Etymology of "nada" and "nadie"

Question

In English, the words to describe the total absence of anything or anybody (and other similar meanings) are nothing and nobody, whose etymologies are quite direct: "no thing" or "not any thing" and the some for "no body" or "not one body".

In Spanish we have nada and nadie. According to RAE's dictionary:

• Nada comes from Latin [res] nata: "born [thing]".
• Nadie comes from nadi which comes from Latin nati: "the born ones".

So what I would like to know is: how did words that meant the existence of someone ("the born ones", "born things") come to mean the total absence of anything or anybody?

Answer 1

This phenomenon is known as the Jespersen's Cycle, and the Spanish case is described here:

Nada 'nothing' and nadie 'nobody' illustrate a completely different linguistic process, one common enough to have its own name: Jespersen's cycle, in honor of Otto Jespersen, the Danish linguist who first wrote about it in 1917. The cycle takes place when a positive element reinforces, and eventually replaces, an existing negative. It's as if the English expression a bit, which reinforces the word not in sentences like I'm not a bit hungry, were to become negative itself. In the case of nadie and nada, the cycle began with the Latin expressions non homines nati 'no people born' and non res nata 'nothing born,' which were roughly equivalent to English 'not a soul' and 'nothing on Earth.' As Latin evolved into Spanish, instead of these phrases becoming compounds, the adjectives nati and nata became stand-alone negatives, in the process undergoing the slight phonetic changes that produced nadie and nada.⁷

• ¿Por qué? 101 Questions About Spanish, Judy Hochberg
  _{Question 40. Why do most Spanish negatives begin with n?}

Answer 2

It looks like the shift in meaning is analogous to the one that made the French word personne come to mean "nobody". Suppose you have a word that means "nothing" but that word is weakened by constant use, or eroded by phonetic change to the point that it's too short or too weak or somehow not enough to indicate "nothing".

As explained in this other website, there was already in Latin an idiom (res nata) that expressed the idea of "the thing or matter under consideration". Two things then took place: 1) other forms of expressing the idea of "nothing" fell out of favor, as indicated above; 2) the existing idiom was associated with negation so much that in time it became a negative itself without the need of an auxiliary negative particle (like French personne).