Intro: Today's oceans are full of fish with fins that couldn't evolve into limbs like ours.
So, who are our ancestors and where did they go?
You've probably heard that every bony land-dwelling creature on Earth evolved from fish
that grew limbs and crawled out of the water many million years ago.
And yet, over 99% of the fish in today's oceans, lakes and rivers have incredibly delicate fins,
formed by a series of spines covered by a thin web of skin.
It's hard to imagine fins like those evolving into sturdy,weight-bearing limbs...
because they didn't. But the fins of a far more foreign fish did.
Very soon after bony fishes first evolved, they branched into two groups,
those with flimsy ray-type fins,
and those with more meaty appendages that were attached to their bodies by a single strong bone.
At first, those so-called lobe-finned fishes far outnumbered their ray-finned relatives,
but just as some of them were becoming adapted for life in the weedy shallows,
global climate fluctuations caused a mass extinction that devastated life in the oceans.
After the catastrophe, the ray-finned fishes recovered and took over the seas.
The lobe-fins never recovered,
but a few survivors in the shallows managed to find their footing out of the water,
and eventually gave rise to every backboned,
four-limbed creature to ever crawl, hop, fly or walk on land.