Do scientists ever consider the possibility that a bird could come from a chemical soup like some other organisms supposedly did? You know through the process of abiogenesis? (Not a full grown bird, but a cell that develops into a bird.)
That appears to be exactly what happened! It's just that those same first cells also have flatworms and nematodes and trilobites and humans as descendants, too. That's why the DNA of everything alive on Earth looks so very similar, and why all of life here uses the same twenty amino acids.
The only organisms that hopped out of that primordial soup have been dead and gone for about 4,000,000,000 years now: the very difficult task of figuring out what they were like can only be done by comparing the life forms we still can see - the fossils and the biochemical clues in living things.