How do we know that the apes we descend from, maintained a tail (maybe vestigal) far the evolution steps ? Is there direct evidence that the tailless monkeys in our evolution steps are indeed tailless ?
All the other apes (chimpanzees, gorillas, orangutans and gibbons) are tailless, which suggests that their ancestors, after they split off from the monkeys, were all tailless.
I find a little tail every Sat night. 
A Swinging good time.
Does your wife know?
The best known early ape (or perhaps close ape relative) wasProconsul,which originated about 23 million years ago. Like later apes, it lacked a tail.
Since being tailless is one of the defining characteristics of apes vs monkeys, we can say that the apes we descended from lacked tails. (A few species of monkeys also lack tails.)
Is “tailed vs. tailless” a distinct branching point in our ancestry? Or have tails gone and come again in some lines of apes/monkeys?
(Isn’t the same true for zebras and other striped horses? There are both striped and unstriped equines along lineages, so that “striped/unstriped” isn’t a primary branching distinction. I think that’s what Stephen Jay Gould said in his essay on the subject.)
AFAWK, yes. As noted, “tailless” is one of the attributes of an ape.
Yep. " Zebra" is a paraphyletic term.
Great question - I had similar thoughts. Also, Colibri - can you please elaborate how did they deduce it had no tail by looking at the skeleton?
I’m no expert, but the skeleton having no tail would be clue #1. That’s not as sarcastic as it sounds–you can tell where the (appropriately named) tailbone attaches and ends if you have a pelvis to look at. I presume they have other skeletons than the ones shown in the Wiki articles, with parts or whole pelvises.
As Speaker says, you can tell by looking at the sacrum and pelvis. Although most internet references said Proconsul was tailless, apparently this has been somewhat controversial. Recent evidence apparently confirms that it didn’t have a tail.
Whether or not Proconsul had a tail, by 15 million years ago it seems that apes had definitely lost their tails.
[QUOTE=Trinopus]
Is “tailed vs. tailless” a distinct branching point in our ancestry? Or have tails gone and come again in some lines of apes/monkeys?
[/QUOTE]
In apes, it seems that once the tail was lost it was never regained. As I mentioned, a few monkeys have also lost their tails independently. I am not aware that any monkey has re-evolved a tail once having lost it.
we did not decend FROM apes, we share acommon ancestor with them.
Well, strictly speaking, we ARE apes. Of course, depending on your taxonomical philosophy, we’re fish, too.
We did not descend from any living ape. But yes, the animals we did descend from are extinct species that are classified as apes. As jayjay says, that makes us taxonomically speaking apes.
Were our ancestors* not apes? They were. Hence, we descend from apes. We can still be apes, descended from apes.
*back at least 20M years.