Carnivorous plant nerd here (of the amateur variety, I have no formal training in evolutionary biology or anything like that). Last time I looked into this the going theory was that the Venus Fly Trap's first carnivorous ancestors were somewhat similar to today's sundews (Drosera) that catch insects by having sticky secretions on their leaves. Keep in mind there is basically no fossil record to work with here, as most carnivorous plants are classed as herbs (ie, non-wood forming plants) so all we have are educated guesses.
Many sundews are capable of moving their leaves once they have caught an insect in order to bring more of the insect's body in contact with the digestive glands on the leaf, but these movements tend to happen over the course of an hour or so and are pretty much imperceptible in real time (with a few notable exceptions, such as Drosera glanduligera). Also bear in mind there are non-carnivorous plants that have also evolved to move rapidly, so this trait isn't exactly unique.
The theory is that the Venus Fly Trap had sticky ancestors that progressively got faster and faster in their movements to trap larger, ground-dwelling bugs like spiders and beetles. Further, instead of digesting their prey on a leaf surface completely exposed to the elements (such as rain, which can wash the prey away), over time the plants that could fully enclose their prey and drown it in digestive enzymes were selected. The previously sticky hairs were converted into the interlocking "teeth" on the margins of the trap, with a few remaining on the inside to serve as the "triggers". The digestive glands remained roughly the same, except they no longer protrude from the surface of the leaf.
After a few million years, boom, you have a Venus Fly Trap. Having grown many types of sundews and many VFT cultivars over the years I've come to appreciate how similar they can be, which isn't terribly surprising as they're both a member of the Droseraceae family taxonomically speaking.
And if you think that's convoluted, you should try explaining the trapping mechanism of Utricularia. That shit is just insane.
Watch the video here:
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