Study pinpoints when bow and arrow came to North America

Radiocarbon results suggest a single origin and rapid diffusion through cultural transition networks.

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[nartreb]

For anyone wondering, "North" in this case means north of 55 degrees lattitude (the lattitude of present-day Ketchikan, Alaska), and "south" means south of that, with the southernmost sample near Monterrey, Mexico.
Samples were almost all west of the Continental divide, with the exceptions in places like New Mexico where the divide is less obvious. Most of the Northern samples were near the coast.
Also, I don't think there were any single sites showing a change from atlatls to bows over time; it was "atlatls found at site A at time T1", then "bows found at site B at time T2", with site A and site B usually hundreds of miles apart.
So we need to be cautious about tying weapon adoption to a particular types of terrain or biomes or target prey. I don't think the data give us that kind of resolution, though associated finds (eg, bones of prey) might help there.

But this is important work. The switch between atlatl use and bow use shows up starkly in Figure 2. Within the "south", there's hardly any overlap at all (in other words, only a few hundred years) and that's very interesting.

 

[nartreb]

I imagine prey type is a significant determinant. Can it be taken down by a single hit with a heavier, but slower weapon? Or is rapid fire from a lighter weapon more effective?

Prey type is definitely a factor, but it's hard to predict how that plays out in practice.

Hunting scenarios rarely allow for "rapid fire". You generally get one shot (per hunter), then the prey bolts. Warfare, though, might be a different story.

My guess is that a larger, heavier weapon is likely to make a larger wound and result in more damage. That means more "stopping power", which would be important if there is a risk of a large animal charging at you. A really heavy spear can also be used hand-held to fend off an attack, and won't bend or break. (There's a reason why spears were the traditional weapon for boar-hunting in Europe.)

In theory an arrow can compensate for lower mass with increased velocity. A simple bow would get you two to four times the projectile velocity of an atlatl, depending on size, with maybe a quarter of the projectile weight. So in terms of kinetic energy (mass times the square of velocity) a bow could easily be advantageous. How that works in practice with flint tips, flexible shafts, and thick animal hides is not entirely clear, but from watching Tod's Workshop on Youtube ("arrows versus armor") I can say that a lot of the projectile's mass is in the shaft, which, being flexible and fragile, won't transfer energy well. A bigger stone point should make a big a difference, and there's a limit to how big an arrowhead can be without completely unbalancing the arrow.

A smaller projectile that travels faster should give you more range, which is an important safety factor. It's also probably easier to fire a bow without breaking concealment than it would be to use an atlatl. You might even have a chance of taking multiple shots before the prey moves out of sight or out of range (or before it can close the distance to stomp you).

My understanding is that hunting large game (deer-sized and up) with a bow often means "wound it with an arrow, then follow its trail for hours while it slowly bleeds to death." If the prey is large enough compared to your arrowhead, it may stop bleeding before you can catch up to it. Smaller game, though, wouldn't have that problem. So for something like rabbits, a bow looks like a much better choice than an atlatl.

So... which game are you hunting? Are you hunting the same game all the time? What's your hunting style - how much risk are you taking? Are you hunting alone or in groups? Is your prey alone or in a herd? How much concealment do you / the prey have? Is your prey cornered, or are you sneaking up on it? Is your weapon choice flexible enough to handle all the likely variations?

And what about the risk that your hunting party runs into a hostile band of humans? If you're carrying only one type of ranged weapon, it better be a good one. Maybe the bow's ability to carry more ammo would be an important factor here. Range too, depending on the environment. (Range doesn't matter too much in thick forests.)

 

[nartreb]

I would’ve assumed, as a first approximation, that the main determinant of kinetic energy would be how much could be transferred from the muscles, rather than the properties of the projectile? Does a bow being powered by both arms do better or worse than a spear using one arm but also at least some of the torso? And does a spear gain advantage from being a single motion rather than draw-hold-loose (these obviously not being compound bows)?

It's not how the energy gets into the device, it's how the device releases the energy. The bow is basically storing energy and releasing it suddenly - suddenly enough to reach higher speeds than you can reach with your arm, even if you extend your arm with an atlatl.
But for the record, you're not really getting any advantage by "using two arms". Each arm independently has to be strong enough to hold the full force of the bow, or you won't be able to draw it.

Historically there have been several schools of thought about the best way to draw a bow, but for many of them, notably the English [Welsh] longbow, one of the more powerful (and certainly among the hardest to draw) bows of history, you'd keep your left elbow and wrist locked straight, basically not using the muscles of your left arm at all, just as you hardly use your leg muscles while standing still with your knees locked. You'd then do a funny-looking thing with your back and hips to get your shoulder, back, and hips involved in helping your right arm pull the bow and hold it.

It is worthwhile to remember that holding a taut bow is tiring, in a way that holding a loaded atlatl is not. I can't really think of a situation where that's a major issue, but it's good to be alert to those kinds of trade-offs, lest we assume a newer technology is universally better.

 

[nartreb]

Certainly didn't seem to be a problem at Agincourt where the slower to load but easier to keep loaded until sighted on a target crossbows underperformed the rapid fire longbows.

The crossbows on the French side didn't get much use at Agincourt. You're probably thinking of Crecy, which was an unfair matchup because the (Genoese mercenary) crossbowmen were ordered to advance without their pavises, and to fight in the rain (which ruins bowstrings). When they finally turned back, the French cavalry mowed them down.
But yeah, infantry battle is a scenario where there's defintiely no point to holding onto a taut bow. Just loose and reload.