Why the bow replaced the atlatl: Tomka 2013 study

This is an archaeology paper: Tomka wants to understand why the bow and arrow replaced the spear-thrower (atlatl) and dart in the Americas, and does so by measuring what a bow of "historical" power can deliver. From the sources he estimates that Native North American bows mostly drew between 40 and 50 pounds (10 of the 17 documented bows) and, by weighing 121 historical arrows in a museum collection, that a typical arrow weighed about 23 grams, with 85% between 16 and 28. Using a machine, he shot a laminated recurve at 40, 45 and 50 pounds with arrows of 23, 28 and 33 grams, measuring speed at 20 metres and penetration into a foam target.

The 23-gram arrow went from 45.5 m/s at 40 pounds to about 54.3 m/s at 50 pounds; at 50 pounds the 33-gram arrow was about 3 m/s slower but penetrated further (at 40 pounds, 13.2 cm against 11.7). Kinetic energy ranged from 17.2 to 32.4 foot-pounds and momentum from 0.23 to 0.38 slug-feet per second. Set against the thresholds that manufacturers and the hunting regulations of some African countries recommend for game of different sizes, according to the author all three weights at 50 pounds were enough for medium game such as deer and pronghorn, and no combination was enough for large or very large game such as bison. The heaviest arrow lost 1.7 m/s over the first 15 metres, and by the author's estimate it would still carry enough momentum for medium game at 45 metres.

From this comes the model: the bow would have been adopted for its speed, useful against agile medium-sized prey, while the dart, slower but far heavier, stayed superior against big game. Abandoning the spear-thrower would then depend on the decline of the largest prey, and bison hunting with the bow would have worked thanks to communal drives and corrals, which let hunters get close and shoot many arrows.

The bow was a laminated Browning Wasp recurve mounted on a purpose-built rig: a winch drew the string to the chosen draw weight (40, 45 or 50 pounds) and a mechanical caliper release let it go. The three arrows were Easton X7 2112 aluminium shafts, all 29 inches long, with an 8.1-gram point and three 4-inch plastic feathers; the base arrow weighed 23 grams, and the other two had 5 and 10 gram metal plugs added behind the point insert. Each arrow and draw weight combination was shot 30 times indoors, with a chronograph at 20 metres and a Yellow Jacket foam target as backstop; shots that missed the chronograph window were repeated. In 10 further trials two chronographs timed the heaviest arrow as it left the bow and at 15 metres, and in another 10 at 10 metres.

The energy and momentum thresholds for each size of prey come from an arrow maker's calculator and from the bowhunting regulations of Namibia, Botswana, South Africa and Zimbabwe, regrouped by the author according to animal weight. The data on the spear-thrower are taken from experiments by others.

Penetration was measured in foam, and the author himself warns that it is only a comparison between arrows, not an estimate of penetration into tissue. The "lethality" thresholds are modern hunting recommendations, not measurements, and they vary considerably from one country to another. There is a single bow, a modern recurve that the author takes to represent the upper end of what a historical longbow of the same draw weight could do, and a mechanical release has nothing in common with a finger release; spread in speed is not reported, only means and charts.

Some figures also need care: in the 10-metre test the units are swapped (the energy, 34.5, is given in slug-feet per second and the momentum, 0.40, in foot-pounds); the 23-gram arrow at 50 pounds, which the text counts among those sufficient for medium game, reaches only 24.4 foot-pounds in the table, just under the threshold of 25; and the 45-metre estimate is an extrapolation from a loss measured over 15 metres. The adoption model is an archaeological hypothesis, which the author himself says is hard to test against the finds.

For target archers the study changes nothing: it is about hunting and prehistory. It remains a tidy illustration of a principle that holds for every bow: at the same draw weight a heavier arrow leaves more slowly but carries more momentum, while a lighter one is faster and flies flatter. Traditional archers will find some concrete orders of magnitude in it: with this recurve the 23-gram arrow went from 45.5 m/s at 40 pounds to about 54.3 at 50, so 10 extra pounds were worth almost 9 m/s, while 10 extra grams on the arrow took away about 3.