Plato playfully toyed with the idea of defining Man as a featherless bi-ped.
I thought of this last night while watching some show about Human Origins on the the History Channel. Humans are distinct from apes in many ways, of course. You've seen the list. Walking upright on two feet (bi-pedalism). Opposable thumb. Tool making. Speaking. Reasoning. Concept forming. Bigger brained. Long lived. Etc.
Paleontologists now think that the first of these traits to appear - the first step, as it were, away from apeness, was walking on two feet. It had the immediate advantage of letting us see farther. It had the subsequent advantage of freeing up our hands to use tools. Bigger brains allowed us to make better tools.
At least, that's the story as they see it now.
In days of yore,
The ape that walked
Came way before
The ape that talked.
On a different note, the TV show said that "Lucy", the famous bipedal fossil hominid, was named after the Beatles song, Lucy In The Sky With Diamonds.
To the tune of Lucy In The Sky:
Picture yourself in the past in a world
Where gigantic predators thrive and abound.
Suddenly someone is there in the grass land
The girl with two feet on the ground.