A wild fact

 Well, maybe more "a wild hypothesis," but this is the kind of stuff I LOVE learning about and I can't believe no professor I EVER had talked about it before (it's an idea that's been around since the 1870s so it's not like it's brand new).

Anyway: if you've had basic botany (or even an extensive basic biology class), you learned about "alternation of generations" in plants, or more fancily called "the haplodiplontic life cycle" 

(a link is here, I don't want to risk a copyright violation by just ganking a picture and I don't feel like trying to draw it all out myself).

Anyway, essentially: in plants there is a multicellular diploid stage (diploid: paired chromosomes; two copies of every gene) BUT ALSO a multicellular haploid (one set of chromosomes; this is the stage that would be eggs or sperm in animals and in animals is unicellular). It's something that always always always trips up and frustrates beginning botany students because while we grok the animal life cycle (because we ARE animals, after all), the plant life cycle is weird to us (and it's especially weird in things like ferns and mosses, where with a little luck, you can see the 'reduced' stage - the gametophyte in ferns, the sporophyte in mosses, and also mosses are BACKWARDS from vascular plants, with the haploid stage being the dominant one)

 Anyway, this was always presented to me as "it just IS this way" which is kind of frustrating and unsatisfying and for years in intro bio I kind of taught it that way but today in my researching plant evolution (which I was never really explicitly taught a lot about ! Despite being a botanist!) I learned a possible reason for it:

It's called the Interpolation Theory or the Antithetic Theory and in short, ti's like this: In algae, they're supposed to be haploid, that is, have only one set of chromosomes. But when they mate, they briefly make a zygote, which is diploid (just like animal zygotes, that then develop into fetuses). But in algae, this zygote is supposed to immediately do meiosis ("reduction division") to go from being diploid back to haploid. BUT. Apparently at some point in the ancient past, an early algal ancestor of the land plants did *mitosis* (simple division, which produces diploid cells instead of haploid ones) instead.

SO THE ALTERNATION OF GENERATIONS LIFE CYCLE WAS AN ALGAL SCREW UP

Yes: it was basically a biological mistake, but it's also possible (likely?) that it's a mistake that allowed land plants to become......so maybe the fact that we are mammals dwelling on land  instead of some kind of possibly-invertebrate Wiwaxia or whatever in the ocean, is because some dumb algae like a billion years ago was all "hurr durr mitosis meiosis what's the difference" and took the wrong pathway and......man, nature is fascinating.

(And yes, it was ALL I could do not to add an "OOPS ALL MITOSIS" slide into my lecture deck because I have a dumb sense of humor that way)

But you better bet I am presenting this hypothesis to the students because somehow "yeah an algae screwed up a billion years ago" is a lot more satisfying to me as an explanation than "*shrug* it just IS this way"