You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Dog

Context:
The term "domestic dog" is generally used for both of the domesticated and feral varieties. The English word dog comes from Middle English dogge, from Old English docga, a "powerful dog breed". The term may possibly derive from Proto-Germanic *dukkōn, represented in Old English finger-docce ("finger-muscle"). The word also shows the familiar petname diminutive -ga also seen in frogga "frog", picga "pig", stagga "stag", wicga "beetle, worm", among others. The term dog may ultimately derive from the earliest layer of Proto-Indo-European vocabulary.

Question:
Dog could also come from the original layer of what vocabulary?

Proto-Indo-European