“Is this the same as saying that the first humans on earth were human gandhabbas?”
Not really. Those early humans “came down” from the Abhassara brahma realm and were very similar to those brahmas.
– Brahmas do not need food. Human gandhabbas of the present time do need to inhale aroma (that is how they get a bit more dense bodies).
– At that time, there was nothing on Earth to provide such aroma.
– After long times, Earth became populated with plants, and then animals. At that time they probably started inhaling aroma, and thus began the gradual process towards physical bodies built with cells. It was a gradual process.
– Even today, when a human bhava first starts, that manomaya kaya (of the gandhabba) is close to that of a brahma, but those gadnhabbas start inhaling aroma soon, and get a “bit more dense” than brahmas who have just a few suddhashtaka (hadaya vatthu and a few pasada rupa).
– Those suddhashtaka are much, much smaller than even an electron, which is one of the smaller fundamental particles in physics.