By the Devonian, the first chapter that would be repeated in so many different variants was already written.

Early life was strange

archaeopteryxx 2599 points 7 years ago*

"Darwin's Dilemma" refers to the absence of known macroscopic fossils (ie. excuding microbial stuff) from beneath the base of the Cambrian period (ie. older than 541 million years) when he was first describing his theory of evolution. Everyone (including Darwin) expected that when fossils were found from before the Cambrian period that they would be the ancestors of modern animals.

Then when those fossils WERE found (Ediacara biota we call them, after the Ediacara Hills in South Australia), they were enthusiastically classified as ancient jellyfish, ancient worms, etc.

But it turns out that's a load of crap. Consensus is growing that almost none of those classifications were accurate. Instead, most Ediacaran organisms seem to have belonged to extinct groups unrelated to anything alive today, and they lived in a world that was so different from the following 540 million years, they had modes of life distinct from any living macroscopic organisms. We were so eager to make everything fit into our neat little evolutionary narrative that for decades we basically glossed over these crazy, alien sort of ecosystems that could be critical to understanding the origins of complex life on other planets as well as our own. We're still just beginning to really understand it though. It's a very controversial field of paleontology.

EDIT: Wow this gained a lot more traction than I expected! Thanks for the interest and all the great questions, and the reddit gold! Usually when I start talking about paleontology people's eyes glaze over. :)

I just wanted to add a disclaimer that this comment, and many of my replies, were written very quickly, without the expectation that they'd get much of an audience, and there were some things I didn't put well that I wanted to set straight!

I shouldn't have used the word "unrelated" in my original post -- many Ediacaran fossils were bizarre "failed experiments" that left no living descendants, and therefore can't be allied with modern groups of macroscopic organisms, but there isn't any reason to suspect they don't share common microbial or multicellular ancestors.

Darwin being wrong about the nature of the fossils that would later be found from before the Cambrian in no way opposes or weakens the theory of evolution. On the contrary, evolutionary theory is critical to the study of the Ediacaran biota. The early evolution of animals was just more convoluted than Darwin anticipated.

Also I should mention that there were some organisms in the Ediacaran that quite possibly were precursors to modern animals. Eg. __Kimberella__ was definitely a bilaterian and has been proposed as an early mollusk, and __Thectardis__ might be a sponge. The weirder ones I was referring to above, for those seeking more information, include the rangeomorphs, erniettomorphs, and trilobozoa, among others.

If anyone wants to learn more about the Ediacaran period and the Ediacara biota, feel free to PM me! I think it's the coolest subject ever (it even pried me away from my childhood obsession with dinosaurs) and it's a shame so few people know about it!

• Reddit community, Scientists of Reddit: What's craziest or weirdest thing in your field that you suspect is true but is not yet supported fully by data?

Proterozoic

'Dickensonia' was the one of the first to move. Ranged from the size of a penny to that of a bathmat.

First digestive system