First, take a very large dense star so similar to a seed that when it implodes from its own mass you've created one frightfully beautiful tree of life called 'Youniverse'. Then, after things cool down a tad, a carbon-rich atmosphere appears on earth which supports the algae, lichens, moss, fungii and their ilk. These, left to their own devices which means a lot of hanky panky, should prosper, simultaneously enriching and creating soils. A young sun will be ideal for the locomotion of energy conecting long chains of carbon thru a process dubbed 'photosynthesis'. Plants unwittingly make dandy little oygen factories for the animals which in their thoughtfulness will eat the plants. As genetic mutations abound everywhere eventually hominoids will appear because life as a monkey was full of bananas. The cats, monkeys, birds and hominoids will find the trees suitable for climbing and escaping the wiliest things like complacency, sabre-tooth flying squirrels, and mean people whose ancestries like trees can also be traced back to algae. Hominoids will learn bipedalism and Tango meets Disco meets potato sack races. Handy for reaching, scratching the privies and generally for commenting on someone else's bad driving in the form of a "T" , arms will become handy for hugging which is why arms are so fucking awesome!!! On those lonely Autumn days when all the Sycamores quit making their chlorophyl and the reds and ambers and the very, very oranges show through their crinkly leaves, yes, the trees Bob, will be irresistable for hugging. True to form, that first homonoid hippy probably felt a tinge of guilt for hugging his first tree after doing what all men do out in the woods without a descent john in sight.