I dare say most of us don't think of plants as particularly dangerous things. Sure, some of them are poisonous, but you generally have to eat them first. A few will give you a nasty rash. Some even eat animals. Unless you're actively interacting with them, though, there's not much they can do to you.
As it turns out, they've already done their worst... and their worst was photosynthesis.
It's worse than it sounds, believe me, and here's why. Let's imagine for a minute that a new species of organism evolved today, one with a really exotic metabolism -- one that, where you and I take in oxygen and release carbon dioxide, took in our familiar air and released hydrogen fluoride. As the article behind that link describes, hydrogen fluoride is truly vicious stuff -- it's violently corrosive and extremely toxic, the kind of acid that dissolves glass outright and kills you with acute fluoride poisoning if the horrid chemical burns don't do it first.
Now imagine that this organism succeeded. Really succeeded. It spread all over the planet. Its breath corroded the very ground it lived on, and now there was a lot of it -- enough to burn off the entire surface of the earth. It wouldn't stop when the soil was seared away and the rocks saturated with fluoride, either: those things would still be alive, and still spitting their poison, only now it'd be building up in the air. In a geological eyeblink, they'd choke the entire atmosphere with toxic acid. I'm not going to describe exactly what would happen to life on Earth in that situation, but you can probably fill in the blanks yourself.
That's what happened when photosynthesis first arose.
When the earth was young, and life was just getting started, there was virtually no oxygen in Earth's atmosphere. Free O2 is reactive stuff; it had all gotten bound up in carbon dioxide. The atmosphere of the time was mostly nitrogen and CO2. Early life on Earth had evolved to be comfortable in that environment; to those early species, oxygen was a corrosive metabolic poison -- much like, say, hydrogen fluoride is to us.
Then, about 2.5 billion years ago, the first photosynthetic organisms evolved. They were something like cyanobacteria -- blue-green algae -- and probably not very efficient by modern standards, but they succeeded, and they started cranking out oxygen by the ton. At first it just corroded the rocks, oxidizing them quickly and thoroughly; those first blue-green algae literally rusted the entire planet, and when the rocks were saturated, they started poisoning the atmosphere.
We have no way of knowing what the death toll was like, as free oxygen built up towards the modern 20% or so -- hard skeletons hadn't evolved yet, so nothing much from that era ever fossilized -- but it was assuredly unimaginable. This Oxygen Catastrophe was the first of Earth's great mass extinctions, and it made the slaughter of the dinosaurs look like barely a blip.
Mercifully, it never happened again. The few species that survived the Oxygen Catastrophe learned to work with free oxygen; aerobic respiration actually turned out to be pretty effective. The staggering diversity of life today is all descended from those survivors.
...Well, it never happened again, until very, very recently. Just an evolutionary eyeblink ago, another species with the ability to transform its whole world's atmosphere arose. It spread across the globe. It started cranking out its chosen gases in geologically-significant quantities. Just now, some individuals have even begun to notice what their species is doing. Some of them are taking steps toward making it stop.
Makes you think twice about how scary climate change might be, doesn't it?