I must disagree.
Our oldest weapon is almost certainly the club or mace, as we could just pick up a likely-feeling piece of branch and hit things with it. But for the vast majority of human history, our manufactured weapon of choice has been a spear. The knife was first developed to skin and butcher the animals we killed by the spear, and then by the atlatl, or bow.
When you took a small blade onto the battlefield with the intent of fighting with it, you carried it as a fallback option. For example in the Roman army, a
pugio was standard equipment. A
dagger is very different than a
knife in that it has a very fine point, and sharpened on both edges. It's optimized to stab. It's also banned in most places in the USA. A knife can be used as a weapon, but it's not designed to be a weapon from the ground up: your average combat knife sees more time opening containers and jamming things one way or another than it sees blood. The honor of "purpose weapon" falls to the dagger. And even a dagger is a very limited weapon next to a sword, which largely replaced it, or any of the polearms which remained popular until after gunpowder started taking over.
Even in World War 1, with the rise of trench-fighting, soldiers needed a melee weapon... But if you look at most "
trench knives", they aren't knives at all, they are daggers and stilettoes.
*shrug*