Define "huge". There are about ~30 mutations on the spike protein from the closest known relative, and there's a fairly strong selective pressure against the wild type (both from vaccine and natural infection). I think it's quite probable that we missed a few months of evolution of a strain circulating in a single-digit percentage of people.
This is particularly true when you consider that this strain has S-gene dropout (Delta does not), and was probably dismissed as Beta and never targeted for sequencing by most labs. That's why I meant by my original comment that we're biased about what we look at.
We're panicking and closing borders and doing untold damage because...we continue to overestimate our ability to see what nature is doing in an unbiased way.
4. Evolution of a lineage that we didn't detect until now because it wasn't common.
We need to stop making the assumption that our surveillance systems are comprehensive. They're pretty bad, actually -- and they're generally incredibly biased in what they're looking at. Even if we were doing unbiased random sampling and sequencing, we'd have to do huge volumes of it to reliably detect rare strains.
(aside: and if we did do that, we'd be in non-stop panic mode, because we'd be finding a new strain every day.)