The first organisms on earth appeared approximately 3.8 billion years ago and were all unicellular. multicellular organisms have only existed for about one billion years.

what evolutionary milestone made it possible for more complex, multicellular organisms to exist?

The phenomenon of endosymbiosis gave rise to more multicellular organisms.

Life on Earth is known to have started from prokaryotes. Prokaryotes were simple, unicellular organisms which managed to survive the harsh conditions of the Earth.

The theory of endosymbiosis provides evidence that with the passage of time, the bigger prokaryotes began engulfing the smaller prokaryotes. As a result, multicellular organisms were formed. Organelles like the mitochondria and chloroplast support the validity of this hypothesis.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.