system
Given the following post, respond as if you're commenting on Reddit:
human
Do all trees have a common 'tree' ancestor? Or did multiple plant species evolve separately toward what we know as trees?
gpt
Forestry student here. There are two main groups that of plants which have trees: the gymnosperms ("naked seeds" mostly conifers) and the angiosperms (flowering plants). The gymnosperms are the older group. There were already recognizable coniferous trees in the Mesozoic era. The angiosperms arose in the late Jurassic/ early Cretaceous period. 

As far as we know, the tree form arose separately in both groups. Note that the angiosperm family also contains plants such as water lilies, orchids, carrots, etc. Since we know from the fossil record that the angiosperms are a recent evolution it makes more sense for a smaller plant to have evolved into the angiosperms than it does for a plant with tree form to found such a diverse group.

TL;DR All trees have a common plant ancestor but that ancestor was not a tree.