As firms have held on to workers during the economic downswing, they will probably be slow to hire more in an upswing that anyway looks likely to be lethargic.
The answer to this riddle lies in the metabolic activity of early photosynthetic life-forms that slowly but surely transformed the chemical composition of Earth's atmosphere.
That's because very large animals have a slow rate of reproduction, so all you have to do is remove a few young females from the herd, and you can, fairly quickly, significantly reduce the population.
MyCn18's hourglass shape is thought to have arisen from the expansion of a fast stellar wind within a slowly expanding cloud which is denser near its equator than its poles.