Lo can mean the following:
It, you or him as a direct object pronoun.
La clave de lo humano translates to the key of the human or even the key of it human.
The first thing you should do is make sure that your adjectives define the noun.
If you were saying the human key/la clave humano. However, because you are asking about the following phrase it would be similar to this:
Can this be translated as "it is the key to being human?"
Es la clave a ser humano
Es la clave del ser humano.
Esto es la clave para ser humano.
It is what defines the key to human life.
Es lo que define la clave de vida humana.
Estos deberían ser suficientes.