In the ancient forms of the Spanish language, almost all consonants were pronounce harder than they are today. The /h/ sound is a very soft sound in the English language, as it was in Spanish. As the Spanish language changed, the consonants became much softer.
"D" is pronounced more like "dth"
"V" is often pronounced like "b"
"Z" in Spain says "th"
In the process of softening /h/, the sound was lost altogether.
Also, it was used in between two vowels to express that they were to be pronounced separately. For instance, he prohibits, originally spelled prohibe was intended to be pronounced pro-ee-beer ( even earlier pro-hee-beer) rather than proy-beer. Today, the "i" has an accent to do the same job. however the "h" is still kept.
Believe it or not, the letter "H" arouses an interesting debate. Modern Spanish linguists encourage the deletion of silent letters from words. They have been successful in words with silent letters other than "h" as the first letter. Also, the argue that the letter "ch" in Spanish should be separated into two separate letters. However, "ch" is universally accepted as one letter throughout the Spanish-speaking world.
Dr. David Yañez
Linguist & Entomologist
The letter H [in some cases that have origin from ancient Greek words] is used for the Latin words in the place of a symbol [accent - δασεία - not in use today] that gave to words a hash tone like in "Helen" or Helios = sun in Greek language