It's impossible to say for certain who the *very first* multicellular organism to invade land was, as the fossil record is incomplete. However, the best candidates are non-vascular plants, also known as bryophytes.

Here's why:

* Fossil evidence: The oldest known land plant fossils are from the Ordovician period, around 470 million years ago. These fossils resemble modern-day liverworts, mosses, and hornworts, which are all bryophytes.

* Adaptations: Bryophytes have adaptations that suggest they were among the first to colonize land. They lack vascular tissue (xylem and phloem) for transporting water and nutrients, meaning they need to live in moist environments. This suggests they evolved from aquatic ancestors.

* Early landforms: The earliest landforms were likely barren and hostile environments. Bryophytes' simple structures and ability to reproduce via spores would have made them well-suited to these conditions.

While it's possible that other organisms, like fungi or even some types of algae, may have made the transition to land before bryophytes, the evidence points towards bryophytes as the most likely candidates for the first multicellular organisms to successfully colonize land.