Earth currently has a second-generation atmosphere.
The first atmosphere was abundant in H2 and He. However, these light gases easily reach escape velocity, and drifted into space. Compared to the noble-gas composition of chondritic meteorites, we find that the atmosphere is extremely depleted in noble gases. Something about the size of Mars probably collided with the earth early on. It was only a glancing blow, and the atmosphere was sucked away by the passing object.