There ARE some organisms that somewhat resemble this ancient type: sea pens, and basket stars. Did these fronds have the ability to curl and move? If they were the first life, there was little to curl about. But inhale and respire and ‘eat’ what they respire?
To a certain extent, I suspect koi succeed so well because they eat what they breathe: even in water that can get pretty mucky green with algae, some of that algae must go into the gut as the water goes through the gills. In the case of these creatures, they could just sit and respire and consume basic rock chemicals directly from the water.
Learning how to utilize an excess of calcium was a biggie. Muscle and contractile tissue is calcium dependent. Skeleton develops because specialized tissue lays down calcium carbonate, a combo of carbon and calcium which is that chalky stuff on your shower. At vinegar ph, it dissolves.
When the early oceans hit a chemistry of 8.3 dkh alkalinity and ph about there (modern oceans about 7.9 ph); a dissolved magnesium level of 1200 and a dissolved calcium of 420, the oceanic chemistry at least locally entered a ‘lock’ of stability, in which as long as that magnesium stays 1200 or better, the calcium and alk will ride at about that level, given a supply of ‘fossil calcium’ or calcium water. H20’s use of calcium stays rock-steady at this reading, particularly as it can access old dead coral skeleton to dissolve it and keep the calcium up so living coral doesn’t deplete it. The early establishment of a calcium supply from, one supposes, dead rock, would enable marine organisms to establish a comfort-level existence in which a microbial population adapts to this situation—and the nature of mg/cal chemistry in water maintains the situation, so that the animal population can continue doing the same thing, generation after generation, and get better at it. There’s no saying that ALL ancient microbes were micro—-there may have been macrobes. Big squishy monocellular life, that at some point anchored successfully where the best food was and thereby got to the dinner trough first, and reliably. Big reliable supply means —it thrives. It divides. It multiplies.
In marine tanks, we hit that magic lock-point chemically, and snail shells stay hard, fish stay healthy, stony coral grows: the water at that ph dissolves just enough calcium out of old coral to keep that level…so long as the magnesium level stays steady at 1200 or a bit above.
Us walkabout on land types have somewhat the same needs: we lay down calcium for bones, our contractile tissues still use calcium as the driving force, and if we run short of magnesium our backs ache and our guts stop moving.
We and corals do have some things in common.