selection pressure for using the rocks intentionally would build which would lead to the evolution of targeting systems.
But rock ammo would be very limiting unless the entire species lived its entire evolutionary span in an area where there were abundant loose stones. I think that unlikely. It might evolve to carry stones but as noted above, humping ammo has a huge cost for something that might not ever be used, or used effectively, and rocks are heavy.
I think projectile weapons require too much specialization, too much expenditure for too little return. If you look at most animal weapons systems such as claws, you see that they perform several non-weapon task as well. Claws, for example, are used for traction, grooming and marking in addition to being weapons. Those additional functions helped claws evolve in the first place (probably traction first) and they help defer the energy cost of the weapon aspect. I have a hard time coming up with any plausible secondary functions that would defer the cost of a projectile weapon system.