When you load existing animation and start to create animation layers, you can access your existing animation in the Animation Layer Editor using the BaseAnimation bar.
To load existing animation to work with layers
Although you have only created one layer, you will also notice the BaseAnimation bar appears in the Animation Layer Editor. The BaseAnimation bar lets you access your existing animation with the layer tools.
The BaseAnimation bar can be locked, but because it is not a layer, it cannot be muted, soloed, or parented to other layers. To mute or solo the animation represented by the BaseAnimation bar, extract it to a layer. See Extract animation to or from an animation layer.