Etymology #1: Eat

At first glance, the English word eat and its Spanish equivalent comer may not resemble one another, but their history is more closely related than meets the eye.

The Modern English eat derives from the Old English etan, a class V strong verb whose past tense was aet (think ate) and past participle was eten (= eaten). Its original meaning was "to eat, devour, consume" and it shares its roots with other Germanic tongues: cf., Old Frisian ita, Old Saxon etan, Middle and Modern Dutch eten, Old High German ezzan, Modern German essen, Old Norse eta and Gothic itan. All of these forms spring from the Proto-Germanic *etanan.

(Asterisks indicate unattested forms)

Comer, on the other hand, took root from a combination of the prefix com- (meaning 'with') plus edere (= 'eat'). According to Sebastián Covarrubios Orozco (1539-1613), author of Tesoro de la Lengua Castellana o Española, "comedere has the suffix con- to remind us that we shouldn't eat alone". Comedere actually meant 'to eat everything' or 'to devour', much in the same way that the con- of consume meant 'to take everything'. Vulgar Latin chose comedere over edere, thus causing its disappearance. Then, through phonetic evolution rules that transformed Latin to Spanish, voiced medial consonants such as b, d, and g disappear, as does the final e, thus reducing the form to comer (cf. sedere --> ser, videre --> ver and cadere --> caer).

Both of these stem from the Proto-Indo-European root *ed-, whose meaning was 'to eat' or 'to bite'.

Unrelated but similarly of interest is the Japanese equivalent, 食べる (taberu). 食べる (taberu) traces its history to 賜ぶ・給ぶ (both read tabu), an honorific verb equivalent to Modern Japanese お与えになる (o-atae ni naru)　（from 与える ataeru） meaning "to give". It was originally used as a humble verb with the meaning 'to receive from a superior', from which the modern meaning of 'receive and eat or drink' evolved.

Sources

1. Online Etymological Dictionary
2. Origen de las Palabras
3. 語源由来辞典 Gogen Yurai Jiten