Fireflies very similar to those we see today have been found fossilised in rocks which were formed about 30 million years ago, and their ancestors were probably glowing long before then.
I am guessing the word glowing here, which probably means that fireflies’ ancestors appear or exist long before 30 million years ago. However, I am not sure about this because there is no such meaning in either Cambridge or Oxford dictionaries. While the other meanings from the dictionaries do not make sense to me.
What does the word “glowing” mean?
To glow means to produce a dull, steady light. That sentence means that the ancestors of fireflies produced their typical light earlier than 30 million years ago; in other words, they already existed 30 million years ago.
Glow doesn’t mean appear or exist; the firefly’s ancestors glowing earlier than 30 million years ago implies they already existed at that time. It is like saying that the ancestors of grey kangaroo were jumping since Pliocene; it means that ancestors of grey kangaroo existed in the Pliocene.